Composite substrate, elastic wave device, and method for producing elastic wave device

Abstract

A composite substrate 10 is formed by bonding together a piezoelectric substrate 12 and a support substrate 14 that has a lower thermal expansion coefficient than the piezoelectric substrate. The support substrate 14 is formed by directly bonding together a first substrate 14a and a second substrate 14b at a strength that allows separation with a blade, the first and second substrates being formed of the same material, and a surface of the first substrate 14a is bonded to the piezoelectric substrate 12, the surface being opposite to another surface of the first substrate 14a bonded to the second substrate 14b.

Claims

1. A composite substrate formed by bonding together a piezoelectric substrate and a support substrate that has a lower thermal expansion coefficient than the piezoelectric substrate, wherein the support substrate is formed by directly bonding together a first substrate and a second substrate at a strength that allows separation with a blade, the first and second substrates being formed of the same material; and a surface of the first substrate is bonded to the piezoelectric substrate, the surface being opposite to another surface of the first substrate bonded to the second substrate.

2. The composite substrate according to claim 1, wherein the first and second substrates are both silicon substrates.

3. The composite substrate according to claim 1, wherein the strength that allows separation with a blade corresponds to a bonding energy per unit area of the first and second substrates in a range of 0.05 to 0.6 J/m.sup.2.

4. The composite substrate according to claim 1, wherein the first and second substrates have a thickness of 100 to 600 m.

5. The composite substrate according to claim 1, wherein iron element and chromium element are contained between the first substrate and the second substrate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic sectional view of a composite substrate 10.

(2) FIG. 2 is a schematic sectional view of steps for producing a composite substrate 10.

(3) FIG. 3 is a schematic sectional view of steps for producing an elastic wave device 30.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(4) Hereinafter, embodiments according to the present invention will be described with reference to drawings. FIG. 1 is a schematic sectional view of a composite substrate 10 according to an embodiment. This composite substrate 10 includes a piezoelectric substrate 12 and a support substrate 14.

(5) The piezoelectric substrate 12 is a substrate that can propagate elastic waves. The material for the piezoelectric substrate 12 may be lithium tantalate (LT), lithium niobate (LN), a lithium niobate-lithium tantalate solid-solution single crystal, rock crystal, lithium borate, zinc oxide, aluminum nitride, langasite (LGS), langatate (LGT), or the like. Of these, LT or LN is preferred. This is because LT and LN allow surface acoustic waves to be propagated at high speeds and have large electromechanical coupling factors and hence are suitable for elastic wave devices for high frequencies and wide-band frequencies. The piezoelectric substrate 12 is not particularly limited in terms of size and may have, for example, a diameter of 50 to 150 mm and a thickness of 0.2 to 50 m.

(6) The support substrate 14 has a lower thermal expansion coefficient than the piezoelectric substrate 12 and is bonded to the back surface of the piezoelectric substrate 12 directly or via an organic adhesive layer. The support substrate 14 has a lower thermal expansion coefficient than the piezoelectric substrate 12, so that variations in the size of the piezoelectric substrate 12 in response to changes in temperature are suppressed, and temperature-dependent changes in frequency characteristics of the composite substrate 10 serving as elastic wave devices can be suppressed. This support substrate 14 is formed by directly bonding together a first substrate 14a and a second substrate 14b at a strength that allows separation with a blade, the first and second substrates 14a and 14b being formed of the same material. The support substrate 14 is bonded to the piezoelectric substrate 12 via a surface of the first substrate 14a, the surface being opposite to the other surface of the first substrate 14a bonded to the second substrate 14b. The material for the support substrate 14 may be silicon, sapphire, aluminum nitride, alumina, borosilicate glass, quartz glass, or the like, and is preferably silicon. The support substrate 14 has a size, for example, a diameter of 50 to 150 mm and a thickness of 200 to 1200 m. The first and second substrates 14a and 14b have a size, for example, a diameter of 50 to 150 mm and a thickness of 100 to 600 m. The support substrate 14 preferably has a higher Young's modulus than the piezoelectric substrate 12.

(7) Table 1 shows the thermal expansion coefficients of representative materials used for the piezoelectric substrate 12 and the support substrate 14.

(8) TABLE-US-00001 TABLE 1 Thermal Expansion Coefficient Material (ppm/K) Piezoelectric Lithium Tantalate (LT) 16.1 Substrate Lithium Niobate (LN) 15.4 Crystal 13.7 Lithium Borate 13 Support Substrate Silicon 3

(9) Hereinafter, a method for producing the composite substrate 10 will be described with reference to FIG. 2. FIG. 2 is a schematic sectional view of steps for producing the composite substrate 10.

(10) First, the first and second substrates 14a and 14b, which are disc-shaped and formed of the same material, are prepared (refer to FIG. 2(a)). The substrates 14a and 14b are directly bonded together to produce the support substrate 14 (refer to FIG. 2(b)). An example of a process of directly bonding together the substrates 14a and 14b is as follows. First, bonding surfaces of the substrates 14a and 14b are washed to remove foreign matter adhering to the bonding surfaces. Subsequently, the bonding surfaces of the substrates 14a and 14b are irradiated with ion beams of an inert gas such as argon, so that remaining impurities (oxide films, adsorbate, and the like) are removed and the bonding surfaces are activated. After that, in a vacuum, the substrates 14a and 14b are bonded together at room temperature. The bonding strength for the substrates 14a and 14b is set to a strength that allows the substrates 14a and 14b to be separated by insertion of a blade having a thickness of 100 m. In order to achieve such a strength, the surface roughness of the bonding surfaces, irradiation time with ion beams, a pressure applied during bonding, and the like are determined on the basis of experiments. In a case where the substrates 14a and 14b are both, for example, silicon substrates, since silicon generally has a bulk strength of 2 to 2.5 J/m.sup.2, the SiSi bonding energy between the substrates 14a and 14b is set to a value lower than the strength, for example, 0.05 to 0.6 J/m.sup.2. A value lower than 0.05 J/m.sup.2 may cause separation during production of elastic wave devices. A value higher than 0.6 J/m.sup.2 may cause hindrance to smooth insertion of a blade.

(11) Subsequently, the support substrate 14 and the piezoelectric substrate 12 are bonded together (refer to FIG. 2(c)). Specifically, the surface of the first substrate 14a of the support substrate 14 is bonded to the back surface of the piezoelectric substrate 12. This bonding may be achieved by direct bonding or bonding via an organic adhesive layer. The direct bonding was described before and hence the description thereof is omitted here. Note that the surface roughness of the bonding surfaces, irradiation time with ion beams, a pressure applied during bonding, and the like are determined such that the bonding strength is equivalent to or higher than a silicon bulk strength of 2 to 2.5 J/m.sup.2. In a case where the bonding via an organic adhesive layer is performed, first, an organic adhesive is uniformly applied to one or both of the surface of the support substrate 14 and the back surface of the piezoelectric substrate 12; and, while the substrates are overlapped, the organic adhesive is solidified to thereby achieve bonding. In this way, the composite substrate 10 is obtained (refer to FIG. 2(d)). The process for direct bonding is not particularly limited to the process described here and may be another process using plasma, neutral atom beams, or the like.

(12) Hereinafter, a method for producing an elastic wave device 30 from the composite substrate 10 will be described with reference to FIG. 3. FIG. 3 is a schematic sectional view of steps for producing the elastic wave device 30.

(13) First, the composite substrate 10 is prepared (refer to FIG. 3(a)). This was described before with reference to FIG. 2 and hence the description thereof is omitted here.

(14) Subsequently, electrodes 31 for elastic wave devices are formed on the surface of the piezoelectric substrate 12 of the composite substrate 10 (refer to FIG. 3(b)). The surface of the piezoelectric substrate 12 is partitioned such that a large number of elastic wave devices are formed. The electrodes 31 for elastic wave devices are formed at positions corresponding to elastic wave devices by photolithographic techniques. As illustrated in FIG. 3(d), each electrode 31 includes IDT electrodes 32 and 34 and reflection electrodes 36.

(15) Subsequently, the second substrate 14b is removed from the first substrate 14a by separation using a blade having a thickness of 100 m (refer to FIG. 3(c)). The surface (separated surface) of the first substrate 14a from which the second substrate 14b has been separated has a sufficiently low surface roughness Ra and hence does not particularly need polishing but may be polished when necessary. The separated surface of the first substrate 14a contains, in addition to elements derived from the material for the first substrate 14a, elements derived from the material for the vacuum chamber used during direct bonding. For example, in a case where the material for the vacuum chamber is stainless steel, the Fe element and the Cr element are contained. The second substrate 14b separated from the first substrate 14a can be recycled for the production of another composite substrate 10.

(16) Finally, dicing along boundaries is carried out to obtain a large number of elastic wave devices 30 (refer to FIG. 3(d)). Upon application of high frequency signals to the IDT electrode 32 on the input side of each elastic wave device 30 produced, an electric field is generated between electrodes and surface acoustic waves are generated and propagate on the piezoelectric substrate 12. The propagated surface acoustic waves can be output as electric signals from the IDT electrode 34, which is disposed on the output side in the propagation direction. That is, the elastic wave device 30 is a surface acoustic wave device.

(17) In the embodiment described above, the support substrate 14 is formed by bonding together the first substrate 14a and the second substrate 14b, which are formed of the same material. Accordingly, the support substrate 14 has a large thickness, compared with a case where the first substrate 14a alone is used as the support substrate 14. As a result, warpage of the composite substrate 10 in response to changes in temperature can be reduced and the strength of the composite substrate 10 can also be increased. After the electrodes 31 for elastic wave devices are formed, the thickness of the support substrate 14 can be easily decreased by removing the second substrate 14b from the first substrate 14a by separation with a blade. This is thus achieved at a low cost, compared with a case where a bulk support substrate having the same thickness as the support substrate 14 is thinned by polishing. As a result, an increase in the production cost of elastic wave devices 30 can be suppressed. The second substrate 14b having been removed can be recycled for producing another composite substrate 10, which also contributes to cost reduction.

(18) Note that the present invention is not limited to the above-described embodiments at all. It is obvious that the present invention can be practiced as various embodiments within the technical scope of the present invention.

EXAMPLES

Example 1

(19) Two silicon substrates having a diameter of 100 mm and a thickness of 250 m were prepared as first and second substrates. Each silicon substrate prepared had two mirror-finished surfaces. Each silicon substrate was washed to remove foreign matter from the surfaces and subsequently introduced into a vacuum chamber formed of stainless steel. The atmosphere within the chamber was adjusted to a vacuum on the order of 110.sup.6 Pa. Within this atmosphere, a surface of each silicon substrate was irradiated with Ar ion beams for 180 sec. Subsequently, the beam-irradiated surfaces of the silicon substrates were overlapped so as to be in contact with each other and a load of 500 kgf was then applied to bond together the silicon substrates. Thus, a support substrate having a total thickness of 500 m was obtained. In addition to this support substrate, a LT substrate having two mirror-finished surfaces, a diameter of 100 mm, and a thickness of 230 m was prepared as a piezoelectric substrate. The LT substrate and the support substrate are washed again and introduced into the vacuum chamber. The atmosphere within the chamber was adjusted to a vacuum on the order of 110.sup.6 Pa. Within this atmosphere, a surface of the LT substrate and a surface of the support substrate (the surface of the first substrate) were irradiated with Ar ion beams for 300 sec. Subsequently, the beam-irradiated surface of the LT substrate and the beam-irradiated surface of the support substrate were overlapped so as to be in contact with each other, and a load of 2000 kgf was then applied to bond together the two substrates. Thus, a composite substrate having a trilayer structure was obtained.

(20) In the composite substrate, the LT substrate serving as the piezoelectric substrate was polished to about 20 m. The amount of warpage of the composite substrate between before and after the polishing was measured and found to be about 25 m. The amount of warpage of the polished composite substrate between before and after heating at 100 C. was measured and found to be about 250 m.

Comparative Example 1

(21) A composite substrate was produced as in Example 1 except that a single silicon substrate having a diameter of 100 mm and a thickness of 250 m was used as the support substrate. The amount of warpage of the composite substrate between before and after the polishing was measured and found to be about 60 m. The amount of warpage between before and after heating at 100 C. was measured and found to be about 1500 m.

(22) Summary of Amounts of Warpage

(23) Table 2 summarizes the results of Example 1 and Comparative Example 1 in terms of amount of warpage. As is obvious from Table 1, the effect of considerably reducing warpage was observed in Example 1 because the support substrate was thicker than that in Comparative Example 1.

(24) TABLE-US-00002 TABLE 2 Amount of Warpage Amount of Warpage between Before and After between Before and After Polishing of LT Substrate Heating at 100 C. Example 1 About 25 m About 250 m Comparative About 60 m About 1500 m Example 1

Example 2

(25) The polished composite substrate in Example 1 was subjected to patterning for electrodes for elastic wave devices (surface acoustic wave devices). A blade was then inserted into the SiSi bonding boundary to divide the bonded substrate. This provided a bilayer composite substrate in which the LT substrate and the Si substrate (first substrate) were bonded together and a monolayer Si substrate (second substrate). The separated surface of the bilayer composite substrate and the separated surface of the monolayer Si substrate were observed with an AFM (atomic force microscope). As a result, the wafers were found to have a surface roughness Ra of about 0.4 nm and the surfaces were in a good condition without the need of polishing. The separated surfaces were subjected to elemental analysis by energy dispersive X-ray spectrometry (EDS). As a result, in addition to the Si element, the Fe element and the Cr element were detected. The Fe element and the Cr element were derived from the vacuum chamber. Entry of these elements occurred during direct bonding between the first substrate and the second substrate.

(26) A crack-opening method was used to measure the bonding energy per unit area. As a result, the bonding energy between the LT substrate and the Si substrate (first substrate) was found to be about 2.5 J/m.sup.2. The bonding energy between the Si substrate (first substrate) and the Si substrate (second substrate) was found to be about 0.2 J/m.sup.2. In general, a silicon bulk strength is 2 to 2.5 J/m.sup.2. The bonding energy between the LT substrate and the Si substrate (first substrate) is equal to or more than the bulk strength. In contrast, the bonding energy between the Si substrate (first substrate) and the Si substrate (second substrate) is lower than the bulk strength, which indicates that the substrates can be separated with a blade. The crack-opening method is a method of inserting a blade at the bonding interface and, on the basis of a distance for which the blade is inserted, the surface energy of the bonding interface is determined. The blade used was a product No. 99077 (blade length: about 37 mm, thickness: 0.1 mm, material: stainless steel) manufactured by FEATHER Safety Razor Co., Ltd.

(27) The present application claims priority from Japanese patent application No. 2013-30161 filed on Feb. 19, 2013, the entire contents of which are incorporated herein by reference.