Method for manufacturing an elastic wave device

Abstract

A method for manufacturing an elastic wave device (30), comprising the following steps: (a) preparing a first substrate (14a) and a second substrate (14b) made of the same material, irradiating bonding surfaces of the first substrate (14a) and the second substrate (14b) with inert gas ion beams, and then bonding the first substrate (14a) and the second substrate (14b) together in vacuum at room temperature to obtain a support substrate (14), such that the bonding strength for the first substrate (14a) and the second substrate (14b) becomes a strength with which the first substrate (14a) and the second substrate (14b) can be separated by inserting a blade having a thickness of 100 µm, and bonding a piezoelectric substrate (12) having a higher thermal expansion coefficient than the material of the first substrate (14a) and the second substrate (14b) to the first substrate (14a) on a side opposite to the bonding surface with the second substrate (14b) to obtain a composite substrate (10); (b) forming electrodes (31) for elastic wave devices on a surface of the piezoelectric substrate (12) of the composite substrate (10); (c) removing the second substrate (14b) from the first substrate (14a) by separation with a blade; and (d) dicing the composite substrate (10) to obtain elastic wave devices (30).

Description

Technical FieldThe invention relates to a method for manufacturing an elastic wave device.Prior ArtIn recent years, for the purpose of improving the temperature characteristic of elastic wave devices, a composite substrate formed by bonding a thin piezoelectric substrate on a supporting substrate having a lower thermal expansion coefficient has been used. For example, DE 10 2006 041 027 A1 discloses a composite substrate having a structure which is formed by bonding together an LT substrate serving as a piezoelectric substrate (LT is the abbreviation for lithium tantalate) and a silicon substrate serving as a carrier substrate via an adhesive layer formed from an epoxy adhesive.DE 10 2010 000 972 A1 likewise proposes a composite substrate having a structure in which a piezoelectric substrate and a silicon substrate serving as a carrier substrate are bonded together via an adhesive layer formed from an epoxy adhesive.The production of elastic wave devices with such a composite substrate has the following problems. First, during manufacturing processes for elastic wave devices at various process temperatures, pattern accuracy deteriorates because the composite substrate largely warp in response to temperature changes, or it becomes difficult to automatically handle the composite substrate. Second, the composite substrate has a low strength and the substrate breaks during a heating process in manufacturing elastic wave devices.In order to solve these problems, it may be considered, for example, to increase the thickness of the supporting substrate in the composite substrate of DE 10 2006 041 027 A1 or DE 10 2010 000 972 A1. If the thickness of the supporting substrate is simply increased, the rigidity is increased and the warpage of the composite substrate is suppressed. In addition, damage due to distortion of the substrate is also reduced.It has also been proposed in U.S. Pat. No. 7,408,286 B1 to form a compensation layer on a surface of the carrier substrate opposite to the other surface of the carrier substrate to which the piezoelectric substrate is bonded, which compensation layer has substantially the same thermal expansion coefficient as the piezoelectric substrate and which compensation layer has a similar thickness as the piezoelectric substrate. In such a composite substrate, the piezoelectric substrate and the compensation layer similarly expand and contract in response to temperature changes, and thus the warpage of the composite substrate is suppressed.Finally, US 2004 / 0 253 795 A1 proposes a method for producing a composite substrate, which comprises the following steps: bonding a first and second substrate, which are both made of quartz, for example, together to form a carrier substrate; bonding a third substrate, which is made of another material, such as silicon, for example, and in which a fracture zone has been incorporated by ion implantation, to a surface of the first substrate; removing a part of the third substrate at the fracture zone by heat treatment or by application of mechanical stress, such that a transfer layer remains on the carrier substrate; and removing the second substrate from the first substrate by separation with a blade.Summary of the InventionWhen the thickness of the supporting substrate in the composite substrate of DE 10 2006 041 027 A1 or DE 10 2010 000 972 A1 is increased to about 500 μm, for example, the above-described problems are solved. However, elastic wave devices made of this composite substrate have an excessively large thickness, which is disadvantageous in the trend of reducing the thickness and leading to a low commercial value. For this reason, the back surfaces of the supporting substrates of the manufactured elastic wave devices must be thinned by polishing. However, polishing involves a high cost, resulting in an increase in the cost of the elastic wave devices. On the other hand, in the composite substrate of U.S. Pat. No. 7,408,286 B1, a compensation layer is added on the rear side of the carrier substrate. Accordingly, the cost of the elastic wave devices made of this composite substrate increases, which is problematic.The present invention has been made to solve these problems. A main object is to achieve a reduction in warpage of a composite substrate and an increase in strength of the composite substrate, and to suppress the corresponding increase in production cost of elastic wave devices.To achieve the above main object, the invention proposes a method of manufacturing an elastic wave device as defined in claim 1In the composite substrate obtained in the method of the present invention, a supporting substrate is formed by directly bonding a first substrate and a second substrate together with a strength allowing separation with a blade, the first and second substrates being formed of the same material. Accordingly, the support substrate has a large thickness compared to the case where the first substrate alone is used as the support substrate. As a result, distortion of the composite substrate in response to temperature changes can be reduced, and the strength of the composite substrate can also be increased. After the electrodes for elastic wave devices are formed, the thickness of the supporting substrate can be easily reduced by removing the second substrate from the first substrate by separation with a blade. This is thus achieved at a low cost as compared with the case where a thick supporting substrate is thinned by polishing. As a result, the corresponding increase in manufacturing cost of elastic wave devices can be suppressed. The second substrate which has been removed can be reused for the production of other composite substrates, which also contributes to a cost reduction.Brief Description of the DrawingsFIG. 1 is a schematic sectional view of a composite substrate 10. FIG. 2 is a schematic sectional view of steps for manufacturing a composite substrate 10. FIG. 3 is a schematic sectional view of steps for manufacturing an elastic wave device 30.DESCRIPTION OF EMBODIMENTSExemplary embodiments according to the invention are described below with reference to the drawings. FIG. 1 is a schematic sectional view of a composite substrate 10.The piezoelectric substrate 12 is a substrate capable of transmitting elastic waves. The material for the piezoelectric substrate 12 may be lithium tantalate (LT), lithium niobate (LN), a lithium niobate-lithium tantalate mixed single crystal, mountain crystal, lithium borate, zinc oxide, aluminum nitride, langasite (LGS), langasate (LGT), or the like. Of these, LT or LN is preferred. This is because LT and LN allow surface acoustic waves to propagate at high speeds and have large electromechanical coupling factors and are thus suitable for elastic wave devices for high frequencies and wide band frequencies. The piezoelectric substrate 12 is not particularly limited in size, and may have, for example, a diameter of 50 to 150 mm and a thickness of 0.2 to 50 μm.The supporting 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 supporting substrate 14 has a lower thermal expansion coefficient than the piezoelectric substrate 12, so that variations in size of the piezoelectric substrate 12 in response to temperature changes can be suppressed, and temperature-dependent changes in the frequency characteristic of the composite substrate 10 serving as elastic wave devices can be suppressed. This support substrate 14 is formed by directly bonding a first substrate 14 aand a second substrate 14 bhaving strength allowing separation with a blade, the first and second substrates 14 aand 14 bbeing formed of the same material. The support substrate 14 is bonded to the piezoelectric substrate 12 via a surface of the first substrate 14 athat is opposite to the other surface of the first substrate 14 ato which the second substrate 14 bis bonded. The material for the support substrate 14 may be silicon, sapphire, aluminum nitride, aluminum oxide, borosilicate glass, fused silica, or the like, and is preferably silicon. The support substrate 14 has a size of, for example, 50 to 150 mm in diameter and 200 to 1200 μm in thickness. The first and second substrates 14 aand 14 bhave a size of, for example, 50 to 150 mm in diameter and 100 to 600 μm in thickness. The supporting substrate 14 preferably has a higher modulus of elasticity than the piezoelectric substrate 12.Table 1 gives the thermal expansion coefficients of representative materials used for the piezoelectric substrate 12 and the supporting substrate 14. Table 1 Table 1Piezoelectric substrateLithium tantalate (LT)16,1Lithium niobate (LN)15,4Crystal13,7Lithium borate13Carrier substrateSilicon Silicon3Next, a method for manufacturing the composite substrate 10 will be described with reference to FIG. 2. FIG. 2 is a schematic sectional view of steps for manufacturing the composite substrate 10.First, the first and second substrates 14 aand 14 bare prepared (see FIG. 2( a) ) which are disk-shaped and formed of the same material. The substrates 14 aand 14 bare directly bonded together to produce the carrier substrate 14 (see FIG. 2( b)). An example of the process for directly bonding the substrates 14 aand 14 btogether is as follows. First, bonding surfaces of the substrates 14 aand 14 bare washed to remove foreign substances adhering to the bonding surfaces. Subsequently, the bonding surfaces of the substrates 14 aand 14 bare irradiated with ion beams of an inert gas such as argon, so that residual impurities (oxide films, adsorbate, and the like) are removed and the bonding surfaces are activated. Thereafter, the substrates 14a and 14b are bonded together in vacuum at room temperature. The bonding strength for the substrates 14 aand 14 bis set to a strength that allows the substrates 14 aand 14 bto be separated by inserting a blade having a thickness of 100 μm. In order to achieve such strength, the surface roughness of the bonding surfaces, the irradiation time with ion beams, a pressure applied during bonding, and the like are determined based on experiments. In the case where the substrates 14 aand 14 bare both silicon substrates, for example, since the silicon generally has a bulk strength of 2 to 2.5 J / m 2 the Si-Si bonding energy between the substrates 14 aand 14 bis set to be lower than this strength, for example, 0.05 to 0.6 J / m 2. A value of less than 0.05 J / m 2 may result in separation during manufacture of elastic wave devices. A value of more than 0.6 J / m 2 may impede the smooth insertion of the blade.Subsequently, the supporting substrate 14 and the piezoelectric substrate 12 are bonded together (see FIG. 2( c)). Namely, the surface of the first substrate 14 aof the supporting substrate 14 is bonded to the back surface of the piezoelectric substrate 12. This bonding may be achieved by direct bonding or by bonding via an organic adhesive layer. The direct bonding has been described before, and therefore, the description thereof will be omitted here. Note that the surface roughness of the bonding surfaces, the irradiation time with ion beams, a pressure applied during bonding, and the like are set such that the bonding strength is similarly high or higher than the bulk silicon strength of 2 to 2.5 J / m 2. In the case where bonding is performed via an organic adhesive layer, first, an organic adhesive is uniformly applied to either the surface of the supporting substrate 14 or the back surface of the piezoelectric substrate 12 or both surfaces; and while the substrates overlap, the organic adhesive is allowed to solidify, thereby achieving bonding. In this way, the composite substrate 10 is obtained (see FIG. 2( d)). The process for the direct bonding is not particularly limited to the process described herein, and may be another process using plasma, neutral atomic beams, or the like.Next, with reference to FIG. 3, a method of manufacturing an elastic wave device 30 from the composite substrate 10 will be described. FIG. 3 is a schematic sectional view of steps for manufacturing the elastic wave device 30.First, the composite substrate 10 is prepared (see FIG. 3( a)). This has been described above with reference to FIG. 2, and therefore, the description thereof will be omitted here.Subsequently, electrodes 31 for elastic wave devices are formed on the surface of the piezoelectric substrate 12 of the composite substrate 10 (see FIG. 3( b)). The surface of the piezoelectric substrate 12 is divided so as to form a large number of elastic wave devices. The electrodes 31 for elastic wave devices are formed by photolithography techniques at locations corresponding to the elastic wave devices. As shown in FIG. 3( d), each electrode 31 includes IDT electrodes 32 and 34 and reflection electrodes 36.Subsequently, the second substrate 14 bis removed from the first substrate 14 aby separation with a blade having a thickness of 100 μm (see FIG. 3( c)). The surface (separation surface) of the first substrate 14 afrom which the second substrate 14 bhas been separated has a sufficiently low surface roughness Ra and thus does not necessarily need to be polished, but may be polished as needed. The separation surface of the first substrate 14 aincludes, in addition to the elements derived from the material for the first substrate 14 a, elements derived from the material for the vacuum chamber used during the direct bonding. In the case where the material for the vacuum chamber is stainless steel, for example, the elements Fe and Cr are contained. The second substrate 14 bseparated from the first substrate 14 acan be reused for the production of a further composite substrate 10.Finally, dicing is performed along the boundaries to achieve a large number of elastic wave devices 30 (see FIG. 3( d)). When high frequency signals are applied to the IDT electrode 32 on the input side of each manufactured elastic wave device 30, an electric field is generated between the electrodes, and surface acoustic waves are generated and propagate on the piezoelectric substrate 12. The propagating surface acoustic waves may be output from the IDT electrode 34 disposed on the output side in the propagating direction as electrical signals. That is, the elastic wave device 30 is a surface acoustic wave device.In the above-described embodiment, the support substrate 14 is formed by bonding the first substrate 14 aand the second substrate 14 bformed of the same material together. Accordingly, the support substrate 14 has a large thickness compared to the case where the first substrate 14 ais used as the support substrate 14 alone. As a result, distortion of the composite substrate 10 in response to temperature changes 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 reduced by removing the second substrate 14 bfrom the first substrate 14 aby cutting with a blade. This will thus be achieved at low cost compared with the case where a bulk carrier substrate having the same thickness as the carrier substrate 14 is thinned by polishing. As a result, an increase in manufacturing cost of elastic wave devices 30 can be suppressed. The second substrate 14b which has been removed can be reused for the production of another composite substrate 10, which also contributes to a cost reduction.It should be noted that the invention is by no means limited to the above-described embodiments. It is obvious that the invention can be practiced in the form of various embodiments within the technical scope of the invention.ExamplesExample 1As the first and second substrates, two silicon substrates having a diameter of 100 mm and a thickness of 250 μm were prepared. Each prepared silicon substrate had two mirror-finished surfaces. Each silicon substrate was washed to remove foreign matter from the surfaces, and then introduced into a vacuum chamber formed of stainless steel. The atmosphere inside the chamber was set to a vacuum on the order of 1×10 -6 Pa. Within this atmosphere, an Ar ion beam was irradiated to a surface of each silicon substrate for 180 seconds. Subsequently, the irradiated surfaces of the silicon substrates were overlapped to be in contact with each other, and then a load of 500 kgf was applied to bond the silicon substrates together. Thus, a support substrate having a total thickness of 500 μm was obtained. In addition to this supporting substrate, as the piezoelectric substrate, an LT substrate having two mirror-finished surfaces, a diameter of 100 mm, and a thickness of 230 μm was prepared. The LT substrate and the support substrate were washed again and introduced into the vacuum chamber. The atmosphere inside the vacuum chamber was set to a vacuum on the order of 1×10 -6 Pa. Within this atmosphere, an Ar ion beam was irradiated to a surface of the LT substrate and a surface of the support substrate (the surface of the first substrate) for 300 seconds. Subsequently, the irradiated surface of the LT substrate and the irradiated surface of the supporting substrate were overlapped to be in contact with each other, and then a load of 2000 kgf was applied to bond the two substrates together. Thus, a composite substrate having a three-layer structure was obtained.In the composite substrate, the LT substrate serving as the piezoelectric substrate was polished to about 20 μm. The distortion of the composite substrate between before and after polishing was measured and found to be about 25 μm. The warpage of the polished composite substrate between before and after heating at 100° C. was measured, and found to be about 250 μm.Comparative Example 1A composite substrate was prepared as in Example 1, except that a single silicon substrate having a diameter of 100 μm and a thickness of 250 μm was used as a supporting substrate. The distortion of the composite substrate between before and after polishing was measured and found to be about 60 μm. The warpage between before and after heating at 100° C. was measured and found to be about 1500 μm.Summary WarpageTable 2 summarizes the results of Example 1 and Comparative Example 1 in terms of warpage. As is apparent from Table 1, in Example 1, the effect of significantly decreasing the warpage was observed because the support substrate was thicker than in Comparative Example 1. Table 2 Table 2Example 1about 25 μmabout 250 μmComparative Example 2about 60 μmabout 1500 μmExample 2The polished composite substrate in Example 1 was subjected to patterning for electrodes for elastic wave devices (surface acoustic wave devices). Then, a blade was inserted into the Si-Si bonding boundary to divide the bonded substrate. This provided a two-layer composite substrate in which the LT substrate and the Si substrate (first substrate) were bonded together, and a single-layer Si substrate (second substrate). The separation surface of the two-layer composite substrate and the separation surface of the single-layer Si substrate were observed with an AFM (Scanning Force Microscope). As a result, it was found that the wafers had a surface roughness Ra of about 0.4 nm and that the surfaces were in a good state without the need for polishing. The separation surfaces were subjected to elemental analysis by energy dispersive X-ray spectroscopy (EDS). As a result, in addition to the element Si, the elements Fe and Cr were detected. The elements Fe and Cr were from the vacuum chamber. The entry of these elements took place during the direct bonding between the first substrate and the second substrate.A crack opening method was used to measure the bonding energy per unit area. As a result, it was found that the bonding energy between the LT substrate and the Si substrate (first substrate) was about 2.5 J / m 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 2. Generally, the bulk silicon strength is 2 to 2.5 J / m 2. The bonding energy between the LT substrate and the Si substrate (first substrate) is equal to or higher 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, indicating that the substrates can be separated with a blade. The crack opening method is a method in which a blade is inserted at the bonding interface, and the surface energy of the bonding interface is determined based on a distance along which the blade is inserted. The blade used was No. 99077 (blade length: about 37 mm, thickness: 0.1 mm, material: stainless steel) manufactured by FEATHER Safety Razor Co., Ltd.Industrial applicabilityThe invention can be applied to acoustic wave devices such as SAW filters.List of reference charactersComposite substrate 10, piezoelectric substrate 12, support substrate 14, first substrate 14 a, second substrate 14 b, elastic wave device (surface acoustic wave device) 30, electrode 31, IDT electrodes 32 and 34, and reflection electrode 36.

Claims

A method for manufacturing an elastic wave device (30), comprising the steps of: (a) preparing a first substrate (14a) and a second substrate (14b) made of the same material, irradiating bonding surfaces of the first substrate (14a) and the second substrate (14b) with ion beams of inert gas, and then bonding the first substrate (14a) and the second substrate (14b) together in vacuum at room temperature to obtain a support substrate (14) so that the bonding strength for the first substrate (14a) and the second substrate (14b) becomes a strength with which the first substrate (14a) and the second substrate (14b) can be separated by inserting a blade having a thickness of 100 μm, bonding a piezoelectric substrate (12) having a higher thermal expansion coefficient than the material of the first substrate (14a) and the second substrate (14b) to the first substrate (14a) on a side opposite to the bonding surface with the second substrate (14b) to obtain a composite substrate (10); (b) forming elastic wave device electrodes (31) on a surface of the piezoelectric substrate (12) of the composite substrate (10); (c) removing the second substrate (14b) from the first substrate (14a) by separation with a blade; and (d) dicing the composite substrate (10) to obtain elastic wave devices (30).The method for manufacturing an elastic wave device (30) according to claim 1, wherein in the step (a), after obtaining the support substrate (14), the support substrate (14) and the piezoelectric substrate (12) are bonded together directly or with resin.

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