JP2007214902A - Surface acoustic wave device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a surface acoustic wave device at low cost, which has high productivity and high effects of improving the frequency-temperature characteristics. <P>SOLUTION: The surface acoustic wave device in which an electrode for energizing/detecting surface acoustic wave or leaked acoustic surface wave is formed on a piezoelectric substrate, is provided at least with a composite piezoelectric chip obtained by processing a composite piezoelectric substrate with the piezoelectric substrate and a ceramic substrate, stuck together via an adhesive into a chip shape; and a mounting substrate for mounting the composite piezoelectric chip by flip chip bonding, wherein the composite piezoelectric chip is mounted so as to make the expansion coefficient αc(ppm/°C) of the surface acoustic wave or the leaked surface acoustic wave over the piezoelectric substrate surface in a propagation direction and the expansion coefficient of the mounting substrate αs(ppm/°C) satisfy the relation αs<αc<αs+6. <P>COPYRIGHT: (C)2007,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 frequency-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.
An example of a piezoelectric substrate that realizes such characteristics is a composite piezoelectric substrate in which a piezoelectric substrate and another substrate are bonded.

As an example of such a composite piezoelectric substrate, an electrode for exciting and detecting an elastic wave is provided on the surface of the piezoelectric material, and the temperature stabilization is characterized in that a composite laminate is joined to the back surface of the piezoelectric material. A surface wave device is disclosed. This surface wave device is such that temperature correction is performed in the piezoelectric material by inducing a controlled stress change in the piezoelectric material (see Patent Document 1).
In this example, “the LiNbO ₃ (lithium niobate) substrate is firmly bonded to the composite laminate to generate a compressive force on the substrate as described above, and this compressive force increases as the temperature increases. It is possible to obtain a means for correcting the influence of temperature on the delay time and the filter center frequency. " This means that the expansion coefficient of the composite laminate as the support substrate is smaller than that of the surface acoustic wave propagation direction of the LiNbO ₃ substrate, which is a piezoelectric material, and stress is generated in the piezoelectric substrate in response to temperature changes. This means that the influence of temperature on the delay time and filter center frequency of the SAW device can be corrected.

Further, an electrical component is disclosed in which a functional element having an electrode provided on the surface of the piezoelectric plate is housed in a package by bonding a rigid plate and a piezoelectric plate using an adhesive (Patent Document). 2).
That is, a surface acoustic wave device using a composite piezoelectric substrate in which a piezoelectric material and a substrate having a smaller expansion coefficient are bonded has improved frequency temperature characteristics, and a rigid plate and a piezoelectric plate are bonded using an adhesive. It is a well-known technique to combine them into an integrated substrate.

Also, a surface acoustic wave device comprising a piezoelectric substrate, an interdigital transducer (IDT) and a bump each having a plurality of electrode fingers and a bus bar that commonly connects these electrode fingers, formed on the piezoelectric substrate. Is a mounting structure of a surface acoustic wave element mounted on a mounting substrate by flip chip bonding via the bump, the mounting substrate having a smaller linear expansion coefficient than the piezoelectric substrate, and A mounting structure of a surface acoustic wave element is disclosed, wherein the bumps are arranged so that thermal expansion and contraction of the piezoelectric substrate due to temperature changes are suppressed by the mounting substrate (Patent) Reference 3).
In the example of this example, LiTaO ₃ (expansion coefficient 16 ppm / ° C.) is used as the piezoelectric body, and alumina (expansion coefficient 7 ppm / ° C.) is used as the mounting base, and the chip is mounted on the mounting substrate by flip chip bonding via bumps. In the surface acoustic wave filter, it is disclosed that the frequency variation due to temperature is improved by −11 kHz / ° C. at an operating frequency of 1.9 GHz.
This improvement effect is preferable because it is improved by about 6 ppm / ° C. in terms of temperature coefficient.

On the other hand, in Non-Patent Document 1, 48 ° rotated Y-cut LiTaO ₃ is used as a piezoelectric body, and an elastic using a composite piezoelectric substrate in which a Si substrate as a supporting substrate is directly bonded to the piezoelectric body via a SiO ₂ layer. A surface wave device is disclosed. The temperature characteristics of the operating frequency of this surface acoustic wave device are as follows. When the composite piezoelectric chip is connected by the bonding wire method, the temperature characteristic of the operating frequency is −12 ppm / ° C., whereas the flip chip bonding method is −22 ppm / ° C. (−35 ppm / ° C.) and temperature characteristics are described as being deteriorated.

JP 51-25951 A Japanese Patent Laid-Open No. 02-62108 JP 2003-324334 A BPAbbot, J. Caron, J. Chocola, K. Lin, S. Malocha, N. Naumenko and P. Welsh, "Advances in Rf SAW Substrates", 2nd International Symposium on Acoustic Wave Devices for Future Mobile Communication Systems, pp. 233-243,2003

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a surface acoustic wave device with high productivity and high effect of improving frequency temperature characteristics at low cost.

In order to solve the above problems, the present invention provides a surface acoustic wave element in which an electrode for exciting and detecting a surface acoustic wave or a leaky surface acoustic wave is formed on a piezoelectric substrate, and includes at least a piezoelectric substrate and a ceramic substrate. A composite piezoelectric chip obtained by processing a composite piezoelectric substrate bonded to each other with an adhesive into a chip shape, and a mounting substrate on which the composite piezoelectric chip is mounted by flip chip bonding. The expansion coefficient αc (ppm / ° C.) in the propagation direction of the leaky surface acoustic wave and the expansion coefficient α s (ppm / ° C.) of the mounting substrate are as follows:
αs <αc <αs + 6
The surface acoustic wave device is provided so as to satisfy the following relationship (claim 1).

  Thus, the surface acoustic wave device of the present invention uses a ceramic substrate as a substrate to be bonded to a piezoelectric substrate in a composite piezoelectric chip, and also bonds the piezoelectric substrate and the ceramic substrate through an adhesive. It can be inexpensive. Also provided is a composite piezoelectric chip obtained by processing a composite piezoelectric substrate obtained by bonding a piezoelectric substrate and a ceramic substrate into a chip shape, and a mounting substrate on which the composite piezoelectric chip is mounted by flip chip bonding. The surface acoustic wave device is mounted so that the expansion coefficient αc in the propagation direction of the wave or leaky surface acoustic wave and the expansion coefficient αs of the mounting substrate satisfy the above relationship, so that the productivity is high and the frequency temperature characteristic improvement effect is high. It can be.

At this time, the ceramic substrate preferably has a thickness of 100 μm or more.
In this way, if the thickness of the ceramic substrate to be bonded to the piezoelectric substrate is 100 μm or more, in-plane variation is suppressed in the thickness of the ceramic substrate, and a good film thickness distribution can be obtained. When bonding to a ceramic substrate, grinding the piezoelectric substrate, etc. to finish it into a composite piezoelectric substrate, variations in the thickness of the piezoelectric substrate are suppressed, resulting in less variation in frequency temperature coefficient and better frequency temperature characteristics. A composite piezoelectric substrate, or a surface acoustic wave element can be used. Further, for example, since it is possible to suppress warpage when heat treatment is applied, problems are unlikely to occur in processes such as patterning and mounting.
As described above, the surface acoustic wave device of the present invention can be manufactured at low cost with high quality and manufacturing yield.

The ceramic substrate preferably has a Young's modulus of 200 GPa or more.
As described above, if the ceramic substrate has a Young's modulus of 200 GPa or more, it is hard and hardly warps even when heated, and thus problems are less likely to occur in processes such as patterning and mounting.

Moreover, it is preferable that the index Ra indicating the surface roughness of the bonding surface of the ceramic substrate and the surface opposite to the bonding surface is 0.02 μm or more and 0.5 μm or less.
In this way, if the index Ra indicating the surface roughness of the bonding surface of the ceramic substrate and the surface opposite to the bonding surface is 0.02 μm or more and 0.5 μm or less, the ceramic substrate is in-plane. It can be precisely adjusted to the same thickness. For this reason, the thickness variation of the piezoelectric substrate bonded to the ceramic substrate can be suppressed, and a composite piezoelectric substrate and a surface acoustic wave element having extremely stable frequency-temperature characteristics in the surface can be obtained. Further, the anchor effect of the adhesive layer becomes remarkable, and good adhesion can be obtained.

At this time, the ceramic substrate preferably has a porosity of 0.1% or more and less than 4%.
Thus, if the porosity of the ceramic substrate is less than 4%, Ra on the bonding surface and the surface opposite to the bonding surface is less likely to be too large, for example, exceeding 0.5 μm. Variations in the thickness of the piezoelectric substrate are suppressed, and a composite piezoelectric substrate and a surface acoustic wave element having a large effect of improving temperature characteristics are obtained.
Moreover, if it is 0.1% or more, a ceramic substrate can be prepared comparatively cheaply, it becomes an inexpensive composite piezoelectric substrate, cost can be reduced, and the anchor effect is also acquired.

Furthermore, it is preferable that the ceramic substrate has the same surface roughness on the bonding surface and the surface opposite to the bonding surface.
As described above, in the ceramic substrate, if the surface roughness of the bonding surface and the surface opposite to the bonding surface is the same, a better film thickness distribution can be obtained, and excellent frequency temperature characteristics can be obtained. A composite piezoelectric substrate having

The piezoelectric substrate is preferably made of any one of LiTaO ₃ , LiNbO ₃ , and Li ₂ B ₄ O ₇ (Claim 7).
Thus, if the piezoelectric substrate is made of any one of LiTaO ₃ , LiNbO ₃ , and Li ₂ B ₄ O ₇ , these are crystal materials having a large electromechanical coupling coefficient. As a composite piezoelectric substrate capable of manufacturing a SAW device with a wide bandwidth and low insertion loss.

The ceramic substrate is preferably composed mainly of alumina.
As described above, if the ceramic substrate is mainly composed of alumina, it can be prepared at a low cost, and since it is hard, it can be a composite piezoelectric substrate in which warpage is suppressed even when heated.

Moreover, it is preferable that the piezoelectric substrate of the composite piezoelectric substrate is removed by 0.1 mm or more and 3 mm or less from the outer periphery after bonding (Claim 9).
Thus, if the piezoelectric substrate of the composite piezoelectric substrate is removed by 0.1 mm or more and 3 mm or less from the outer periphery after bonding, it is preferable that processing distortion and cracks that easily occur on the outer periphery of the piezoelectric substrate are less likely to occur.

Furthermore, it is preferable that the composite piezoelectric substrate has the same reflectance at a wavelength of 365 nm on the piezoelectric substrate side as that of the single piezoelectric substrate.
Thus, in the composite piezoelectric substrate, if the reflectance at the wavelength 365 nm on the piezoelectric substrate side is the same as that of the piezoelectric substrate alone, for example, photolithography such as Al having a line width of 0.5 μm is a piezoelectric substrate alone. It is preferable because it can be performed in the same manner as described above.

The composite piezoelectric substrate preferably has a thickness of 300 μm or less.
Thus, a composite piezoelectric substrate having a thickness of 300 μm or less is preferable because it can be sufficiently mounted on a thin mobile phone or the like in recent years.

With such a surface acoustic wave device of the present invention, a surface acoustic wave device with high productivity and high effect of improving frequency temperature characteristics can be obtained. In addition, since the substrate to be bonded to the piezoelectric substrate is a ceramic substrate and bonded with an adhesive, the substrate can be made inexpensive.
In particular, if the thickness of the ceramic substrate to be bonded to the piezoelectric substrate is, for example, 100 μm or more, the surface acoustic wave device has small warpage due to heat, an extremely large temperature improvement effect, and small variation in characteristics within the substrate surface. Is possible.

Hereinafter, embodiments of 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 an embodiment of a surface acoustic wave device according to the present invention.
The surface acoustic wave element 8 includes a composite piezoelectric chip 1 obtained by processing a composite piezoelectric substrate obtained by bonding a piezoelectric substrate 2 and a ceramic substrate 3 through an adhesive (adhesive layer) 4 into a chip shape, and the composite piezoelectric chip 1. And a mounting substrate 6 mounted by flip chip bonding via the bumps 5. Further, an electrode 7 for exciting and detecting a surface acoustic wave or a leaky surface acoustic wave is formed on the piezoelectric substrate 2.
The expansion coefficient αc (ppm / ° C.) in the propagation direction of the surface acoustic wave or leaky surface acoustic wave on the surface of the piezoelectric substrate 2 and the expansion coefficient αs (ppm / ° C.) of the mounting substrate 6 are αs <αc <αs + 6. It is implemented so as to satisfy the following relationship.

By having such a configuration, the surface acoustic wave element 8 of the present invention has high productivity and high frequency temperature characteristic improvement effect.
That is, as in the present invention, the relationship of αs <αc <αs + 6 between the expansion coefficient αc in the propagation direction of the surface acoustic wave or leakage surface acoustic wave on the surface of the piezoelectric substrate 2 and the expansion coefficient αs of the mounting substrate 6 is satisfied. In the surface acoustic wave element 8 on which the composite piezoelectric substrate is mounted by flip chip bonding, the frequency temperature coefficient is improved by about several ppm / ° C. to several tens ppm / ° C. as compared with the case of mounting by the chip and wire method, for example. In addition, since it is mounted by flip chip bonding, productivity can be increased.

  Since the composite piezoelectric substrate used in the present invention has the adhesive layer 4 as described above, the stress on the surface of the composite piezoelectric substrate due to the heat of the piezoelectric substrate is relieved, but the composite piezoelectric substrate is flip-chip mounted as described above. Then, since the stress value and the stress distribution on the surface of the piezoelectric body are dramatically recovered, the frequency temperature characteristics can be greatly improved as compared with the case where the flip chip mounting is not performed.

  Here, when αc is smaller than αs, as in Non-Patent Document 1, the result is that the frequency temperature characteristics when mounted by flip-chip bonding are deteriorated compared to when mounted by the chip-and-wire method. In addition, both the frequency temperature characteristics improvement effect and high productivity cannot be achieved. When αc is larger than αs + 6 (ppm / ° C.), the effect of improving the frequency temperature coefficient is small. Therefore, both high frequency improvement effect and high productivity can be achieved by mounting so as to satisfy the relationship of αs <αc <αs + 6 as in the present invention. In order for the expansion coefficient to satisfy the above relationship, for example, the thickness of the piezoelectric substrate 2, the thickness of the adhesive layer 4 that bonds the piezoelectric substrate 2 and the ceramic substrate 3 that is the support substrate, the chip size, and the like are adjusted. That's fine.

Next, the components of the composite piezoelectric chip 1 will be specifically described.
The composite piezoelectric chip 1 shown in FIG. 1 is formed by processing a composite piezoelectric substrate in which a piezoelectric substrate 2 and a ceramic substrate 3 are bonded together with an adhesive 4 into a chip shape as described above.
FIG. 2 is a schematic sectional view showing an example of the embodiment of the composite piezoelectric substrate 9 used in the surface acoustic wave element 8 according to the present invention. The composite piezoelectric substrate 9 is formed by bonding a piezoelectric substrate 2 and a ceramic substrate 3 having a thickness of, for example, 100 μm or more with an adhesive 4, and the surface of the piezoelectric substrate 2 has a surface acoustic wave. Alternatively, a metal electrode 7 for exciting a leaky surface acoustic wave is formed.
With such a configuration, stress is generated in the piezoelectric substrate 2 in accordance with a temperature change, and correction can be performed effectively, and a uniform frequency temperature characteristic improvement effect can be obtained in the plane.

At this time, if the ceramic substrate 3 having a thickness of, for example, 100 μm or more is used as shown in FIG. 2, the thickness of the ceramic substrate 3 can be precisely adjusted in the surface to suppress variation, and a good thickness distribution can be obtained. It is possible to prepare a ceramic substrate 3 having the same. As a result, when the ceramic substrate 3 and the piezoelectric substrate 2 are bonded to finish the composite piezoelectric substrate 9, even if the piezoelectric substrate 2 is subjected to processing such as grinding, the thickness of the piezoelectric substrate 2 varies. Therefore, the composite piezoelectric substrate 9 having excellent frequency temperature characteristics can be obtained. This naturally leads to obtaining a surface acoustic wave element 8 having excellent frequency-temperature characteristics.
Further, even if heat is applied, a large warp is unlikely to occur, and the composite piezoelectric substrate 9 has almost no warp, and processes such as patterning and mounting can be easily performed.
The upper limit of the thickness of the ceramic substrate 3 is not particularly limited, and the thicker it is, the more difficult it is to deform. Therefore, for example, it can be 10 mm or less, but if it is about 500 μm, the above effect can be obtained sufficiently, and more preferably 200 μm or less. As a result, the overall thickness of the composite piezoelectric substrate can be reduced, and the cost can be reduced more than necessary. What is necessary is just to determine suitably according to a use etc.

  Since ceramic is widely used as a packaging material, it can be manufactured at low cost. Bonding via the adhesive 4 is one factor that makes the composite piezoelectric substrate 9 inexpensive, and has the advantage that it can be firmly bonded. Examples of the adhesive 4 include an ultraviolet curable adhesive mainly composed of epoxy methacrylate. The adhesive 4 is not limited to this, and may be composed mainly of epoxy.

  Further, the ceramic substrate 3 preferably has a Young's modulus of, for example, 200 GPa or more. If it is 200 GPa or more, it is sufficiently hard, and even when heat is applied, it can be assumed that warpage hardly occurs. The upper limit is not particularly limited, and is preferably as large as possible. However, for example, about 400 GPa is sufficient.

  Further, the ceramic substrate 3 will be described. The index Ra indicating the surface roughness of the bonding surface and the surface opposite to the bonding surface is 0.02 μm to 0.5 μm, particularly about 0.3 μm. Is preferred. And it is better that the surface roughness of both surfaces is substantially the same, and the thickness variation of the ceramic substrate 3 is further suppressed. If Ra is 0.02 μm or more, an anchor effect is obtained at the joint surface and there is no peeling. On the other hand, if Ra is 0.5 μm or less, the surface roughness is large, so that there are no thickness variations and particle problems.

  When preparing the ceramic substrate 3 to be bonded, for example, if only the bonding surface is processed into a mirror surface, for example, a thickness variation of about 8 μm occurs, and this is used to bond the piezoelectric substrate 2 to the composite piezoelectric substrate. When finished to 9, the thickness variation of the piezoelectric substrate 2 becomes about 8 μm, and the characteristic variation becomes relatively large.

On the other hand, if the surface roughness Ra on both surfaces of the ceramic substrate 3 is within the above range, the thickness of the ceramic substrate 3 can be precisely adjusted to the same thickness within the substrate surface. For example, the thickness variation can be suppressed to 1 μm or less. If the thickness variation of the ceramic substrate 3 is reduced as described above, the thickness variation of the ceramic substrate 3 is small even if the bonded piezoelectric substrate 2 is subjected to grinding or the like. Variations in the thickness of the substrate 2 can also be suppressed, and the frequency temperature characteristics of the composite piezoelectric substrate 9 are improved.
At this time, if the ceramic substrate 3 has the same surface roughness on both sides, it is possible to prepare a ceramic substrate 3 having a more suppressed variation in thickness, and the composite piezoelectric substrate 9 having more excellent characteristics. It can be.

  The ceramic substrate 3 may have a porosity of, for example, 0.1% or more and less than 4%. When the porosity is less than 4%, the surface roughness tends to be less than 1 μm in Ra value, the effect of improving the temperature characteristics becomes relatively large, and the stability of the frequency temperature characteristics becomes high. If it is 0.1% or more, the ceramic substrate 3 can be made relatively inexpensive, and the cost can be reduced in the manufacture of the composite piezoelectric substrate 9. For this reason, if the porosity is in the above range, the thickness variation of the ceramic substrate 3 can be suppressed, and the composite piezoelectric substrate 9 with higher quality can be obtained.

  And as a material of the ceramic substrate 3, it is good that a main component is an alumina. If it has an alumina as a main component, it can be set as the composite piezoelectric substrate 9 which is comparatively cheap and is hard, so that warpage is suppressed even when heated. For example, if the alumina is 92.0%, the Young's modulus is about 250 GPa, and if it is 99.9%, the Young's modulus is about 400 GPa and is sufficiently hard.

The piezoelectric substrate 2 is preferably made of any one of LiTaO ₃ , LiNbO ₃ , and Li ₂ B ₄ O ₇ . Since these are crystal materials having a large electromechanical coupling coefficient, a composite piezoelectric substrate 9 capable of manufacturing a SAW device having a wide bandwidth as a frequency selection filter and a small insertion loss can be obtained.

The composite piezoelectric substrate 9 preferably has the same reflectance at a wavelength of 365 nm on the piezoelectric substrate 2 side as that of the piezoelectric substrate alone. Such a case is preferable because photolithography such as Al having a line width of 0.5 μm can be performed in the same manner as in the case of a single piezoelectric substrate.
On the other hand, when a Si substrate, for example, is used instead of the ceramic substrate 3, reflection occurs from the bonding interface of the composite piezoelectric substrate 9, and thus the reflectance is usually larger than that of the piezoelectric substrate alone, and an exposure abnormality occurs in photolithography. It is not preferable.

Such a composite piezoelectric substrate 9 can be produced, for example, by applying the adhesive 4 to one or both of the piezoelectric substrate 2 and the ceramic substrate 3 and bonding them firmly under vacuum to firmly bond them. It is preferable to clean the surface of each substrate before bonding so that no foreign matter enters the adhesive 4, and the surface is subjected to a hydrophilic treatment with an ammonia-hydrogen peroxide aqueous solution or a plasma treatment. For example, the adhesive force may be increased by heating the substrate to 100 ° C. and pretreating with a short wave UV light having a wavelength of 200 nm or less and ozone (preferably high concentration ozone).
The size of the composite piezoelectric substrate 9 is not particularly limited, and can be, for example, 100 mm in diameter, but may be larger or smaller.

Then, after the composite piezoelectric substrate 9 is chamfered, the surface side of the piezoelectric substrate 2 is scraped off by grinding and lapping, and further polished so that the piezoelectric substrate 2 has a predetermined thickness (for example, about 20 μm). To process.
Further, at this time, it is better to remove the outer periphery of the piezoelectric substrate 2 of the composite piezoelectric substrate 9 by a width of 0.1 mm or more and 3 mm or less after the bonding and chamfering as described above. This removal step can be performed using, for example, a special chamfering wheel. If the composite piezoelectric substrate 9 has been removed from the outer periphery of the piezoelectric substrate 2 by 0.1 mm or more and 3 mm or less, it is difficult for processing distortion or cracks to occur in the piezoelectric substrate 2 in the subsequent grinding process of the piezoelectric substrate 2 or the like. Become. If it is less than 0.1 mm, the effect of preventing these occurrences is weak, and if it is removed more than 3 mm, it will be removed more than necessary, resulting in low production efficiency. By removing in the range of 0.1 mm or more and 0.3 mm or less, it is possible to efficiently suppress the generation of processing distortion and cracks, and the composite piezoelectric substrate 11 is less prone to cracks.

Thereafter, the metal electrode 7 can be formed by forming a film of a metal material on the composite piezoelectric substrate 9, that is, on the piezoelectric substrate 2 by a method such as vapor deposition, sputtering, or CVD, and patterning by etching or the like. is there. The metal electrode 7 is preferably made of, for example, Al, Cu, or an alloy thereof.
Further, if the composite piezoelectric substrate 9 having a thickness of 300 μm or less is formed by cutting the back surface of the composite piezoelectric substrate 9, that is, the ceramic substrate 3 side, the composite piezoelectric substrate 9 can be mounted on a thin mobile phone or the like. Can be widened. In the composite piezoelectric substrate 9 of the present invention, the ceramic substrate 3 is bonded to the piezoelectric substrate 2 with the adhesive 4 as described above. The ceramic substrate 3 is very hard in the first place, and therefore is shaved using a back grind or the like. However, if the thickness of the ceramic substrate 3 is particularly 100 μm or more, warping and cracking are unlikely to occur, and the device characteristics of the composite piezoelectric substrate 9 are unlikely to fluctuate.

  Then, the composite piezoelectric substrate thus manufactured is diced and cut into chips, and the surface acoustic wave device 8 of the present invention uses such a composite piezoelectric chip 1 and bumps 5 made of, for example, Au or Sn. And mounted on the mounting substrate 6 by conventional flip chip bonding. If the mounting substrate 6 is made of alumina (expansion coefficient 8 ppm / ° C.) or low expansion ceramic (expansion coefficient 5.5 ppm / ° C.), the expansion coefficient is an appropriate value, and the expansion coefficients αc and αs are the above-mentioned values. It is easy to adjust and mount so as to satisfy the above relationship, but a mounting substrate made of other materials may be used.

Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples, but the present invention is not limited thereto.
Example 1
The ceramic substrate as the support substrate has a diameter of 4 inches (100 mm) and a thickness of 290 μm. The surface roughness Ra of the bonded surface and the opposite surface is both 0.3 μm, and the porosity is An alumina substrate having 2%, Young's modulus of 380 GPa, and resistivity of 10 ¹⁵ Ωcm was prepared (see Table 1). FIG. 3 shows an observation view of the surface of the alumina substrate with an electron microscope.
In addition, a 36-degree rotated Y-cut lithium tantalate (LiTaO ₃ ) substrate having a diameter of 4 inches (100 mm) is prepared as the piezoelectric substrate, and the roughness of the front and back surfaces is obtained by double-side rough polishing so that the thickness of the piezoelectric substrate becomes 180 μm. Was finished to be 0.13 μm.

The alumina substrate was pretreated with short-wave UV light of 200 nm or less and high-concentration ozone while heating to 60 ° C. The alumina substrate was spin-coated with an ultraviolet curable adhesive mainly composed of epoxy methacrylate and applied uniformly on the bonding surface.
Further, the bonding surface of the LiTaO ₃ substrate is washed and the adhesive is applied in the same manner, and the adhesive application surface of the alumina substrate and the adhesive application surface of the LiTaO ₃ substrate are set at a pressure of 1 × 10 ⁻³ mbar. Bonding was performed under vacuum.
Next, the bonded composite piezoelectric substrate was irradiated with ultraviolet rays having an illuminance of 50 mW / cm ² for 10 minutes to cure the adhesive. At this time, the adhesive layer was uniformly 5 μm in thickness within the bonded substrate surface.
Thereafter, this bonded substrate was cured at a temperature of 130 ° C. for 2 hours in an N ₂ atmosphere.

Then, after chamfering the composite piezoelectric substrate, about 0.5 mm of the outer periphery of the LiTaO ₃ substrate which is a piezoelectric substrate was scraped off with a special chamfering wheel. Then, scraping 130 .mu.m LiTaO ₃ surface side of the substrate (bonding surface opposite) by lap and grinding, the thickness of the LiTaO ₃ substrate was set to 20μm by further polishing.
The reflectance at 365 nm on the piezoelectric substrate side of this composite piezoelectric substrate was the same value as that of the LiTaO ₃ substrate alone.

As shown in Table 2 below, the thickness of the alumina substrate was very small as 290 μm ± 0.5 μm, and the thickness of the LiTaO ₃ substrate of the composite piezoelectric substrate was as extremely small as 20 ± 0.5 μm. Further, no processing strain or cracks were observed in the LiTaO ₃ substrate.

  Next, a 1-port SAW resonator pattern made of an Al electrode (thickness 0.07 μm, electrode width 0.5 μm) was formed on the composite piezoelectric substrate by plasma etching. FIG. 4 shows an observation view of the one-port SAW resonator with an electron microscope.

The surface of this composite piezoelectric substrate is affixed to a protective tape, and the back side of the composite substrate, that is, the alumina substrate is scraped off by about 150 μm by a grinding machine, and a patterned composite piezoelectric substrate having a thickness of about 160 μm (alumina substrate thickness is about 140 μm) Was made. With such a thickness, it can be sufficiently mounted on a thin mobile phone or the like these days.
When this composite piezoelectric substrate was heated at 170 ° C., the warpage was as small as 0.8 mm (see Table 2).
Further, even when this composite piezoelectric substrate was heated at 290 ° C., the substrate did not crack.

The composite piezoelectric substrate was cut into 2 mm square, and the chip was exposed to air at 260 ° C. for 5 minutes. Thereafter, cracking and peeling of the LiTaO ₃ substrate did not occur even when subjected to a heat cycle of −40 ° C. to 85 ° C. for 1000 cycles.

Further, in the composite piezoelectric chip obtained by processing the composite piezoelectric substrate on which the above electrode is formed into a chip shape in the same manner, the direction of leakage surface acoustic wave propagation on the surface on which the electrode of the LiTaO ₃ substrate is formed is the X direction. The expansion coefficient was determined by in-situ observation, and it was αc = 10 ppm / ° C.
Next, this composite piezoelectric chip was flip-chip connected to a mounting substrate made of an alumina ceramic substrate (expansion coefficient αs = 8 ppm / ° C.) via solder bumps made of Ag and Sn, and packaging was performed.

As shown in Table 2, the temperature coefficient of the antiresonance frequency of the 1-port resonator in which the composite piezoelectric chip is flip-chip connected is as small as −13 ppm / ° C., and the effect of improving the frequency temperature characteristic is high.
Further, the variation of the temperature coefficient in the substrate surface was 1 ppm / ° C. or less, and the stability of the frequency temperature characteristic was high.
The resonance characteristics of the SAW resonator obtained in Example 1 are shown in FIG.

When the thickness of the LiTaO ₃ substrate is set to 15 μm and 25 μm by the same method as described above, the expansion coefficient in the X direction, which is the leaky surface acoustic wave propagation direction, of the surface on which the electrode of the LiTaO ₃ substrate is formed is observed in situ. Was found to be αc = 9 ppm / ° C. and 10.5 ppm / ° C.
Next, the composite piezoelectric chip on which electrodes are formed in the same manner as described above is flip-chip connected to a mounting substrate made of an alumina ceramic substrate (expansion coefficient αs = 8 ppm / ° C.) via solder bumps made of Ag and Sn. I did the packaging.
The temperature coefficient of the anti-resonance frequency of the 1-port SAW resonator in which the composite piezoelectric chip is flip-chip connected is as small as −7 ppm / ° C. and −18 ppm / ° C., respectively, and the variation of the temperature coefficient in the substrate surface is 1 ppm / ° C. or less. It was.
Thus, it can be seen that a surface acoustic wave element having a good frequency temperature characteristic and a high quality is obtained.

(Example 2)
The ceramic substrate, which is a support substrate, has a diameter of 4 inches (100 mm) and a thickness of 100 μm. Both the surface roughness Ra of the bonded surface and the opposite surface are 0.02 μm, and the porosity is An alumina substrate having a 0.1% Young's modulus of 400 GPa and a resistivity of 10 ¹⁵ Ωcm was prepared (see Table 1).
A composite piezoelectric substrate was fabricated from the prepared ceramic substrate in the same manner as in Example 1 except for the following two points, and a 1-port SAW resonator comprising an Al electrode (thickness 0.07 μm, electrode width 0.5 μm) was prepared. A pattern was formed by plasma etching. As a first point, the outer periphery of the piezoelectric substrate after bonding was set to 3 mm (0.5 mm in Example 1). As a second point, in Example 1, the alumina substrate was scraped off by 150 μm after the pattern was formed. However, in Example 2, since the thickness was sufficiently thin, this grinding was not performed.

The composite piezoelectric substrate was processed into a chip shape.
At this time, the expansion coefficient in the X direction, which is the leakage acoustic surface wave propagation direction, of the surface on which the electrode of the LiTaO ₃ substrate was formed was determined by in-situ observation, and αc = 9 ppm / ° C.
Next, the composite piezoelectric chip on which the electrodes were formed was flip-chip connected to a mounting substrate made of an alumina ceramic substrate (expansion coefficient αs = 8 ppm / ° C.) via solder bumps made of Ag and Sn, and packaging was performed. .

As shown in Table 2, the temperature coefficient of the anti-resonance frequency of the 1-port resonator in which the composite piezoelectric chip is flip-chip connected is as small as −12 ppm / ° C., and the effect of improving the frequency temperature characteristic is high.
Further, the variation of the temperature coefficient in the substrate surface was 1 ppm / ° C. or less, and the stability of the frequency temperature characteristic was high.

Further, regarding the surface acoustic wave element thus obtained, the following three items were measured in the same manner as in Example 1. The thickness variation of the ceramic substrate: 100 μm ± 0.6 μm, the thickness variation of the piezoelectric substrate : 20 μm ± 0.5 μm, warpage due to heating (170 ° C.): 0.9 mm (see Table 2).
Both showed good results, and a high-quality surface acoustic wave device could be obtained as in Example 1.

(Example 3-9)
A substrate different from the alumina substrate prepared in Example 2 in the following points was prepared as a ceramic substrate as a support substrate, and a surface acoustic wave element was produced in the same procedure as in Example 2.
Example 3 ... surface roughness Ra (bonding surface: 0.07 μm, opposite surface: 0.3 μm)
Example 4 ... surface roughness Ra (both sides 0.7 μm)
Example 5: Surface roughness Ra (both sides 0.01 μm)
Example 6 Young's modulus (50 GPa: LTCC (low temperature cofired ceramic) mainly composed of low-temperature calcined alumina)
Example 7 Young's modulus (180 GPa: ceramics mainly composed of alumina)
Example 8: Porosity and surface roughness Ra (porosity 5%, surface roughness Ra is 1 μm on both sides)
Example 9 Young's modulus (200 GPa: ceramics mainly composed of alumina)
Example 10 Ceramic substrate thickness (90 μm)
And when it measured about the said item like Example 2, the result shown in Table 3 was brought.

Examples 1-10 are examples in which the present invention was implemented. In either case, a surface acoustic wave device having a good frequency temperature characteristic can be obtained with a small frequency temperature coefficient value of about -13 to -16, a high frequency temperature characteristic improvement effect, and the like. Moreover, since it is mounted by flip chip bonding, it can be obtained with high productivity.
Furthermore, in these surface acoustic wave elements of the present invention, a ceramic substrate is used as a support substrate to be bonded to the piezoelectric substrate, and is bonded with an adhesive, so that it can be obtained at low cost.

  At this time, as can be seen from Tables 2 and 3, the surface acoustic wave device of Example 1-9 in which the thickness of the ceramic substrate as the support substrate is 100 μm or more is that of Example 10 in which the thickness is less than 100. Compared with, the variation in temperature coefficient in the substrate surface is smaller, the stability is higher, and the frequency temperature characteristic is better. Further, with respect to the warp of the composite piezoelectric substrate portion, Example 1-9 has a better result.

  Furthermore, in Example 1-9, regarding the four items of thickness variation of the ceramic substrate, thickness variation of the piezoelectric substrate, variation in temperature coefficient within the substrate surface, and warpage due to heating, the ceramic substrate used for bonding was: An example in which the surface roughness Ra is 0.02 μm or more and 0.5 μm or less on both surfaces, and both surfaces have the same surface roughness, the Young's modulus is 200 GPa or more, and the porosity is 0.1% or more and less than 4%. The composite piezoelectric substrate of Example 1, Example 2 and Example 9 is Example 3 in which the surface roughness is different on both sides, Example 4 in which the surface roughness is greater than 0.5 μm on both sides, and the surface roughness is on both sides Both composites of Example 5 in which both are smaller than 0.02 μm, Examples 6 and 7 whose Young's modulus is less than 200 GPa, and whose porosity is 4% or more and whose surface roughness exceeds 1 μm on both sides Better than piezoelectric substrate The results are shown.

When 20 composite piezoelectric substrates were produced in the same manner as in Example 2 except that the step of removing the outer periphery of the piezoelectric substrate after bonding was omitted, cracks occurred in the outer peripheral portion of the three piezoelectric substrates.
Further, when the removal was 0.08 mm from the outer periphery, cracks occurred in the outer peripheral portion of the piezoelectric substrate in one of the 20 substrates. These caused cracks in the heat treatment process.
Compared to Example 1-10, it can be seen that by removing 1 mm or more from the outer periphery of the piezoelectric substrate after bonding, it is possible to effectively prevent cracks and the like from being generated in the piezoelectric substrate. In consideration of cost and productivity, the upper limit of removal is preferably about 3 mm.

(Comparative Example 1-3)
An alumina substrate similar to the alumina substrate prepared in Example 10 was prepared as a ceramic substrate as a support substrate (Comparative Example 1). In addition, substrates different from the alumina substrate prepared in Example 10 in the following points were prepared (Comparative Examples 2 and 3). Then, a substrate different from that of Example 10 was prepared as a mounting substrate, and a surface acoustic wave element was produced in the same procedure as in Example 10.
Comparative example 2 ... surface roughness Ra (both sides 0.7 μm)
Comparative Example 3 Young's modulus (180 GPa: ceramics mainly composed of alumina)
In Comparative Example 1-3, the coefficient of expansion in the X direction, which is the leakage surface acoustic wave propagation direction, of the surface on which the electrode of the LiTaO ₃ substrate was formed was determined by in-situ observation, and each was αc = 9 ppm / ° C. The expansion coefficient αs of the mounting board was 10 ppm / ° C. That is, in Comparative Example 1-3, unlike the relationship between αs and αc in the present invention (αs <αc <αs + 6), αs> αc.
And when it measured about the said item like Example 10, it became a result shown in Table 4.

  Thus, as shown in Table 4, the frequency temperature coefficient is −28 ppm / ° C. in Comparative Example 1, −36 ppm / ° C. in Comparative Example 2, −30 ppm / ° C. in Comparative Example 3, and the value in any example is The effect of improving frequency temperature characteristics is low. It can be seen that the frequency temperature coefficient shows a large value as compared with Examples 1-10 (Tables 2 and 3) according to the present invention.

(Comparative Example 4)
A gadolinium gallium garnet (GGG) substrate having a diameter of 4 inches (100 mm) and a thickness of 200 μm was prepared. Next, a 36-degree rotated Y-cut lithium tantalate (LiTaO ₃ ) substrate having a diameter of 4 inches (100 mm) was processed to have a thickness of 0.2 mm (200 μm) and a surface Ra of 0.12 μm by double-sided lapping.

Next, the surface of the GGG substrate is cleaned, and the substrate is further pretreated with short-wave UV light having a wavelength of 200 nm or less and high-concentration ozone while being heated to 100 ° C., and an ultraviolet curable adhesive mainly composed of epoxy methacrylate is spin-coated. Then, it was uniformly applied on one surface. Next, the back surface of the LiTaO ₃ substrate is washed, and the adhesive is applied in the same manner. The adhesive application surface of the GGG substrate and the adhesive application surface of the LiTaO ₃ substrate are subjected to a vacuum of 1 × 10 ⁻³ mbar. We pasted together.

Next, the bonded composite piezoelectric substrate was irradiated with ultraviolet rays having an illuminance of 50 mW / cm ² for 10 minutes to cure the adhesive. At this time, the adhesive layer was uniformly 5 μm thick within the substrate surface. Then, after chamfering the composite piezoelectric substrate, scraping 155μm the surface of the LiTaO ₃ substrate by grinding and lap, and as the thickness of the LiTaO ₃ substrate is 20μm through further polished.

  Next, the composite piezoelectric substrate is processed into a chip shape, and a 1-port SAW resonator pattern made of an Al electrode (thickness 0.07 μm, electrode width 0.5 μm) is formed on the composite piezoelectric chip by plasma etching. did. .

At this time, the expansion coefficient αc in the X direction ± 0.5 °, which is the leakage acoustic surface wave propagation direction, of the surface on which the electrode of the LiTaO ₃ substrate is formed is heated and cooled to change the temperature change of the electrode width. As a result of in-situ observation, αc = 15 ppm / ° C.

Next, the composite piezoelectric chip on which the electrode was formed was flip-chip connected to a mounting substrate made of an alumina ceramic substrate (expansion coefficient αs = 8 ppm / ° C.) via a solder bump made of Sn, and packaging was performed. That is, unlike the relationship between αs and αc (αs <αc <αs + 6) in the present invention, αc> αs + 6.
The temperature coefficient of the antiresonance frequency of the 1-port resonator in which the composite piezoelectric chip was flip-chip connected was −39 ppm / ° C., and there was almost no improvement in temperature characteristics.

  In addition, the temperature dependence of the anti-resonance frequency of the 1-port surface acoustic wave resonator in which the composite piezoelectric chip is mounted by chip-and-wire connection is investigated by changing the ambient temperature from -40 ° C to 85 ° C, and the temperature coefficient is examined. As a result, the temperature coefficient of the anti-resonance frequency was −40 ppm / ° C. and there was no effect of improving the temperature characteristics.

(Comparative Example 5)
The temperature coefficient of the antiresonance frequency of a 1-port resonator in which the composite piezoelectric chip prepared in Example 1 (LiTaO ₃ substrate thicknesses of 20, 15, and 25 μm) is mounted by the chip-and-wire method is −17 ppm / ° C., − Although it was 10 ppm / ° C. and −22 ppm / ° C. and good temperature characteristics, the mounting operation is complicated and the productivity is not high.

As described above, as in the present invention, a ceramic substrate is used as a support substrate to be bonded to a piezoelectric substrate, and a composite piezoelectric substrate bonded through an adhesive is used, and the surface acoustic wave or leakage elastic surface of the piezoelectric substrate surface is used. As long as the expansion coefficient αc in the wave propagation direction and the expansion coefficient αs of the mounting substrate are mounted so as to satisfy the relationship of αs <αc <αs + 6, the product is manufactured with high productivity and at low cost. In addition, a surface acoustic wave device having a small frequency temperature coefficient, a high frequency temperature characteristic improvement effect, and excellent frequency temperature characteristics can be obtained.
Furthermore, in particular, when the thickness of the ceramic substrate is, for example, 100 μm or more, the variation in the in-plane frequency temperature coefficient is small, and the frequency temperature characteristics can be stabilized. Become. And the warp caused by heat becomes small.

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

1 is a schematic cross-sectional view showing an example of an embodiment of a surface acoustic wave element according to the present invention. It is the schematic sectional drawing which showed an example of the composite piezoelectric substrate used for the surface acoustic wave element concerning this invention. It is an observation figure by an electron microscope of an alumina substrate (porosity 2%, surface roughness Ra = 0.3 μm). 1-port SAW resonance consisting of an Al electrode (thickness 0.07 μm, electrode width 0.5 μm) formed on a composite piezoelectric substrate of 36 ° Y-cut LiTaO ₃ substrate / adhesive layer (5 μm) / alumina substrate (porosity 2%) structure It is an observation figure by the electron microscope of a child. 6 is a graph showing the resonance characteristics of the 1-port SAW resonator obtained in Example 1.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Composite piezoelectric chip, 2 ... Piezoelectric substrate, 3 ... Ceramic substrate,
4 ... Adhesive (adhesive layer), 5 ... Bump, 6 ... Mounting substrate, 7 ... Electrode,
8 ... Surface acoustic wave element, 9 ... Composite piezoelectric substrate.

Claims (11)

A surface acoustic wave device in which an electrode for exciting and detecting surface acoustic waves or leaky surface acoustic waves is formed on a piezoelectric substrate, and is a composite piezoelectric substrate in which at least a piezoelectric substrate and a ceramic substrate are bonded together with an adhesive And a mounting substrate on which the composite piezoelectric chip is mounted by flip chip bonding, and an expansion coefficient αc in the propagation direction of the surface acoustic wave or leakage surface acoustic wave on the surface of the piezoelectric substrate is provided. ppm / ° C.) and an expansion coefficient αs (ppm / ° C.) of the mounting substrate,
αs <αc <αs + 6
The surface acoustic wave device is mounted so as to satisfy the following relationship.   The surface acoustic wave device according to claim 1, wherein the ceramic substrate has a thickness of 100 μm or more.   The surface acoustic wave device according to claim 1, wherein the ceramic substrate has a Young's modulus of 200 GPa or more.   The index Ra indicating the surface roughness of the bonding surface of the ceramic substrate and the surface opposite to the bonding surface is 0.02 μm or more and 0.5 μm or less. The surface acoustic wave element according to any one of the above.   5. The surface acoustic wave device according to claim 1, wherein the ceramic substrate has a porosity of 0.1% or more and less than 4%.   The elastic surface according to any one of claims 1 to 5, wherein the ceramic substrate has the same surface roughness of a bonding surface and a surface opposite to the bonding surface. Wave element. The surface acoustic wave according to claim 1, wherein the piezoelectric substrate is made of any one of LiTaO ₃ , LiNbO ₃ , and Li ₂ B ₄ O _7. element.   The surface acoustic wave device according to any one of claims 1 to 7, wherein the ceramic substrate is mainly composed of alumina.   The surface acoustic wave device according to any one of claims 1 to 8, wherein the piezoelectric substrate of the composite piezoelectric substrate is removed from the outer periphery by 0.1 mm or more and 3 mm or less after bonding. .   10. The surface acoustic wave device according to claim 1, wherein the composite piezoelectric substrate has a reflectance at a wavelength of 365 nm on the piezoelectric substrate side that is the same as that of the piezoelectric substrate alone. . The surface acoustic wave device according to claim 1, wherein the composite piezoelectric substrate has a thickness of 300 μm or less.

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