JP4956569B2 - Surface acoustic wave device - Google Patents

Description

  The present invention relates to a surface acoustic wave element having good frequency temperature characteristics.

  Surface acoustic wave (Surface Acoustic Wave) elements are widely used as bandpass filters in communication devices such as mobile phones. Along with the improvement in performance of mobile phones and the like, higher performance is also required for filters using surface acoustic wave elements.

  However, the surface acoustic wave element has a problem that the pass band moves due to a temperature change. In particular, lithium niobate and lithium tantalate, which are widely used at present, have a large electromechanical coupling coefficient, and are advantageous for realizing broadband filter characteristics. However, lithium niobate and lithium tantalate are inferior in temperature stability.

  For example, the temperature coefficient of frequency change of lithium tantalate is −35 ppm / ° C., and the frequency variation is large in the temperature range of −30 to + 85 ° C. For this reason, it is necessary to reduce the temperature coefficient of the frequency change.

Patent Document 1 describes a device in which a SAW propagation substrate and a support substrate are bonded with an organic thin film layer. The propagation substrate is, for example, a lithium tantalate substrate having a thickness of 30 μm, and this is bonded to a glass substrate having a thickness of 300 μm with an organic adhesive having a thickness of 15 μm.
JP2001-53579

Patent Document 2 describes a SAW device in which a lithium tantalate substrate (thickness: 125 μm) and a quartz glass substrate (thickness: 125 μm) are bonded together with an adhesive.
JP2006-42008

Patent Documents 3, 4, and 5 also describe SAW devices in which a SAW propagation substrate and a support substrate are bonded.
JP-A-6-326553 Patent No. 3777482 US Pat. No. 7,105,980

Furthermore, Patent Document 6 describes that an oxide layer having a thickness of 0.1 to 40 μm is formed on both surfaces of a silicon support substrate, and then a piezoelectric substrate is bonded onto the support substrate to manufacture a SAW device. Yes. The Si oxide film is necessary to reduce the warp of the composite piezoelectric substrate 1.
JP2005-229455

  However, none of the documents solves the problem of movement of the passband accompanying a change in temperature, but rather moves away from the solution.

  Patent Document 1 (0025) and (0037) describe the temperature coefficient of the frequency of a SAW device in which a lithium tantalate substrate is bonded to a support substrate. There is almost no improvement in properties. For example, in the case of a 2 GHz SAW filter, a shift of ± MHz is observed in the temperature range of −30 to + 85 ° C. This corresponds to ± 7% of the required bandwidth. Therefore, it can be seen that when the adhesive layer is provided between the lithium tantalate propagation substrate and the glass supporting substrate, the temperature characteristic of the frequency is hardly improved.

  Patent Document 2 (0037) describes that “the frequency temperature characteristic of the surface acoustic wave element can be improved”, but no improved data is described.

  Even in the description of (0062) of Patent Document 3, it is specified that bonding the surface acoustic wave propagation substrate to the support substrate is not practical.

  Judging from the above, it is common sense that a surface acoustic wave substrate having a structure in which, for example, a lithium tantalate propagation substrate is bonded to a support substrate is constantly practical, and it is impossible to reduce the temperature coefficient of frequency in particular. is there.

  In Patent Document 6, it is necessary to flow an oxygen gas at a high temperature of, for example, about 1000 ° C. and leave the Si substrate for several tens of hours to form a surface oxide film. However, this method not only oxidizes the surface of the Si substrate, but also dissolves Si itself into SiO2. For this reason, a Si defect layer is generated at the interface between Si and SiO2, and the adhesive strength of this portion is lowered. In addition, the thickness of the adhesive layer between the Si support substrate and the piezoelectric substrate is all 3 μm in the examples, and when it is thinner than 1.5 μm, the adhesive force is insufficient, and peeling occurs at 250 ° C. (0028).

  An object of the present invention is to reduce the temperature coefficient of the frequency of a surface acoustic wave device using a piezoelectric single crystal propagation substrate.

The present invention
A support substrate made of silicon or borosilicate glass and having a thickness of 200 to 500 μm,
A rotating Y-cut X-propagating substrate having a thickness of 10 to 50 μm made of a lithium tantalate single crystal,
Acrylic resin adhesive layer formed by spin coating with a thickness of 0.1 μm to 1.0 μm for bonding the support substrate and the propagation substrate, and a surface acoustic wave made of aluminum or an aluminum alloy provided on the propagation substrate The present invention relates to a surface acoustic wave device including a filter or a resonator.

Contrary to the common knowledge of those skilled in the art, the present inventor has continued research on a structure in which a lithium tantalate single crystal propagation substrate is bonded to a support substrate. Here, it tried trying thinning of the acrylic resin contact bonding layer which was overlooked conventionally. Such a trial has been denied from the description of (0062) of Patent Document 3, for example.

  However, contrary to expectations, it has been found that the propagation substrate adheres well to the support substrate and the temperature coefficient of frequency is significantly reduced. That is, when the thickness of the organic adhesive is 0.1 to 1.0 μm, the temperature characteristics due to the difference in thermal expansion coefficient between the propagation substrate and the support substrate are considerably improved. On the other hand, when the thickness of the adhesive is greater than 1 μm, the stress due to the difference in thermal expansion coefficient between the propagation substrate and the support substrate is absorbed by the organic adhesive, and the effect of improving the temperature characteristics cannot be obtained. . In addition, when the thickness of the adhesive layer is less than 0.1 μm, the frequency characteristic of the frequency seems to deteriorate again due to the influence of the void, and the temperature characteristic is not improved as the adhesive layer becomes thinner. It was confirmed.

  The surface acoustic wave device of the present invention includes a surface acoustic wave filter or a resonator. The elastic surface is a bandpass filter as described later. The resonator is a surface acoustic wave oscillation element, and includes both a 1-port type and a 2-port type.

In the present invention, the material of the support substrate is silicon or borosilicate glass. By adopting these, it is possible to reduce the difference in thermal expansion from the propagation substrate and further improve the temperature characteristics of the frequency.

Preferably, an oxide film is not formed on the surface of the support substrate, whereby the adhesive force between the support substrate and the propagation substrate is increased, and peeling and cracking of the support substrate and the propagation substrate can be prevented even at high temperatures. From this point of view, it is preferable that the support substrate is made of silicon, and there is no silicon oxide film on the surface. Note that the presence or absence of the surface oxide film on the support substrate is observed by a transmission electron microscope (TEM).

In the present invention, the material of the propagation substrate is a large lithium tantalate of the electromechanical coupling constant.

The material of the organic adhesive layer for bonding the support substrate and the propagation substrate is an acrylic resin.

  In the present invention, the thickness t of the organic adhesive layer is set to 0.1 μm or more and 1.0 μm or less. From the viewpoint of further improving the temperature characteristics of the frequency of the surface acoustic wave device, the thickness of the organic adhesive layer is preferably 0.1 μm or more, and more preferably 0.5 μm or less.

Propagating substrate made of a piezoelectric single crystal, a lithium tantalate, a surface acoustic wave propagation direction and X, the cut angle and the rotation Y-cut plate.

FIG. 1 is a cross-sectional view schematically showing a manufacturing process of an adhesive for a surface acoustic wave device.
As shown in FIG. 1A, a support substrate 1 is prepared. As shown in FIG. 1B, an organic adhesive 2 is applied to the surface of the support substrate 1, and a substrate 3 made of a piezoelectric single crystal is bonded as shown in FIG. Next, as shown in FIG. 1D, the substrate 3 is processed and thinned to obtain a propagation substrate 3A having a thickness T2.

  Next, as shown in FIGS. 2A and 2B, the input electrode 4 and the output electrode 5 are formed on the propagation substrate 3 </ b> A to obtain the transversal surface acoustic wave element 6. A surface acoustic wave propagates from the input electrode 4 toward the output electrode 5 as indicated by an arrow 7. This portion becomes a surface acoustic wave filter.

  In a surface acoustic wave filter for a mobile phone, a resonance type surface acoustic wave element is mainly used. 3A and 3B relate to this example. FIG. 3A is a plan view showing an electrode pattern of a resonance type surface acoustic wave element, and FIG. 3B is a cross-sectional view taken along line A-A ′ of FIG.

  Electrodes 16, 17, and 18 are formed on the propagation substrate 10 to obtain a resonant surface acoustic wave device. In this example, the propagation substrate 10 is bonded onto the support substrate 12 via the organic adhesive layer 14. As described above, the support substrate 12, the adhesive layer 14, and the propagation substrate 10 are configured according to the present invention.

Method of forming an organic adhesive layer is spin coating.

Here, the material of the input electrode and an output electrode of aluminum, an aluminum alloy.

  The thickness T1 of the support substrate 1 is preferably 100 μm or more, and more preferably 200 μm or more, from the viewpoint of improving temperature characteristics. T1 is preferably 500 μm or less from the viewpoint of product size reduction.

  The thickness T2 of the propagation substrate 3A is preferably 10 to 50 μm, particularly preferably 30 to 50 μm, from the viewpoint of improving the temperature characteristic of the frequency.

Example 1
According to the manufacturing method shown in FIG. 1, a surface acoustic wave element 6 as shown in FIG. 2 was produced.
However, the substrate 3 was a 36 ° Y-cut X-propagating lithium tantalate substrate in which the SAW propagation direction was X and the cutting angle was a rotating Y-cut plate. The linear expansion coefficient in the SAW propagation direction X is 16 ppm / ° C. A single crystal silicon substrate was used as the support substrate 1. The linear expansion coefficient in the SAW propagation direction X of the support substrate 1 is 3 ppm / ° C. The thickness T1 of the support substrate 1 was 350 μm, the thickness of the piezoelectric single crystal substrate 3 was 350 μm, and the substrates were bonded to each other at 180 ° C. using an organic adhesive (acrylic). Next, the thickness of the piezoelectric single crystal substrate 3 was reduced to 30 μm by grinding. An input electrode 4 and an output electrode 5 made of metal aluminum were formed on the obtained propagation substrate 3A.

  However, the thickness t of the organic adhesive layer 2 was variously changed from 0.05 μm to 15 μm. For each element, the thermal expansion coefficient of the surface acoustic wave element and the frequency temperature characteristic (Temperature Coefficient of Frequency) at the resonance point were measured and are shown in Table 2 and FIG.

  As can be seen from this result, it was found that the frequency temperature characteristic (Temperature Coefficient of Frequency) is significantly improved significantly by setting the thickness of the organic adhesive layer to 0.1 to 1.0 μm.

(Example 2)
Next, the surface acoustic wave element shown in FIG. 3 was produced in the same manner as in Example 1. When the same experiment as Example 1 was conducted about the obtained element, the temperature-temperature characteristic (Temperature Coefficient of Frequency) was critical by setting the thickness of the organic adhesive layer to 0.1 to 1.0 μm. It was confirmed that it was significantly improved.

(A), (b), (c), (d) is typical sectional drawing which shows the manufacturing process of the adhesive body for surface acoustic wave elements. FIG. 2A is a cross-sectional view schematically showing the surface acoustic wave element 6, and FIG. 2B is a plan view schematically showing the surface acoustic wave element 6. (A) is a top view which shows a resonance-type surface acoustic wave element, (b) is the sectional view on the A-A 'line of (a). It is a graph which shows the relationship between the thickness of an adhesive bond layer, a thermal expansion coefficient, and the temperature characteristic of a frequency.

  DESCRIPTION OF SYMBOLS 1,12 Support substrate 2,14 Organic adhesive layer 3A, 10 Propagation substrate 4 Input electrode 5 Output electrode 6 Surface acoustic wave element 7 Surface acoustic wave 16, 17, 18 Electrode t Thickness of organic adhesive layer T1 of support substrate Thickness T2 Thickness of propagation board

Claims (6)

A support substrate made of silicon or borosilicate glass and having a thickness of 200 to 500 μm,
A rotating Y-cut X-propagating substrate having a thickness of 10 to 50 μm made of lithium tantalate single crystal
An acrylic resin adhesive layer formed by spin coating with a thickness of 0.1 μm to 1.0 μm for bonding the support substrate and the propagation substrate, and aluminum or an aluminum alloy provided on the propagation substrate A surface acoustic wave device comprising a surface acoustic wave filter or a resonator.   The element according to claim 1, wherein the support substrate is made of silicon.   The element according to claim 1, wherein a thickness of the X propagation substrate is 30 μm or more.   The element according to claim 1, wherein the X propagation substrate is a 36 ° Y-cut X propagation substrate.   The element according to any one of claims 1 to 4, wherein an oxide film is not formed on a surface of the support substrate. The element according to claim 1, wherein the surface acoustic wave filter or the resonator is made of aluminum.

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