JP2006246050A - Composite piezoelectric wafer and surface acoustic wave device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve temperature characteristics and electric characteristic of a piezoelectric wafer forming an SAW apparatus and to manufacture a highly reliable piezoelectric wafer especially having these excellent characteristics and capable of preventing electrostatic failures or the like. <P>SOLUTION: A composite piezoelectric wafer is provided with first and second wafers consisting of a piezoelectric material and integrally formed by sticking the first and second wafers to each other so that faces having the same polarity are joined with each other. The thickness of the first wafer is 0.1 to 0.5 λ. Both the wafers are integrated by direct junction or surface activation junction. In the first wafer, it is preferable to improve a pyroelectric characteristic, to add an additive and to set volume resistivity to 3.6×10<SP>10</SP>to 1.5×10<SP>14</SP>Ωcm. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a composite piezoelectric wafer and a surface acoustic wave device, and more particularly to a composite wafer obtained by bonding and integrating a plurality of piezoelectric wafers, and a surface acoustic wave device manufactured using the wafer.

  A surface acoustic wave device (hereinafter referred to as a SAW device) using a surface acoustic wave (SAW) generated by a piezoelectric effect is small and light and has excellent reliability. Therefore, it is a transmission / reception filter in a communication terminal such as a mobile phone. Recently, it has been widely used for antenna demultiplexers. Such a SAW device is provided with a cross-fingered electrode (Interdigital Transducer / hereinafter referred to as IDT) on a piezoelectric substrate, a high frequency signal is applied to the IDT to excite a surface acoustic wave, and the propagated signal Is converted to a high frequency signal again to extract a specific frequency.

  By the way, in the SAW device using such a piezoelectric substrate, since the piezoelectric substrate itself has temperature characteristics, for example, when a filter is configured, there arises a problem that the characteristics of the device fluctuate such that the center frequency of the filter shifts due to temperature change. Sometimes. In particular, with the improvement of high frequency and high performance of communication terminals, further improvement in characteristics of mounted devices is required, and various methods for improving the temperature characteristics of the electrode structure and connection structure of the SAW device or the piezoelectric substrate itself have been proposed. Proposed.

For example, there are two methods for improving the temperature characteristics of a piezoelectric wafer: (1) a method of forming a SiO ₂ thin film having a temperature coefficient opposite to that on a piezoelectric substrate such as LiTaO _3; A method of suppressing expansion and contraction due to temperature change of the piezoelectric substrate by pasting a material having an expansion coefficient, or (3) forming a domain-inverted layer by proton exchange treatment on the surface of the piezoelectric substrate on which the surface acoustic wave is propagated, thereby providing a short circuit effect There are ways to get it.

Further, there are the following documents as proposals for improving the characteristics.
JP 2004-343359 A JP 2004-297893 A Japanese Patent Laid-Open No. 10-51262 JP 2004-254114 A JP 2004-35396 A JP 2004-336600 A IEICE Tech. Bulletin US93-49 (1993-09 / pages 1-5) "Measurement of temperature characteristics of SH type surface acoustic wave on LiTaO3 36 [deg.] Rotated Y plate with polarization reversal layer" Tohoku University / Sakai, Akira Nakamura Author

However, the above (1) method of depositing the SiO ₂ thin film has problems that it is difficult to control the thickness of the SiO ₂ film during deposition, and the propagation loss of the surface acoustic wave is increased. In addition, in the above (2) method of attaching a material having a low expansion coefficient to the piezoelectric substrate, it is difficult to control at the bonding interface, and the SAW device is manufactured after several hundred degrees of temperature such as an IDT formation process. In the process, cracks and cracks occur due to the difference in thermal expansion coefficient between the bonded materials, and there is a difficulty in reducing the yield. Furthermore, in the method (3) based on proton exchange, it is not easy to form a domain-inverted layer having a uniform thickness, and the manufacturing process must be complicated.

  On the other hand, a piezoelectric substrate generally has pyroelectricity as well as piezoelectricity, and therefore a non-uniform charge distribution is generated on the substrate surface with a temperature change (rise), deteriorating the electrical characteristics of the device, or being fine due to accumulated charges. In some cases, problems such as discharge between the electrodes of the IDT portion and damage to the electrode fingers may be caused. Moreover, these problems cannot be solved even by the method described in the above document.

  Accordingly, an object of the present invention is to improve the temperature characteristics and electrical characteristics of the piezoelectric wafer forming the SAW device, and in particular, these excellent characteristics and a highly reliable piezoelectric that can prevent electrostatic breakdown and the like. This is in that the wafer can be manufactured by a simpler method.

  In order to solve the above problems and achieve the object, a composite piezoelectric wafer according to the present invention includes a first piezoelectric wafer made of a piezoelectric material and a second piezoelectric wafer made of a piezoelectric material, The piezoelectric wafer and the second piezoelectric wafer are bonded and integrated so that the same polar surfaces are bonded to each other.

  When the polarization of the surface of the piezoelectric substrate (piezoelectric wafer) is reversed, the electric field short-circuit effect and the phase velocity of the substrate surface decrease, thereby improving temperature characteristics (such as TCD) and electrical characteristics such as Q value. The present invention utilizes this property, and a first piezoelectric wafer (hereinafter referred to as a first wafer) and a second piezoelectric wafer (hereinafter referred to as a second wafer), both of which are made of a piezoelectric material, have the same polar surface. The two pieces (plus faces or minus faces) are bonded and integrated so as to be joined. As a result, an electric field short-circuit layer is formed at the boundary between both wafers, and temperature characteristics (TCD and the like) and electrical characteristics such as Q value can be improved.

The kind of piezoelectric material that forms the first wafer and the second wafer is not particularly limited. For example, a piezoelectric single crystal such as lithium tantalate (LiTaO ₃ / hereinafter, sometimes referred to as LT), lithium niobate (LiNbO ₃ / hereinafter, sometimes referred to as LN), quartz, or the like may be used. It may be formed of piezoelectric ceramics such as lead zirconate-based piezoelectric ceramics.

  It should be noted that configuring both wafers with the same material (for example, both LT or both LN) eliminates the difference in thermal expansion coefficient between the bonded wafers (the first wafer and the second wafer), and at the time of device manufacture and various This is preferable from the viewpoint of preventing the occurrence of cracks and cracks during actual use after mounting on an electronic device. However, the present invention is not limited to one in which both the first and second wafers are made of the same material, and one in which each wafer is formed from a different material (for example, one of the two wafers is made of the other by LT). Are formed by LN, etc.).

  The first wafer and the second wafer can be bonded by, for example, direct bonding or surface activated bonding. According to direct bonding, it is possible to perform strong bonding by applying heating below the Curie point without using an adhesive or the like. Also, according to surface activated bonding, it is possible to form a composite wafer in which both wafers are firmly bonded by removing the surface layer that hinders bonding and directly bonding the bonds of atoms on the wafer surface. I can do it.

  The first wafer has a thickness of 0.1λ to 1.5λ.

  As for the surface acoustic wave excited by the piezoelectric substrate, for example, about 90% or more of Rayleigh waves are propagated on the surface layer within a depth of 1λ from the substrate surface. In addition, even with a leaky wave of the SH type or the like, wave energy can be concentrated on the substrate surface by appropriately selecting the electrical boundary condition on the substrate surface. For this reason, when the thickness of the first wafer is 0.1λ to 1.5λ, the effect of improving the temperature characteristics and the electrical characteristics due to the electric field short-circuit effect can be obtained satisfactorily.

  In order to set the thickness of the first wafer to the above value, for example, after bonding the first wafer to the second wafer, the first wafer is processed to have the above value by a method such as cutting and polishing. The first wafer may be processed in advance to the above thickness before bonding to the two wafers, and then bonded to the second wafer. The thickness of the second wafer is not particularly limited.

  In the composite piezoelectric wafer of the present invention, it is preferable to perform pyroelectric property improvement processing on the first wafer.

  Piezoelectric substrates generally have pyroelectric properties, and non-uniform charge distribution occurs on the surface of the substrate with changes in temperature. When this charge is accumulated, it is actually used during the manufacture of SAW devices and after being mounted on various electronic devices. At times, there is a concern that the electrode may be damaged due to characteristic deterioration or discharge of the IDT portion. If the pyroelectric characteristics are improved on the first wafer, such a problem can be avoided and a SAW device having excellent temperature characteristics and reliability can be manufactured.

  A specific method for improving the pyroelectric characteristics is not particularly limited. For example, when a single crystal of the piezoelectric material constituting the first wafer is generated in the Czochralski method, an additive ( For example, it can be carried out by adding at least one of Fe, Mn, Cu, and Ti) and reducing the volume resistance of the wafer. Further, the volume resistance of the wafer may be reduced by pulling up the piezoelectric single crystal and slicing it to perform a reduction process for removing oxygen in the crystal.

  In the composite piezoelectric wafer, it is desirable to add an additive to the first wafer from another viewpoint.

  That is, when an additive is added, a color corresponding to the additive is attached to the wafer itself. On the other hand, the second wafer to which no additive is added (for example, in the case of an LT wafer) generally has a transparent or white color. Therefore, after bonding the first wafer and the second wafer, for example, when the first wafer on which an IDT (SAW element) will be formed is cut and polished to a predetermined thickness, The thickness of one wafer can be easily visually checked, and the first wafer can be processed accurately and easily.

  In this case, examples of the additive to be added include Fe, Mn, Cu, and Ti. One or more of these may be mixed into the piezoelectric material forming the first wafer.

Furthermore, in the composite piezoelectric wafer, it is desirable that the first wafer has a volume resistivity of 3.6 × 10 ¹⁰ Ω · cm or more and 1.5 × 10 ¹⁴ Ω · cm or less. In order to reduce the volume resistivity to such a value, one or more additives of Fe, Mn, Cu or Ti may be added to the first wafer.

Conventional piezoelectric wafers used in the manufacture of SAW devices generally have a high volume resistivity, which tends to cause a non-uniform charge distribution and can cause discharge between IDT electrode fingers. On the other hand, in the present invention, the volume resistivity of the first wafer, which is the wafer on the side where the SAW device is formed, is preferably set to 1.5 × 10 ¹⁴ Ω · cm or less. This makes it possible to improve the characteristics of the SAW device by making the charge distribution uniform, and to prevent the occurrence of discharge breakdown and the like.

On the other hand, the volume resistivity of the first wafer is 3.6 × 10 ¹⁰ Ω · cm or more. Thereby, it is possible to prevent the electrode fingers of the IDT from being short-circuited.

  A SAW device according to the present invention is manufactured using any one of the above-described composite piezoelectric wafers, and includes a piezoelectric substrate formed by the composite piezoelectric wafer and a cross-finger electrode provided on the piezoelectric substrate. The interdigitated electrodes are provided on the piezoelectric substrate so as to be disposed on the surface of the first wafer.

  Examples of the SAW device include various devices that use surface acoustic waves generated on a piezoelectric substrate such as an IDT or a SAW resonator including a reflector and a SAW filter, a SAW filter, and a SAW duplexer.

  According to the present invention, it is possible to improve the temperature characteristics and electrical characteristics of a piezoelectric wafer forming a SAW device, and in particular, to provide a highly reliable piezoelectric wafer that can prevent electrostatic breakdown and the like with these good characteristics. It becomes possible to manufacture by a simpler method.

  Other objects, features and advantages of the present invention will become apparent from the following description of embodiments of the present invention.

FIG. 1 shows a composite piezoelectric wafer according to an embodiment of the present invention. As shown in the figure, this composite piezoelectric wafer 11 is obtained by integrating a first wafer 12 made of a piezoelectric material and a second wafer 13 made of the same piezoelectric material, and these first and second wafers. 12 and 13 are both formed of a 39 ° Y-cut X-propagation LT (LiTaO ₃ ) single crystal wafer in this example, and iron (Fe) is added to the first wafer 12 as an additive.

  The method for manufacturing the first and second wafers 12 and 13 (single crystal) is not particularly limited. For example, the LT raw material is melted, the seed crystal is immersed in the melt, and the single crystal is pulled up. Can be obtained by the Czochralski method. However, for the first wafer 12, an additive (Fe) is added to the melt.

Further, the pyroelectric characteristics are improved for the first wafer 12 and the volume resistivity is set to a value within the range of 3.6 × 10 ^{10 to} 1.5 × 10 ¹⁴ Ω · cm. The pyroelectric treatment and the volume resistivity of 1 × 10 ¹³ Ω · cm or less prevent the occurrence of discharge breakdown due to the accumulated charge accompanying the temperature change, while the volume resistivity is 3.6 × 10 ¹⁰ Ω · cm. This is to prevent a short circuit from occurring between the IDT electrodes formed on the surface of the first wafer.

  As a specific method for improving the pyroelectric characteristics, for example, an additive (for example, Fe, Mn, Cu, Ti, etc.) may be added to the molten LT raw material. Further, after the LT single crystal is pulled up and sliced, reduction treatment for removing oxygen in the crystal may be performed. On the other hand, in order to set (lower) the volume resistivity to the above value, for example, one or more of Fe, Mn, Cu and Ti may be added to the first wafer 12 as an additive.

The first wafer 12 and the second wafer 13 are bonded and integrated so that the minus surfaces (may be plus surfaces) are in contact with each other. As a result, an electric field short-circuit layer 15 is formed at the boundary surface between the wafers 12 and 13. The thickness t ₁ of the first wafer 12 is, for example, about 20 μm. The thickness is required to be about several λ depending on the wavelength (λ) of the surface acoustic wave in order to effectively give the electric field short circuit effect to the surface acoustic wave propagating on the substrate surface layer. The thickness t ₂ of the second wafer 13 is not particularly limited, but is about 300 μm, for example.

The first wafer 12 and the second wafer 13 can be joined by direct joining. 2A to 2D and FIG. 3 sequentially show the steps of manufacturing the composite piezoelectric wafer 11 by this method. As shown in FIG. 2A and FIG. 3, first, a first wafer 120 (which is later processed to have a predetermined thickness t ₁ by grinding or the like, but is denoted by reference numeral 120 to indicate a state before processing) and a first wafer 120. The two wafers 13 are washed with a processing solution (chemicals such as acid and pure water) 1 to remove impurities on the surfaces of the wafers 120 and 13 and to make the surfaces of the wafers 120 and 13 hydrophilic (FIG. 3). Step S301).

Next, as shown in FIG. 2B, the first wafer 120 and the second wafer 13 are superposed so that the minus surfaces are aligned with each other (step S302 in FIG. 3), and then heated in the furnace 2 (FIG. 3). In step S303, the wafers 120 and 13 are bonded (step S304 in FIG. 3). Then, as shown in FIG. 2D, the first wafer 120 is ground until the thickness thereof becomes t ₁ (= 20 μm, for example) (step S305 in FIG. 3).

At this time, the second wafer 13 is transparent or white, but the first wafer 120 has an orange color due to the addition of Fe. Therefore, even after the wafers 120 and 13 are integrated, the first wafer 120 can be easily visually identified, and the first wafer 120 can be accurately ground to the predetermined thickness t ₁ . . In FIG. 2D, reference numeral 12a indicates a portion that is removed by grinding.

As described above, according to the present embodiment, the electric field short-circuit layer 15 can be easily formed at a uniform depth t ₁ (for example, 20 μm / several λ) from the surface of the first wafer 12 that is the formation surface of the SAW device. The composite piezoelectric wafer 11 having excellent temperature characteristics and electrical characteristics such as the Q value can be manufactured by the electric field short-circuit effect.

  The bonding of the first wafer 12 and the second wafer 13 can also be performed by surface activated bonding. 4A to 4D and FIG. 5 sequentially show steps for manufacturing the composite piezoelectric wafer 11 by surface activated bonding.

  In this method, first, the surfaces of the first wafer 120 and the second wafer 13 covered with the contaminant S as shown in FIG. 4A are formed on the surface of the first wafer 120 and the second wafer 13 in a vacuum atmosphere 5 as shown in FIG. 4B and FIG. The surface is cleaned and activated by sputter etching using the active gas 6 (step S501 in FIG. 5).

Next, the first wafer 120 and the second wafer 13 whose surfaces are activated as shown in FIG. 4C are superposed in a vacuum atmosphere as shown in FIG. 4D (step S502 in FIG. 5). 13 are joined (step S503 in the figure). At this time, the same polar surfaces of both wafers are overlapped as in the case of the direct bonding. Then, the first wafer 120 is ground until the thickness t ₁ is reached in the same manner as in the direct bonding method.

  According to such surface activated bonding, the surface layer of the wafer that hinders bonding is removed, and bonding is performed in a vacuum atmosphere, whereby the bonds of the surface atoms are directly bonded to each other at room temperature or 100 ° C. or lower. A strong bond can be formed by applying a certain amount of heating.

FIG. 6 is a line showing the relationship between the depth of the electric field short-circuit layer (thickness t ₁ of the first wafer) and the delay time temperature coefficient TCD of the SH type surface acoustic wave in the composite piezoelectric wafer manufactured based on the above embodiment. FIG. In the figure, each point of the rhombus indicates an experimental value, and a solid line indicates a theoretical value obtained by numerical calculation. The depth of the electric field short-circuit layer is a value t normalized by the wavelength λ of the surface acoustic wave. ₁ / λ is used.

As can be seen from this figure, it is preferable that the depth (first wafer thickness) t ₁ / λ of the electric field short-circuit layer is 0.1 to 1.5 in terms of improving temperature characteristics.

  FIG. 7 is a perspective view showing an example of a SAW element configured using the composite piezoelectric wafer 11 according to the embodiment. As shown in the figure, this SAW element 21 is formed by forming a conductor pattern including an IDT (intersecting finger electrode) 22, an electrode pad 23, and a conductor line (not shown) on the surface of a piezoelectric substrate 31. Are formed by the composite piezoelectric wafer. In the piezoelectric substrate 31, the surface layer 32 on which the IDT 22 and the like are formed is formed by the first wafer 12, and the electric field short-circuit layer 15 is formed on the boundary surface with the lower surface layer 33 formed by the second wafer 13. Is provided.

  Further, FIG. 8 shows a SAW filter formed using the SAW element 21 shown in FIG. 7, and this filter 41 is formed by flip-chip mounting the SAW element 21 on a base substrate 42 via bumps 44. The lid 43 is hermetically sealed. The SAW element 21 includes the electric field short-circuit layer 15, whereby the filter 41 has excellent temperature characteristics and electrical characteristics.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to these, It is clear to those skilled in the art that a various change can be made within the range as described in a claim. .

It is a side view which shows the composite piezoelectric wafer which concerns on one Embodiment of this invention. It is a conceptual diagram which shows one process in the manufacturing method (direct bonding) of the composite piezoelectric wafer which concerns on the said embodiment. It is a conceptual diagram which shows one process following the process of the said FIG. 2A. It is a conceptual diagram which shows one process following the process of the said FIG. 2B. It is a conceptual diagram which shows one process following the process of the said FIG. 2C. It is a flowchart which shows the manufacturing process (direct bonding) of the composite piezoelectric wafer which concerns on the said embodiment. It is a conceptual diagram which shows one process in the manufacturing method (surface activation joining) of the composite piezoelectric wafer which concerns on the said embodiment. It is a conceptual diagram which shows one process following the process of the said FIG. 4A. It is a conceptual diagram which shows one process following the process of the said FIG. 4B. It is a conceptual diagram which shows one process following the process of the said FIG. 4C. It is a flowchart which shows another production process (surface activation joining) of the composite piezoelectric wafer which concerns on the said embodiment. It is a diagram which shows the relationship between the depth of the electric field short circuit layer (thickness of a 1st wafer) in the composite piezoelectric wafer which concerns on the said embodiment, and the delay time temperature coefficient TCD of SH surface acoustic wave. It is a perspective view which shows an example of the SAW element produced using the composite piezoelectric wafer which concerns on this invention. It is sectional drawing which shows an example of the SAW filter comprised using the SAW element shown in the said FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Composite piezoelectric wafer 12,120 1st wafer 13 2nd wafer 15 Electric field short circuit layer 21 SAW element 22 IDT (interstitial finger electrode)
23 Electrode pad 31 Piezoelectric substrate 41 SAW filter

Claims (8)

A first piezoelectric wafer made of a piezoelectric material;
A second piezoelectric wafer made of a piezoelectric material;
With
A composite piezoelectric wafer, wherein the first piezoelectric wafer and the second piezoelectric wafer are bonded and integrated so that the same polar surfaces are bonded to each other. The composite piezoelectric wafer according to claim 1, wherein the first piezoelectric wafer has a thickness of 0.1λ to 1.5λ. The composite piezoelectric wafer according to claim 1 or 2, wherein the first piezoelectric wafer and the second piezoelectric wafer are integrated by direct bonding. The composite piezoelectric wafer according to claim 1 or 2, wherein the first piezoelectric wafer and the second piezoelectric wafer are integrated by surface activation bonding. 5. The composite piezoelectric wafer according to claim 1, wherein the first piezoelectric wafer is a piezoelectric wafer that has undergone pyroelectric property improvement processing. The composite piezoelectric wafer according to claim 1, wherein an additive is added to the first piezoelectric wafer to color the wafer. The first piezoelectric wafer has a volume resistivity of 3.6 × 10 ¹⁰ Ω · cm or more and 1.5 × 10 ¹⁴ Ω · cm or less. The composite piezoelectric wafer according to Item. A piezoelectric substrate formed by the composite piezoelectric wafer according to any one of claims 1 to 7,
An interdigitated electrode provided on the piezoelectric substrate;
A surface acoustic wave device comprising:
The surface acoustic wave device is characterized in that the interdigitated electrodes are provided on the piezoelectric substrate so as to be disposed on a surface of the first piezoelectric wafer.

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