Laminate and SAW device

Abstract

To provide a laminate in which a piezoelectric substrate and a ceramic substrate (base substrate) are bonded together with sufficient bonding force.SOLUTION: A laminate has a ceramic substrate 10 composed of polycrystalline ceramic and having a supporting principal surface 11 and a piezoelectric substrate 20, coupled by van der Waals forces at the coupling principal surface 22 with respect to the supporting principal surface 11 and comprising a piezoelectric material. The ceramic substrate 10 includes a supporting principal surface amorphous layer 19 formed to include the supporting principal surface 11. The piezoelectric substrate 20 includes a coupling principal surface amorphous layer 29 that is formed to include the coupling principal main surface 22. The thickness t1 of the supporting principal surface amorphous layer 19 is smaller than the thickness t2 of the coupling principal surface amorphous layer 29.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a laminate and a SAW device.

[0002] This application claims priority from Japanese Application No. 2017-060774, filed on March 27, 2017, and incorporates by reference all of the contents of the above-mentioned Japanese application. [Background technology]

[0003] SAW devices (Surface Acoustic Wave Devices) are placed inside communication devices such as mobile phones to remove noise contained in electrical signals. SAW devices have the function of extracting only electrical signals of a desired frequency from the input electrical signals. SAW devices have a structure in which electrodes are formed on a piezoelectric substrate. Furthermore, for the purpose of heat dissipation during use, the piezoelectric substrate is placed on a base substrate made of a highly heat-dissipating material.

[0004] The base substrate can be made of, for example, single crystal sapphire. However, using a single crystal sapphire substrate as the base substrate raises the problem of increased manufacturing costs for SAW devices. In response to this problem, a SAW device has been proposed that uses a ceramic substrate made of polycrystalline spinel as the base substrate, and has a structure in which a piezoelectric substrate and a ceramic substrate with reduced surface roughness Ra (arithmetic mean roughness) are bonded together by van der Waals forces. This reduces the manufacturing costs of SAW devices (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2011-66818 Summary of the Invention

[0006] The laminate of the present disclosure includes a ceramic substrate made of polycrystalline ceramic and having a main support surface, and a piezoelectric substrate made of a piezoelectric material and bonded to the main support surface at a main bonding surface by van der Waals forces. The ceramic substrate includes a main support surface amorphous layer formed to include the main support surface. The piezoelectric substrate includes a main bonding surface amorphous layer formed to include the main bonding surface. The thickness of the main support surface amorphous layer is smaller than the thickness of the main bonding surface amorphous layer. [Brief explanation of the drawings]

[0007] [Figure 1] 1 is a schematic cross-sectional view showing the structure of a laminate including a ceramic substrate and a piezoelectric substrate. [Figure 2] FIG. 2 is a schematic cross-sectional view showing a structure in the vicinity of the interface between a ceramic substrate and a piezoelectric substrate. [Figure 3] 1 is a flowchart showing an outline of a method for manufacturing a laminate and a SAW device. [Figure 4] 5A to 5C are schematic cross-sectional views for explaining a method for manufacturing a laminate and a SAW device. [Figure 5] 5A to 5C are schematic cross-sectional views for explaining a method for manufacturing a laminate and a SAW device. [Figure 6] 5A to 5C are schematic cross-sectional views for explaining a method for manufacturing a laminate and a SAW device. [Figure 7] 5A to 5C are schematic diagrams for explaining a method for manufacturing a laminate and a SAW device. [Figure 8] 1 is a schematic diagram showing the structure of a SAW device. DETAILED DESCRIPTION OF THE INVENTION

[0008] [Problem to be solved by this disclosure] In conventional laminates, the bonding strength between the piezoelectric substrate and the ceramic substrate may be insufficient. Therefore, one of the objects of the present disclosure is to provide a laminate in which the piezoelectric substrate and the ceramic substrate are bonded with sufficient bonding strength, and a SAW device including the laminate.

[0009] [Effects of this disclosure] According to the laminate of the present disclosure, it is possible to provide a laminate in which a piezoelectric substrate and a ceramic substrate are bonded with sufficient bonding strength.

[0010] [Description of the embodiments of the present disclosure] First, embodiments of the present invention will be described. The laminate of the present application includes a ceramic substrate made of polycrystalline ceramic and having a main support surface, and a piezoelectric substrate made of a piezoelectric material and bonded to the main support surface at a bonding main surface by van der Waals forces. The ceramic substrate includes an amorphous layer on a main support surface formed to include the main support surface. The piezoelectric substrate includes an amorphous layer on a bonding main surface formed to include the bonding main surface. The thickness of the amorphous layer on the main support surface is smaller than the thickness of the amorphous layer on the bonding main surface.

[0011] In the laminate of the present application, an amorphous layer having a smaller thickness than that of the piezoelectric substrate is formed on the ceramic substrate side, and the main supporting surface of the ceramic substrate and the main bonding surface of the piezoelectric substrate are bonded together by van der Waals forces.

[0012] According to the inventors' investigations, the bonding strength due to van der Waals forces can be increased by reducing the thickness of the amorphous layer formed on the polycrystalline ceramic substrate side compared to the piezoelectric substrate side made of a single-crystal piezoelectric material. This can be attributed to the following reasons, for example: Numerous crystal planes with different crystal orientations are exposed on the main surface of a polycrystalline ceramic substrate. The characteristics of the crystal planes vary depending on the crystal orientation. Therefore, forming an amorphous layer on the main surface of the ceramic substrate that is thicker than the amorphous layer on the piezoelectric substrate side increases the variation in characteristics within the main surface of the ceramic substrate, thereby reducing the bonding strength with the piezoelectric substrate. By reducing the thickness of the amorphous layer on the ceramic substrate side compared to the amorphous layer on the piezoelectric substrate side, the bonding strength due to van der Waals forces can be improved. Thus, the laminate of the present application can provide a laminate in which the piezoelectric substrate and the ceramic substrate are bonded with sufficient bonding strength.

[0013] In the above laminate, the thickness of the amorphous layer on the support main surface may be 0.3 nm or more and 3.0 nm or less. By making the thickness of the amorphous layer on the support main surface 0.3 nm or more, it becomes easier to form a uniform amorphous layer on the support main surface. By making the thickness of the amorphous layer on the support main surface 3.0 nm or less, it becomes easier to ensure the flatness of the support main surface. From the viewpoint of more reliably forming a uniform amorphous layer, it is more preferable that the thickness of the amorphous layer on the support main surface be 0.5 nm or more. From the viewpoint of more reliably ensuring the flatness of the support main surface, it is more preferable that the thickness of the amorphous layer on the support main surface be 2.0 nm or less.

[0014] In the above laminate, the thickness of the bonding principal surface amorphous layer may be 0.5 nm or more and 5.0 nm or less. By making the thickness of the bonding principal surface amorphous layer 0.5 nm or more, it becomes easier to form a uniform amorphous layer on the bonding principal surface. By making the thickness of the bonding principal surface amorphous layer 5.0 nm or less, it becomes easier to ensure the flatness of the bonding principal surface. From the viewpoint of more reliably forming a uniform amorphous layer, it is more preferable that the thickness of the bonding principal surface amorphous layer be 1.2 nm or more. From the viewpoint of more reliably ensuring the flatness of the bonding principal surface, it is more preferable that the thickness of the bonding principal surface amorphous layer be 3.0 nm or less.

[0015] In the laminate, the ceramic substrate may be made of one or more materials selected from the group consisting of spinel (MgAlO), alumina (AlO), magnesia (MgO), silica (SiO), mullite (3AlO·2SiO), cordierite (2MgO·2AlO·5SiO), calcia (CaO), titania (TiO), silicon nitride (SiN), aluminum nitride (AlN), and silicon carbide (SiC). These materials are suitable for use in the ceramic substrate of the present invention.

[0016] In the laminate, the piezoelectric substrate may be made of lithium tantalate (LiTaO3) or lithium niobate (LiNbO3), which are suitable materials for forming the piezoelectric substrate of the present invention.

[0017] The SAW device of the present application comprises the laminate of the present application and an electrode formed on the main surface of the piezoelectric substrate opposite to the ceramic substrate.

[0018] The SAW device of the present invention includes the laminate of the present invention in which a piezoelectric substrate and a ceramic substrate made of polycrystalline ceramic are bonded with sufficient bonding strength, thereby making it possible to provide a SAW device in which the piezoelectric substrate and the ceramic substrate are bonded with sufficient bonding strength.

[0019] [Details of the embodiments of the present disclosure] Next, one embodiment of the laminate according to the present invention will be described below with reference to the drawings. In the following drawings, the same or corresponding parts are designated by the same reference numerals, and description thereof will not be repeated.

[0020] 1, laminate 1 in the present embodiment includes base substrate 10 as a ceramic substrate and piezoelectric substrate 20. Piezoelectric substrate 20 is made of a single crystal piezoelectric material such as single crystal lithium tantalate or single crystal lithium niobate. Base substrate 10 is made of polycrystalline ceramic made of one or more materials, preferably any one material, selected from the group consisting of spinel, alumina, magnesia, silica, mullite, cordierite, calcia, titania, silicon nitride, aluminum nitride, and silicon carbide.

[0021] The base substrate 10 has a supporting main surface 11. The piezoelectric substrate 20 has an exposed main surface 21, which is one of its main surfaces, and a bonding main surface 22, which is the main surface opposite to the exposed main surface 21. The piezoelectric substrate 20 is disposed so that the bonding main surface 22 contacts the supporting main surface 11 of the base substrate 10. The base substrate 10 and the piezoelectric substrate 20 are bonded by van der Waals forces.

[0022] 2, the base substrate 10 includes a supporting principal surface amorphous layer 19 formed to include the supporting principal surface 11. The piezoelectric substrate 20 includes a bonding principal surface amorphous layer 29 formed to include the bonding principal surface 22. The thickness t1 of the supporting principal surface amorphous layer 19 is smaller than the thickness t2 of the bonding principal surface amorphous layer 29.

[0023] In the laminate 1, the thickness t1 of the supporting principal surface amorphous layer 19, which is the amorphous layer on the base substrate 10 side, is smaller than the thickness t2 of the bonding principal surface amorphous layer 29, which is the amorphous layer on the piezoelectric substrate 20 side. As a result, in the laminate 1, the bonding strength between the base substrate 10 and the piezoelectric substrate 20 due to van der Waals forces is improved.

[0024] The thickness t1 of the support main surface amorphous layer 19 is preferably 0.3 nm or more and 3.0 nm or less. By setting the thickness t1 of the support main surface amorphous layer 19 to 0.3 nm or more, it becomes easy to form a uniform amorphous layer on the support main surface 11. By setting the thickness t1 of the support main surface amorphous layer 19 to 3.0 nm or less, it becomes easy to ensure the flatness of the support main surface 11.

[0025] The thickness t2 of the bonding principal surface amorphous layer 29 is preferably 0.5 nm or more and 5.0 nm or less. By setting the thickness t2 of the bonding principal surface amorphous layer 29 to 0.5 nm or more, it becomes easy to form a uniform amorphous layer on the bonding principal surface 22. By setting the thickness t2 of the bonding principal surface amorphous layer 29 to 5.0 nm or less, it becomes easy to ensure the flatness of the bonding principal surface 22.

[0026] Next, a method for manufacturing the laminate 1 and a SAW device using the laminate 1 according to this embodiment will be described. Referring to FIG. 3, the method for manufacturing the laminate 1 and the SAW device according to this embodiment first performs a substrate preparation step (S10). In this step (S10), referring to FIG. 4, a base substrate 10 is prepared, which is made of polycrystalline ceramic composed of one or more materials selected from the group consisting of spinel, alumina, magnesia, silica, mullite, cordierite, calcia, titania, silicon nitride, aluminum nitride, and silicon carbide. For example, a base substrate 10 is prepared which is made of polycrystalline ceramic composed of any one material selected from the above group. Specifically, when preparing a base substrate 10 made of polycrystalline spinel, for example, magnesia powder and alumina powder are mixed to prepare a raw material powder, which is then molded to produce a green body. The green body can be produced, for example, by performing preforming by press molding followed by cold isostatic pressing (CIP). Next, the green body is sintered. The sintering process can be performed by, for example, vacuum sintering, HIP (Hot Isostatic Press), or the like. This results in a sintered body made of polycrystalline spinel. The sintered body is then diced to obtain base substrate 10 having the desired shape (thickness) (see FIG. 4).

[0027] Furthermore, in step (S10), a piezoelectric substrate 20 made of a single crystal piezoelectric material such as single crystal lithium tantalate or single crystal lithium niobate is prepared, as shown in Fig. 4. Piezoelectric substrate 20 is prepared, for example, by slicing a single crystal of lithium tantalate or lithium niobate (see Fig. 4).

[0028] Next, a polishing step is performed as step (S20). In this step (S20), referring to Fig. 4, polishing is performed on main supporting surface 11 of base substrate 10 and main bonding surface 22 of piezoelectric substrate 20 prepared in step (S10). The polishing step includes, for example, rough polishing, normal polishing, and finish polishing.

[0029] Next, an amorphous layer forming step is carried out as step (S30). In this step (S30), a supporting principal surface amorphous layer 19 and a bonding principal surface amorphous layer 29 are formed on the base substrate 10 and the piezoelectric substrate 20 that have been polished in step (S20), respectively. Specifically, with reference to FIG. 4, for example, the base substrate 10 and the piezoelectric substrate 20 are washed and dried, and then inserted into a chamber, and the pressure inside the chamber is reduced. The pressure inside the chamber is, for example, 10 -6 The pressure is set to about 10 Pa. Then, as shown by the arrows in Figure 4, an Ar (argon) beam, for example, is irradiated onto support main surface 11 and bonding main surface 22. This disrupts the atomic arrangement in the vicinity of support main surface 11 and bonding main surface 22, forming support main surface amorphous layer 19 and bonding main surface amorphous layer 29. In this embodiment, the Ar beam is irradiated so that thickness t1 of support main surface amorphous layer 19 is smaller than thickness t2 of bonding main surface amorphous layer 29.

[0030] Next, a bonding step is performed as step (S40). In this step (S40), the base substrate 10, on whose main surface an amorphous layer has been formed in step (S30), and the piezoelectric substrate 20 are bonded together. Specifically, with reference to FIGS. 4 and 1, the base substrate 10 and the piezoelectric substrate 20 are bonded together so that the main bonding surface 22 of the piezoelectric substrate 20 and the main supporting surface 11 of the base substrate 10 are in contact with each other. This bonds the base substrate 10 and the piezoelectric substrate 20 together by van der Waals forces. As a result, the laminate 1 of the present embodiment is obtained.

[0031] In this embodiment, the thickness t1 of the supporting principal surface amorphous layer 19 is smaller than the thickness t2 of the bonding principal surface amorphous layer 29. As a result, the method for manufacturing the laminate 1 described above manufactures the laminate 1 in which the piezoelectric substrate 20 and the base substrate 10 are bonded with sufficient bonding strength.

[0032] Next, a method for manufacturing a SAW device using the laminate 1 will be described. Referring to FIG. 3, following step (S40), a thickness reduction step is performed as step (S50). In this step (S50), referring to FIGS. 1 and 5, processing is performed to reduce the thickness of the piezoelectric substrate 20 of the laminate 1 obtained in step (S40). Specifically, for example, a grinding process is performed on the exposed main surface 21 of the piezoelectric substrate 20. This reduces the thickness of the piezoelectric substrate 20 to a thickness suitable for a SAW device.

[0033] Next, an electrode forming step is performed as step (S60). In step (S60), referring to FIGS. 5 to 7, comb-shaped electrodes are formed on the exposed main surface 21 of the piezoelectric substrate 20. FIG. 6 is a cross-sectional view taken along line VI-VI in FIG. 7. Specifically, referring to FIGS. 6 and 7, a conductive film made of a conductive material such as Al is formed on the exposed main surface 21 of the piezoelectric substrate 20, the thickness of which has been adjusted to an appropriate value in step (S50). The conductive film can be formed by, for example, sputtering. After that, a resist is applied to the conductive film to form a resist film, which is then exposed and developed to form openings in regions other than those corresponding to the desired shapes of the input electrode 30 and the output electrode 40. Then, using the resist film with the openings as a mask, for example, wet etching is performed to form a plurality of pairs of input electrode 30 and output electrode 40, as shown in FIGS. 6 and 7. Note that FIGS. 6 and 7 show the regions corresponding to the pairs of input electrode 30 and output electrode 40. The electrode spacing of the comb-tooth electrodes in the input electrode 30 and the output electrode 40 can be determined appropriately depending on the frequency of the signal to be output.

[0034] Next, a chipping step is carried out as step (S70). In this step (S70), the laminate 1, in which a plurality of pairs of input electrodes 30 and output electrodes 40 are formed, is cut in the thickness direction to separate it into a plurality of chips, each including a pair of input electrodes 30 and output electrodes 40.

[0035] Thereafter, referring to FIGS. 7 and 8, input wiring 51 and output wiring 61 are formed on the chip fabricated in step (S70), thereby completing SAW device 100 (SAW filter) in the first embodiment.

[0036] Referring to FIG. 8, the SAW device 100 of this embodiment comprises a laminate 1 including a base substrate 10 and a piezoelectric substrate 20 bonded by van der Waals forces, an input electrode 30 and an output electrode 40 which are a pair of comb-shaped electrodes formed on the exposed main surface 21 of the piezoelectric substrate 20 so as to be in contact with each other, an input wiring 51 connected to the input electrode 30, and an output wiring 61 connected to the output electrode 40.

[0037] The input electrode 30 includes a first portion 31 and a second portion 32. The first portion 31 includes a linear base portion 31A and a plurality of linear protrusions 31B protruding from the base portion 31A in a direction perpendicular to the extension direction of the base portion 31A. The second portion 32 includes a linear base portion 32A extending parallel to the base portion 31A and a plurality of linear protrusions 32B protruding from the base portion 32A in a direction perpendicular to the extension direction of the base portion 32A and extending between adjacent protrusions 31B. The protrusions 31B and the protrusions 32B are arranged at a predetermined interval.

[0038] The output electrode 40 includes a first portion 41 and a second portion 42. The first portion 41 includes a linear base portion 41A and a plurality of linear protrusions 41B protruding from the base portion 41A in a direction perpendicular to the extension direction of the base portion 41A. The second portion 42 includes a linear base portion 42A extending parallel to the base portion 41A and a plurality of linear protrusions 42B protruding from the base portion 42A in a direction perpendicular to the extension direction of the base portion 42A and extending between adjacent protrusions 41B. The protrusions 41B and the protrusions 42B are arranged at a predetermined interval.

[0039] When an AC voltage, which is an input signal, is applied from the input wiring 51 to the input electrode 30, a surface acoustic wave is generated on the exposed principal surface 21 (surface) of the piezoelectric substrate 20 due to the piezoelectric effect and transmitted to the output electrode 40. The input electrode 30 and the output electrode 40 have a comb-like shape, as shown in FIG. 1 , with the spacing between the protrusions 31B and 32B and the spacing between the protrusions 41B and 42B being constant. Therefore, in the direction from the input electrode 30 to the output electrode 40, the electrode-formed regions of the exposed principal surface 21 of the piezoelectric substrate 20 are spaced at a predetermined period (electrode period). Therefore, the surface acoustic wave generated by the input signal is excited most strongly when its wavelength matches the electrode period, and is attenuated as the wavelength deviates from the electrode period. As a result, only signals with wavelengths close to the electrode period are output via the output electrode 40 and the output wiring 61.

[0040] Here, the temperature of the piezoelectric substrate 20 rises during the above operation. In the SAW device 100 of this embodiment, the base substrate 10 made of a material with high heat dissipation properties is arranged so as to be in contact with the piezoelectric substrate 20. Therefore, the SAW device 100 has high reliability. Furthermore, in the SAW device 100 of this embodiment, the piezoelectric substrate 20 and the base substrate 10 are bonded with sufficient bonding strength. Therefore, the SAW device 100 has high reliability. [Example]

[0041] A laminate 1 was fabricated by carrying out steps (S10) to (S40) of the above embodiment, and an experiment was conducted to confirm the bonding strength between the base substrate 10 (ceramic substrate) and the piezoelectric substrate 20. Specifically, in step (S10), a base substrate 10 made of polycrystalline spinel and a piezoelectric substrate 20 made of single-crystal lithium tantalate were prepared, and steps (S20) to (S40) were carried out to fabricate the laminate 1. Two types of laminate 1 (sample A and sample B) were fabricated by changing the Ar beam irradiation conditions in step (S30).

[0042] The bond strength between the base substrate 10 and the piezoelectric substrate 20 was confirmed for each of Sample A and Sample B by a crack opening method. Furthermore, for each of Sample A and Sample B, the laminate 1 was cut in the thickness direction of the substrate, and the vicinity of the bonded interface was observed using a STEM (Scanning Transmission Electron Microscope) to measure the thicknesses of the support principal surface amorphous layer 19 and the bonding principal surface amorphous layer 29. The thicknesses of the support principal surface amorphous layer 19 and the bonding principal surface amorphous layer 29 were measured for five fields of view for each of Sample A and Sample B. The experimental results are shown in Tables 1 and 2.

[0043] [Table 1]

[0044] [Table 2]

[0045] Tables 1 and 2 show the experimental results for Sample A and Sample B, respectively. Referring to Tables 1 and 2, in both Sample A and Sample B, the thickness of the support principal surface amorphous layer 19 was smaller than the thickness of the bonding principal surface amorphous layer 29. More specifically, the thickness of the support principal surface amorphous layer 19 was less than half the thickness of the bonding principal surface amorphous layer 29. Furthermore, in both Sample A and Sample B, the thickness of the support principal surface amorphous layer was 0.3 nm or more and 3.0 nm or less, and the thickness of the bonding principal surface amorphous layer was 0.5 nm or more and 5.0 nm or less. Furthermore, an investigation of the bonding strength confirmed that both Sample A and Sample B had sufficient bonding strength. On the other hand, a separately prepared sample in which the thickness of the support principal surface amorphous layer 19 was thicker than the thickness of the bonding principal surface amorphous layer 29 did not achieve sufficient bonding strength. The above experimental results confirm that the laminate of the present invention can provide a laminate in which the piezoelectric substrate and the ceramic substrate (base substrate) are bonded with sufficient bonding strength.

[0046] It should be understood that the embodiments and examples disclosed herein are illustrative in all respects and are not limiting in any respect. The scope of the present invention is defined not by the above description but by the scope of the claims, and it is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0047] 1 laminate, 10 base substrate, 11 supporting main surface, 19 supporting main surface amorphous layer, 20 piezoelectric substrate, 21 exposed main surface, 22 bonding main surface, 29 bonding main surface amorphous layer, 30 input side electrode, 31 first portion, 31A base portion, 31B protruding portion, 32 second portion, 32A base portion, 32B protruding portion, 40 output side electrode, 41 first portion, 41A base portion, 41B protruding portion, 42 second portion, 42A base portion, 42B protruding portion, 51 input side wiring, 61 output side wiring, 100 SAW device.

Claims

1. a ceramic substrate made of polycrystalline ceramic and having a main supporting surface; a piezoelectric substrate made of a piezoelectric material, the piezoelectric substrate being bonded to the supporting main surface at a bonding main surface by van der Waals forces; the ceramic substrate includes a supporting primary surface amorphous layer formed to include the supporting primary surface; the piezoelectric substrate includes a bonding principal surface amorphous layer formed to include the bonding principal surface, the thickness of the supporting major surface amorphous layer is smaller than the thickness of the bonding major surface amorphous layer; the ceramic substrate is made of one or more materials selected from the group consisting of spinel, alumina, magnesia, silica, mullite, cordierite, calcia, titania, silicon nitride, aluminum nitride, and silicon carbide; the piezoelectric substrate is made of single-crystal lithium tantalate or single-crystal lithium niobate, A laminate, wherein the thickness of the amorphous layer on the supporting main surface is 3.0 nm or less.

2. The laminate according to claim 1 ; an electrode formed on a main surface of the piezoelectric substrate opposite to the ceramic substrate.

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