WO2011158636A1 - Composite substrate - Google Patents

Abstract

Disclosed is a composite substrate provided with a piezoelectric substrate, a support substrate made of spinel, and an organic adhesive layer for adhering the piezoelectric substrate and the support substrate, wherein the support substrate comprises an adhesive surface to be adhered to the piezoelectric substrate and the adhesive surface has an Rt (maximum cross sectional height of a roughness curve) of 5 nm or more and 50 nm or less.

Description

Composite board

The present invention relates to a composite substrate.

Conventionally, elasticity such as surface acoustic wave devices that can function as filter elements and oscillators used in mobile phones and the like, Lamb wave elements using piezoelectric thin films, and thin film resonators (FBAR: Film Bulk Acoustic Resonator) Wave devices are known. As such an acoustic wave device, a device in which a supporting substrate and a piezoelectric substrate that propagates an acoustic wave are bonded together and a comb-shaped electrode capable of exciting a surface acoustic wave is provided on the surface of the piezoelectric substrate is known. Of these, a substrate obtained by bonding a support substrate and a piezoelectric substrate is referred to as a composite substrate. As the support substrate, a sapphire substrate is often used. However, since sapphire has a high hardness, it has been disadvantageous in that it is difficult to cut into a predetermined shape and the cost is high. In consideration of these drawbacks, it has recently been proposed to use a spinel substrate as a support substrate (Patent Document 1).
JP 2011-66818 A

However, as described in paragraph 0021 of Patent Document 1, it has been difficult to form a piezoelectric substrate using an adhesive on the main surface of the spinel substrate. For this reason, in Patent Document 1, the spinel substrate and the piezoelectric substrate are directly joined using van der Waals force without using an adhesive.

The present invention has been made to solve such problems, and has as its main object to provide a composite substrate in which a piezoelectric substrate and a support substrate made of spinel are firmly bonded via an organic adhesive layer. .

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

The composite substrate of the present invention is
A piezoelectric substrate;
A support substrate made of spinel;
An organic adhesive layer for bonding the piezoelectric substrate and the support substrate;
With
Of the support substrate, the bonding surface with the piezoelectric substrate has Rt (maximum cross-sectional height of the roughness curve) of 5 nm to 50 nm.

In this composite substrate, since the Rt of the bonding surface of the support substrate to the piezoelectric substrate is 5 nm or more and 50 nm or less, the piezoelectric substrate and the support substrate made of spinel are firmly bonded through the organic adhesive layer. Moreover, since the organic adhesive layer is present compared to the case where the piezoelectric substrate and the support substrate are directly bonded, an effect of preventing cracking due to stress relaxation can be obtained.

1 is a perspective view of a composite substrate 10. FIG. FIG. 2 is a cross-sectional view taken along line AA in FIG. 1.

In the composite substrate of the present invention, the piezoelectric substrate is, for example, one selected from the group consisting of lithium tantalate, lithium niobate, lithium niobate-lithium tantalate solid solution, lithium borate, langasite, and quartz. It is good also as a board | substrate which becomes. The size of the piezoelectric substrate is not particularly limited. For example, the diameter may be 50 to 150 mm and the thickness may be 10 to 500 μm.

In the composite substrate of the present invention, the organic adhesive layer may be, for example, an acrylic adhesive layer or an epoxy adhesive. The thickness of the organic adhesive layer is not particularly limited, but for example, 0.1 to 1.0 μm is preferable because good frequency temperature characteristics can be obtained.

In the composite substrate of the present invention, the support substrate is a substrate made of spinel. Rt of the bonding surface of the support substrate with the piezoelectric substrate is 5 nm or more and 50 nm or less. If the Rt of the adhesion surface is less than 5 nm, the piezoelectric substrate and the support substrate may be peeled off when the composite substrate is processed at a high temperature, which is not preferable. If it exceeds 50 μm, the piezoelectric substrate is not processed when the composite substrate is processed at a high temperature. This is not preferable because the substrate may burst. The spinel is preferably a polycrystalline spinel that is an oxide of magnesium and aluminum. The size of the support substrate is not particularly limited, and for example, the diameter may be 50 to 150 mm and the thickness may be 100 to 500 μm.

An example of a method for producing the composite substrate of the present invention will be described below. First, the surface of the support substrate is polished so that Rt is 5 nm to 50 nm. Subsequently, the bonding surface of the piezoelectric substrate and the support substrate is washed, and impurities (oxide, adsorbed material, etc.) adhering to the bonding surface are removed. Next, an organic adhesive is uniformly applied to at least one of the joint surfaces of both substrates. Examples of the coating method include spin coating. Thereafter, the two substrates are bonded together, and when the organic adhesive is a thermosetting resin, it is cured by heating, and when the organic adhesive is a photocurable resin, it is cured by irradiation with light. Thus, when indirectly bonding through an organic adhesive layer, it is preferable that the organic adhesive layer has a thickness of 0.1 to 1.0 μm. In this way, the composite substrate of the present invention can be obtained.

The composite substrate of the present invention is used for an acoustic wave device. As an acoustic wave device, a surface acoustic wave device, a Lamb wave element, a thin film resonator (FBAR), and the like are known. For example, a surface acoustic wave device includes an input-side IDT (Interdigital-Transducer) electrode (also referred to as a comb-shaped electrode or an interdigital electrode) that excites surface acoustic waves on the surface of a piezoelectric substrate and an output-side IDT that receives surface acoustic waves. And an electrode. When a high frequency signal is applied to the IDT electrode on the input side, an electric field is generated between the electrodes, and a surface acoustic wave is excited and propagates on the piezoelectric substrate. Then, the propagated surface acoustic wave can be taken out as an electric signal from the IDT electrode on the output side provided in the propagation direction. Such an acoustic wave device employs a reflow process when mounted on a printed wiring board, for example. In this reflow process, when lead-free solder is used, the acoustic wave device is heated to about 260 ° C. However, since the acoustic wave device using the composite substrate of the present invention is excellent in heat resistance, the piezoelectric substrate and the support substrate Generation of cracks is suppressed.

In the composite substrate of the present invention, the piezoelectric substrate may have a metal film on the back surface. The metal film plays a role of increasing the electromechanical coupling coefficient in the vicinity of the back surface of the piezoelectric substrate when a Lamb wave element is manufactured as an elastic wave device. In this case, the Lamb wave element has a structure in which comb electrodes are formed on the surface of the piezoelectric substrate, and the metal film of the piezoelectric substrate is exposed by the cavity provided in the support substrate. Examples of the material of such a metal film include aluminum, an aluminum alloy, copper, and gold. When a Lamb wave element is manufactured, a composite substrate including a piezoelectric substrate that does not have a metal film on the back surface may be used.

In the composite substrate of the present invention, the piezoelectric substrate may have a metal film and an insulating film on the back surface. The metal film serves as an electrode when a thin film resonator is manufactured as an acoustic wave device. In this case, the thin film resonator has a structure in which electrodes are formed on the front and back surfaces of the piezoelectric substrate, and the metal film of the piezoelectric substrate is exposed by using the insulating film as a cavity. Examples of the material for such a metal film include molybdenum, ruthenium, tungsten, chromium, and aluminum. Examples of the material for the insulating film include silicon dioxide, zinc oxide, phosphorous silica glass, and boron phosphorous silica glass.

[Example 1]
1 is a perspective view of the composite substrate 10, and FIG. 2 is a cross-sectional view taken along the line AA of FIG. The composite substrate 10 is used for a surface acoustic wave device, and is formed in a circular shape with one portion being flat. This flat portion is a portion called an orientation flat (OF), and is used for detection of a wafer position and direction when various operations are performed in the manufacturing process of the surface acoustic wave device. The composite substrate 10 includes a piezoelectric substrate 12 made of lithium tantalate (LT) capable of propagating elastic waves, and a support substrate 14 made of spinel (cubic polycrystalline spinel, MgAl ₂ O ₄ ) bonded to the piezoelectric substrate 12. And an adhesive layer 16 for joining the substrates 12 and 14 together. The piezoelectric substrate 12 has a thickness of 20 μm and a diameter of 4 inches (about 100 mm). The piezoelectric substrate 12 is a 42 ° Y-cut X-propagation LT substrate (42Y-X LT). The support substrate 14 has a thickness of 250 μm and a diameter of 4 inches. The adhesive layer 16 is a layer in which an acrylic adhesive is solidified and has a thickness of 0.6 μm.

A method for manufacturing such a composite substrate 10 will be described below. First, a support substrate made of polycrystalline spinel having a diameter of 4 inches was prepared as a support substrate. In addition, a 42 ° Y-cut X-propagation LT substrate having a diameter of 4 inches was prepared as a piezoelectric substrate. Then, the adhesive surface of the piezoelectric substrate with the support substrate was polished and polished so that the surface roughness Rt was 3 nm. The thickness of the piezoelectric substrate after polishing was 250 μm. On the other hand, the adhesive surface of the support substrate with the piezoelectric substrate was polished and polished with microdiamond so that Rt was 5 nm. The thickness of the support substrate after polishing was 250 μm. Rt was measured in accordance with JIS B601 (2001), with the measurement range being a region surrounded by a 10 μm × 10 μm square. Subsequently, an acrylic adhesive was applied to one side of each substrate using a spin coater. Then, both substrates were bonded so that the adhesive-coated surfaces of both substrates were facing each other, and held at 280 ° C. for 30 minutes. Thereby, the bonded substrate by which both the board | substrates were adhere | attached through the contact bonding layer formed by hardening | curing an acrylic adhesive was obtained. Among the bonded substrates, the piezoelectric substrate was polished and polished until the thickness became 20 μm, and 10 composite substrates 10 of Example 1 were obtained.

[Example 2]
Ten composite substrates 10 were obtained in the same manner as in Example 1, except that the adhesive surface of the support substrate with the piezoelectric substrate was polished and polished so that Rt was 45 nm.

[Comparative Example 1]
Ten composite substrates 10 were obtained in the same manner as in Example 1, except that the adhesive surface of the support substrate with the piezoelectric substrate was polished and polished so that Rt was 2 nm.

[Comparative Example 2]
Ten composite substrates 10 were obtained in the same manner as in Example 1, except that the adhesive surface of the support substrate with the piezoelectric substrate was polished and polished so that Rt was 60 nm.

[Evaluation]
Ten composite substrates 10 of Examples 1 and 2 and Comparative Examples 1 and 2 were each left in an oven at 280 ° C. for 1 hour. The results are shown in Table 1. As apparent from Table 1, no defects were found in the composite substrates 10 of Examples 1 and 2, and the piezoelectric substrate and the support substrate were firmly bonded. On the other hand, in Comparative Example 1, the piezoelectric substrate and the support substrate were peeled off. In Comparative Example 2, the piezoelectric substrate burst. The reason for the rupture of the piezoelectric substrate is that since Rt exceeded 50 nm, there was a microhole with a deep depth locally, and the piezoelectric substrate and the support substrate were bonded together in a state where air was caught in the microhole. It is considered that the air was expanded by the high temperature treatment, and the piezoelectric substrate was ruptured. Since the arithmetic average roughness Ra is an average value, it does not serve as an index as to whether or not a minute hole having a deep depth exists. For example, even if Ra is 40 nm, deep microholes with a depth exceeding 50 nm may exist.

This application is based on US provisional application 61/354837 filed Jun. 15, 2010, the entire contents of which are incorporated herein by reference.

The composite substrate of the present invention can be used for an elastic wave device such as a surface acoustic wave device, a Lamb wave element, and a thin film resonator (FBAR).

Claims (3)

A piezoelectric substrate;
A support substrate made of spinel;
An organic adhesive layer for bonding the piezoelectric substrate and the support substrate;
With
Of the support substrate, the adhesive surface with the piezoelectric substrate has an Rt (maximum cross-sectional height of the roughness curve) of 5 nm or more and 50 nm or less.
Composite board. The spinel is a polycrystalline spinel that is an oxide of magnesium and aluminum.
The composite substrate according to claim 1. The organic adhesive layer has a thickness of 0.1 μm or more and 1.0 μm or less.
The composite substrate according to claim 1 or 2.

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