JP5180104B2 - Surface acoustic wave device - Google Patents

Description

  The present invention relates to a surface acoustic wave device.

  2. Description of the Related Art Conventionally, a surface acoustic wave element is known in which a support substrate and a piezoelectric substrate that propagates an acoustic wave are bonded together and a comb-shaped electrode that can excite a surface acoustic wave is provided on the surface of the piezoelectric substrate. Here, by attaching a support substrate having a thermal expansion coefficient smaller than that of the piezoelectric substrate to the piezoelectric substrate, the change in size of the piezoelectric substrate when the temperature changes is suppressed, and the frequency characteristics of the surface acoustic wave element are reduced. The change is suppressed. For example, the surface acoustic wave element described in Patent Document 1 has a structure in which a piezoelectric substrate and a support substrate are bonded together with an adhesive layer. This surface acoustic wave element further suppresses the occurrence of spurious by roughening the surface (back surface) of the piezoelectric substrate to be bonded to the support substrate. That is, a bulk wave, which is a kind of elastic wave generated in the vicinity of the comb-shaped electrode, reaches the back surface of the piezoelectric substrate, but is scattered because the back surface is roughened. In this manner, the bulk wave is prevented from reflecting on the back surface of the piezoelectric substrate and reaching the comb-shaped electrode, thereby suppressing the occurrence of spurious.

JP 2001-53579 A

  However, in the surface acoustic wave element described in Patent Document 1, if the back surface of the piezoelectric substrate is roughened, the thickness of the adhesive layer that bonds the piezoelectric substrate and the support substrate may not be sufficiently reduced. In this case, although it is possible to scatter bulk waves on the back surface of the piezoelectric substrate and suppress spurious generation, the original role of the supporting substrate is to suppress changes in the size of the piezoelectric substrate when the temperature changes. In some cases, it could not be fully achieved.

  The present invention has been made in view of the above-described problems, and for a surface acoustic wave element in which a piezoelectric substrate and a support substrate are bonded together, while suppressing changes in the size of the piezoelectric substrate when the temperature changes, The main purpose is to suppress the occurrence of

  The present invention adopts the following means in order to achieve the above-mentioned object.

The surface acoustic wave device of the composite substrate of the present invention is
A piezoelectric substrate capable of propagating elastic waves;
A comb electrode provided on the surface of the piezoelectric substrate and capable of exciting a surface acoustic wave;
A support substrate that is bonded to the piezoelectric substrate, has a back surface that is rougher than the back surface of the piezoelectric substrate and has a roughness that scatters bulk waves propagating from the piezoelectric substrate, and has a smaller thermal expansion coefficient than the piezoelectric substrate; ,
An organic adhesive layer for bonding the piezoelectric substrate and the support substrate;
It is equipped with.

  In the surface acoustic wave device according to the present invention, when the bulk wave propagates from the piezoelectric substrate to the support substrate, the back surface of the support substrate has a roughness that scatters the bulk wave, and the bulk wave is reflected by this back surface to form a comb shape. Reaching the electrode is suppressed, and spurious generation is suppressed. Furthermore, it is the back surface of the support substrate that has a roughness that scatters the bulk waves propagated from the piezoelectric substrate, and the back surface of the support substrate is rougher than the back surface of the piezoelectric substrate. For this reason, the thickness of the organic adhesive layer can be reduced, and the effect of suppressing the change in the size of the piezoelectric substrate when the temperature is changed by the support substrate can be obtained. In particular, in Patent Document 1, the thickness of the organic adhesive layer needs to be equal to or greater than the difference between the highest and lowest roughness irregularities that scatter surface acoustic waves. In the present invention, the thickness is thinner than the difference. can do.

FIG. 5 is a cross-sectional view schematically showing a manufacturing process of the surface acoustic wave element from the preparation of the LT substrate 10 and the silicon substrate 12 to the roughening of the back surface of the silicon substrate 12. It is the top view and A-A 'sectional view of a surface acoustic wave element. It is the top view and B-B 'sectional view of other surface acoustic wave elements. 3 is a measurement result of frequency characteristics of the surface acoustic wave elements of Example 1 and Comparative Example 1.

  In the surface acoustic wave element of the present invention, the piezoelectric substrate can propagate an acoustic wave. Examples of the material of the piezoelectric substrate include lithium tantalate, lithium niobate, lithium niobate-lithium tantalate solid solution single crystal, and the like. The size of the piezoelectric substrate is not particularly limited, and for example, the vertical length is 0.5 to 2 mm, the horizontal length is 0.5 to 2 mm, and the thickness is 10 to 60 μm.

  In the surface acoustic wave device of the present invention, the comb-shaped electrode is provided on the surface of the piezoelectric substrate and can excite surface acoustic waves. Examples of the material of the comb electrode include aluminum, aluminum alloy, copper, and gold.

  In the surface acoustic wave device according to the present invention, the support substrate is bonded to the piezoelectric substrate, and has a back surface that is rougher than the back surface of the piezoelectric substrate and has a roughness that scatters bulk waves that have propagated from the piezoelectric substrate. The coefficient is smaller than that of the piezoelectric substrate. Examples of the material of the support substrate include silicon, sapphire, aluminum nitride, alumina, borosilicate glass, and quartz glass. Moreover, the magnitude | size of a support substrate is although it does not specifically limit, For example, length is 0.5-2 mm, width is 0.5-2 mm, and thickness is 0.25-0.5 mm. Here, the roughness of the back surface of the support substrate is such that the arithmetic average roughness Ra is 0.25 to 0.55 μm, and the maximum height Rz is 9 to 12 times the arithmetic average roughness Ra. preferable. When the roughness of the back surface of the support substrate satisfies this condition, the effect of scattering the bulk wave propagating from the piezoelectric substrate is sufficiently obtained, and it is satisfactorily attracted when the composite substrate is attracted to the electrostatic chuck. It is because it can be made. As a method of satisfying this condition with respect to the roughness of the back surface of the support substrate, for example, polishing is performed using abrasive grains having a number of 800 to 1200.

  In the surface acoustic wave device of the present invention, the organic adhesive layer bonds the support substrate and the piezoelectric substrate. Examples of the material of the organic adhesive layer include a solidified epoxy adhesive or acrylic adhesive. Furthermore, the thickness of the organic adhesive layer is preferably 0.1 to 1.0 μm. In this way, changes in frequency characteristics with respect to temperature changes can be relatively suppressed. If the thickness of the organic adhesive layer exceeds 1.0 μm, the difference in thermal expansion coefficient between the piezoelectric substrate and the support substrate is absorbed by the organic adhesive layer, and the effect of suppressing the change in frequency characteristics with respect to temperature change is not obtained. Absent. Moreover, when the thickness of the organic adhesive layer is less than 0.1 μm, it is not preferable because the effect of suppressing the change of the frequency characteristic with respect to the temperature change cannot be obtained due to the influence of the void.

[Example 1]
FIG. 1 is a cross-sectional view schematically showing the manufacturing process of the composite substrate of this example. First, a silicon substrate 12 having an orientation flat portion (OF portion), a diameter of 100 mm, and a thickness of 350 μm was prepared as a substrate before being cut out into a support substrate. In addition, a lithium tantalate substrate (LT substrate) 10 having an OF portion, a diameter of 100 mm, and a thickness of 250 μm was prepared as a substrate before being cut out into a piezoelectric substrate (see FIG. 1A). The LT substrate 10 is a 36 ° Y-cut X-propagation LT substrate in which the propagation direction of surface acoustic waves (SAW) is X and the cutting angle is a rotating Y-cut plate. Next, an epoxy-based adhesive 13 was applied to the back surface of the LT substrate 10 by spin coating, and was superposed on the surface of the silicon substrate 12 and heated at 180 ° C. to obtain a bonded substrate 20. The organic adhesive layer 14 of the bonded substrate 20 is formed by solidifying the epoxy adhesive 13 (see FIG. 1B). At this time, the thickness of the organic adhesive layer 14 was 0.3 μm.

  Subsequently, it grind | polished until the thickness of LT board | substrate 10 became 30 micrometers with the grinder (refer FIG.1 (c)). As the polishing machine, a machine that performs mirror polishing after reducing the thickness as follows was used. That is, when the thickness is reduced, the bonded substrate 20 is sandwiched between the polishing platen and the pressure plate, and a slurry containing abrasive grains is supplied between the bonded substrate 20 and the polishing platen. A pressure plate was used to rotate the pressure plate while pressing the laminated substrate 20 against the surface plate by using a plate. 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, A substrate whose surface was mirror-polished before being cut into a piezoelectric substrate was used. First, the surface of the LT substrate of the bonded substrate 20 was pressed against the surface of the platen, and the rotation was performed at a rotation speed of 100 rpm and the polishing duration was 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 20 against the surface plate is 0.2 MPa, rotation of rotation Mirror polishing was performed with a speed of 100 rpm, a revolution speed of 100 rpm, and a polishing duration of 60 minutes.

  Subsequently, the back surface of the silicon substrate 12 of the bonded substrate 20 is pressed against the surface of the platen, the count is changed to the number 800 polishing abrasive, the bonded substrate 20 is pressed against the surface of the platen, and the rotational speed of rotation is 100 rpm. The back surface of the silicon substrate 12 was roughened for 15 minutes (FIG. 1D). At this time, the arithmetic average roughness Ra of the back surface of the silicon substrate 12 was 0.55 μm, and the maximum height Rz was 5.0 μm (approximately 9 times the arithmetic average roughness Ra 0.55 μm). At this time, a stylus type surface roughness meter was used to measure the arithmetic average roughness Ra and the maximum height Rz. Here, if the back surface of the silicon substrate 12 is roughened before the LT substrate 10 and the silicon substrate 12 are bonded together in the step of FIG. 1B, the silicon substrate 12 is warped, making it difficult to bond the two substrates. Here, since the back surface of the silicon substrate 12 is roughened after the two substrates are bonded together, the surface acoustic wave element can be manufactured without causing such a problem.

  Subsequently, using a general photolithography technique, the material is aluminum, and the finally produced surface acoustic wave element has a shape that functions as a bandpass filter with a center frequency of 2 GHz at room temperature, and has a thickness of 0.1 μm. A plurality of comb electrodes were formed on the surface of the LT substrate. In addition, for each comb-shaped electrode, two reflectors were formed so as to sandwich the comb-shaped electrode. Subsequently, each surface acoustic wave element was cut into individual shapes by dicing. Each surface acoustic wave element was cut out so that the vertical length was 1 mm and the horizontal length was 1 mm. The surface acoustic wave device thus obtained is shown in FIG. FIG. 2A is a plan view of the obtained surface acoustic wave element, and FIG. 2B is a cross-sectional view taken along line A-A ′. This surface acoustic wave element has comb-shaped electrodes 16 and 17 and a reflector 18 on the surface of the LT substrate 10 as shown in the figure. As shown in FIG. 3, a structure having comb electrodes 115 and 116 on the surface of the LT substrate 10 may be employed. FIG. 3A is a plan view of the surface acoustic wave element, and FIG. 3B is a cross-sectional view along B-B ′.

[Comparative Example 1]
After the LT substrate 10 is polished in the step of FIG. 1C, the surface acoustic wave is obtained in the same manner as in Example 1 except that the comb electrodes 16 and 17 and the reflector 18 are formed without roughening the back surface of the silicon substrate 12. An element was produced. At this time, the arithmetic mean roughness Ra of the back surface of the silicon substrate 12 was 0.3 nm, and the maximum height Rz was 1 nm. At this time, the same surface roughness meter as in Example 1 was used for the measurement of the arithmetic average roughness Ra and the maximum height Rz.

[Evaluation]
The surface acoustic wave elements of Example 1 and Comparative Example 1 were connected to a network analyzer, and the attenuation of the surface acoustic wave element was measured by changing the frequency. The measurement results are shown in FIG. In the figure, the solid line is the measurement result of the surface acoustic wave element of Example 1, and the dotted line is the measurement result of the surface acoustic wave element of Comparative Example 1. As shown in the figure, spurious on the high frequency side (approximately 2.02 to 2.2 GHz) appearing in the measurement result of Comparative Example 1 is suppressed in the measurement result of Example 1.

[Examples 2 and 3]
A surface acoustic wave element was produced in the same manner as in Example 1 except that the number of the abrasive grains used in the step of FIG. The arithmetic average roughness Ra of the back surface of the silicon substrate 12 was 0.35 μm when the number of the abrasive grains was 1000 and was 0.25 μm when the number was 1200. Further, the maximum height Rz of the back surface of the silicon substrate 12 is 3.5 μm (10 times the arithmetic average roughness Ra 0.35 μm) when the number of abrasive grains is 1000, and 3.0 μm (arithmetic average) when the number is 1200 Roughness 12 times of Ra 0.25). In these cases, the same frequency characteristics as in Example 1 were obtained.

[Comparative Example 2]
A surface acoustic wave element was produced in the same manner as in Example 1 except that the number of the abrasive grains used in the step of FIG. The arithmetic average roughness (Ra) of the back surface of the silicon substrate 12 was 0.2 μm. The maximum height Rz of the back surface of the silicon substrate 12 was 2.4 μm (12 times the arithmetic average roughness Ra). Also in this case, the same frequency characteristic as in Comparative Example 1 was obtained, and it was confirmed that spurious generation could not be suppressed.

  Here, with respect to Examples 1 to 3 and Comparative Example 2 described above, Table 1 shows the measurement results of the count of the abrasive grains, the arithmetic average roughness Ra and the maximum height Rz of the back surface of the silicon substrate 12.

[Example 4]
A surface acoustic wave element was produced in the same manner as in Example 1 except that the thickness of the organic adhesive layer 14 was changed as shown in Table 2. And the thermal expansion coefficient and the frequency temperature characteristic were measured. The measurement results are shown in Table 2. Here, the linear thermal expansion coefficient in the propagation direction X of the SAW of the LT substrate is 16 ppm / ° C. The linear expansion coefficient in the SAW propagation direction X of the single crystal silicon substrate is 3 ppm / ° C. As is apparent from the results in Table 1, it was found that the frequency temperature characteristics (temperature characteristics) are significantly improved by setting the thickness of the organic adhesive layer to 0.1 to 1.0 μm.

  10 Lithium tantalate substrate (LT substrate), 12 Silicon substrate, 13 Organic adhesive, 14 Organic adhesive layer, 16, 17, 115, 116 Comb electrode, 18 Reflector, 20 Bonded substrate.

Claims (4)

A piezoelectric substrate capable of propagating elastic waves;
A comb electrode provided on the surface of the piezoelectric substrate and capable of exciting a surface acoustic wave;
A support substrate that is bonded to the piezoelectric substrate, has a back surface that is rougher than the back surface of the piezoelectric substrate and has a roughness that scatters bulk waves propagating from the piezoelectric substrate, and has a smaller thermal expansion coefficient than the piezoelectric substrate; ,
An organic adhesive layer for bonding the piezoelectric substrate and the support substrate;
A surface acoustic wave device comprising: The piezoelectric substrate is made of a material selected from the group consisting of lithium tantalate, lithium niobate and lithium niobate-lithium tantalate solid solution single crystal,
The support substrate is made of a material selected from the group consisting of silicon, sapphire, aluminum nitride, alumina, borosilicate glass, and quartz glass.
The surface acoustic wave device according to claim 1. The organic adhesive layer has a thickness of 0.1 to 1.0 μm.
The surface acoustic wave device according to claim 1. The roughness for scattering the bulk wave has an arithmetic average roughness Ra of 0.25 to 0.55 μm, and a maximum height Rz of 9 to 12 times the arithmetic average roughness Ra.
The surface acoustic wave device according to claim 1.

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