Composite substrate, piezoelectric element, and method for manufacturing composite substrate

Abstract

The composite substrate disclosed in the present invention comprises: a piezoelectric substrate having a first surface as an element forming surface and a second surface as the back surface thereof; a sapphire substrate having a third surface arranged opposite to the second surface and a fourth surface as the back surface thereof; and an aluminum oxide layer having a fifth surface opposite to the second surface and a sixth surface opposite to the third surface, and bonding the second surface and the third surface. The arithmetic mean roughness Ra of the third surface is 0.1 μm or more and 0.5 μm or less. The arithmetic mean roughness Ra of the fifth surface is 0.1 μm or less and is smaller than the arithmetic mean roughness Ra of the third surface.

Description

Technical Field

[0001] The present disclosure relates to a composite substrate having a structure in which a piezoelectric substrate and a sapphire substrate are bonded together, a piezoelectric element including the composite substrate, and a method for manufacturing the composite substrate. Background Art

[0002] In recent years, there has been a demand for miniaturization and high performance of piezoelectric elements such as surface acoustic wave elements used in communication devices such as mobile phones. As a small and high-performance piezoelectric element, a piezoelectric element having a structure in which element electrodes are formed on a piezoelectric substrate of a composite substrate in which a piezoelectric substrate and a supporting substrate are bonded has been proposed. Sapphire substrates are excellent in mechanical strength, insulation, and heat dissipation, and are relatively excellent as supporting substrates.

[0003] In a composite substrate, the stray caused by the reflection of the bulk wave at the bonding interface between the piezoelectric substrate and the supporting substrate becomes a problem. In order to solve this problem, Patent Document 1 discloses a composite substrate in which the surface of the supporting substrate is roughened by grinding. In addition, Patent Document 2 discloses a composite substrate using a supporting substrate having a concave-convex structure in a pyramid shape formed by wet etching. However, there is a problem that if the surface roughness of the supporting substrate is increased in order to reduce the reflection of the bulk wave, the bonding strength between the supporting substrate and the piezoelectric substrate is reduced.

[0004] Prior Art Literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Publication No. 2014-147054

[0007] Patent Document 2: Japanese Patent Application Publication No. 2018-61226 Summary of the invention

[0008] Problems to be solved by the invention

[0009] An object of the present disclosure is to provide a composite substrate and a piezoelectric element in which the bonding strength between a piezoelectric substrate and a supporting substrate is high and reflection of bulk waves at a bonding surface is reduced.

[0010] Means for solving problems

[0011] The composite substrate disclosed in the present invention comprises: a piezoelectric substrate having a first surface as an element forming surface and a second surface as the back surface thereof; a sapphire substrate having a third surface arranged opposite to the second surface and a fourth surface as the back surface thereof; and an aluminum oxide layer having a fifth surface opposite to the second surface and a sixth surface opposite to the third surface, and bonding the second surface and the third surface. The arithmetic mean roughness Ra of the third surface is not less than 0.1 μm and not more than 0.5 μm. The arithmetic mean roughness Ra of the fifth surface is not more than 0.1 μm and is smaller than the arithmetic mean roughness Ra of the third surface.

[0012] The manufacturing method of the composite substrate disclosed in the present invention comprises: a preparation step of preparing a piezoelectric substrate having a first surface as an element forming surface and a second surface as its back side, and a sapphire substrate having a third surface and a fourth surface as its back side; a roughening step of processing the third surface so that the arithmetic mean roughness Ra is greater than 0.1 μm and less than 0.5 μm; an aluminum oxide layer forming step of forming an aluminum oxide layer on the roughened third surface, and processing the exposed surface of the aluminum oxide layer located on the opposite side of the sapphire substrate, i.e., the fifth surface, so that the arithmetic mean roughness Ra is less than 0.1 μm and smaller than the arithmetic mean roughness Ra of the third surface; and a bonding step of directly bonding the fifth surface of the aluminum oxide layer and the second surface of the piezoelectric substrate.

[0013] The composite substrate disclosed in the present invention comprises: a piezoelectric substrate having a first surface as an element forming surface and a second surface as its back side; a sapphire substrate having a third surface arranged opposite to the second surface and a fourth surface as its back side; and a bonding layer comprising any one of the same material as the piezoelectric substrate, aluminum oxide, and an oxide having an intermediate thermal expansion coefficient between the piezoelectric substrate and aluminum oxide, having a fifth surface opposite to the second surface and a sixth surface opposite to the third surface, and bonding the second surface and the third surface, the arithmetic mean roughness R of the second surface is greater than 0.1 μm and less than 0.5 μm, and the arithmetic mean roughness Ra of the sixth surface is less than 0.1 μm and less than the arithmetic mean roughness Ra of the second surface.

[0014] The manufacturing method of the composite substrate disclosed in the present invention comprises: a preparation step of preparing a piezoelectric substrate having a first surface as an element forming surface and a second surface as its back side, and a sapphire substrate having a third surface and a fourth surface as its back side; a roughening step of processing the second surface so that the arithmetic mean roughness Ra is greater than 0.1 μm and less than 0.5 μm; a bonding layer forming step of forming a bonding layer on the roughened second surface, the bonding layer comprising any one of the same material as the piezoelectric substrate, aluminum oxide, and an oxide having a thermal expansion coefficient intermediate between the piezoelectric substrate and aluminum oxide, processing the exposed surface of the bonding layer located on the opposite side of the piezoelectric substrate, i.e., the sixth surface, so that the arithmetic mean roughness Ra is less than 0.1 μm and smaller than the arithmetic mean roughness Ra of the second surface; and a bonding step of directly bonding the sixth surface of the bonding layer to the third surface of the sapphire substrate.

[0015] Effects of the Invention

[0016] According to the present disclosure, it is possible to provide a composite substrate and a piezoelectric element in which the bonding strength between a piezoelectric substrate and a sapphire substrate is high and the reflection of bulk waves at the bonding surface is reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a schematic cross-sectional view showing a composite substrate according to one embodiment of the present disclosure.

[0018] Figure 2 This is a schematic cross-sectional view showing a composite substrate according to another embodiment of the present disclosure.

[0019] Figure 3 This is a schematic cross-sectional view showing one embodiment of a method for manufacturing a composite substrate according to the present disclosure.

[0020] Figure 4 This is a schematic cross-sectional view showing another embodiment of the method for producing a composite substrate according to the present disclosure. DETAILED DESCRIPTION

[0021] <Composite substrate and piezoelectric element>

[0022] A composite substrate and a piezoelectric element according to an embodiment of the present disclosure will be described with reference to the accompanying drawings. Figure 1 In the figure, a schematic cross-sectional view of a composite substrate 1 according to one embodiment is shown. The composite substrate 1 comprises: a piezoelectric substrate 2 having a first surface 2a as an element forming surface and a second surface 2b as the back surface thereof; a sapphire substrate 3 having a third surface 3a arranged opposite to the second surface 2b; and a bonding layer 4 bonding the second surface 2b and the third surface 3a. The bonding layer 4 comprises any one of the same material as the piezoelectric substrate 2, aluminum oxide, and an oxide having a thermal expansion coefficient intermediate between that of the piezoelectric substrate 2 and that of aluminum oxide. Figure 1 In the figure, the bonding layer 4 is described as an "aluminum oxide layer 4" containing aluminum oxide. The arithmetic mean roughness Ra of the third surface 3a of the sapphire substrate 3 is greater than 0.1 μm and less than 0.5 μm, and the arithmetic mean roughness Ra of the fifth surface 4a of the aluminum oxide layer 4, which is the surface on the piezoelectric substrate 2 side, is less than 0.1 μm and smaller than the arithmetic mean roughness Ra of the third surface 3a.

[0023] A piezoelectric element according to one embodiment of the present disclosure includes a composite substrate 1 according to one embodiment. Examples of the piezoelectric element include an oscillator used in an oscillation circuit, a surface elastic wave element, a boundary elastic wave element, a bulk wave element, and other elastic wave elements used in a filter circuit, etc. The second surface 2b and the third surface 3a are opposed to each other via the aluminum oxide layer 4 as described above.

[0024] Hereinafter, a composite substrate 1 according to an embodiment will be described in detail. A composite substrate 1 according to an embodiment includes a piezoelectric substrate, a sapphire substrate 3, and an aluminum oxide layer 4. The piezoelectric substrate 2 has a first surface 2a as an element forming surface, and a second surface 2b as its back surface and bonded to the aluminum oxide layer 4. The sapphire substrate 3 has a third surface 3a arranged opposite to the second surface 2b of the piezoelectric substrate 2, and a fourth surface 3b as its back surface. The aluminum oxide layer 4 has a fifth surface 4a bonded to the second surface 2b of the piezoelectric substrate 2, and a sixth surface 4b bonded to the third surface 3a of the sapphire substrate 3. The aluminum oxide layer 4 bonds the piezoelectric substrate 2 and the sapphire substrate 3 without an adhesive or the like.

[0025] The element electrodes are formed on the first surface 2a of the piezoelectric substrate 2, and the composite substrate 1 is used as a piezoelectric element such as a surface acoustic wave element. In the following description, for the sake of convenience, the piezoelectric substrate 2 is described as a substrate for a surface acoustic wave element. The piezoelectric substrate 2 is not limited to this, and can also be a substrate for other purposes and functions such as a sensor substrate such as a vibration sensor or a substrate for a signal transmitter.

[0026] The first surface 2a of the composite substrate 1 according to one embodiment is a component forming surface such as a component electrode, the second surface 2b and the third surface 3a are bonding surfaces, and the fourth surface 3b is a back surface. The component forming surface is a portion where functional parts such as component electrodes are located as described above. The component electrodes are, for example, comb-shaped electrodes arranged to mesh with each other. The signal transmitted between the comb-tooth electrodes is filtered by the surface elastic waves between the comb-tooth electrodes.

[0027] Conventionally, in a surface acoustic wave element having a composite substrate, there is a problem that a so-called stray noise is generated at a frequency higher than a passband (a frequency band through which a bandpass filter allows a signal to pass without attenuation). This noise is caused by the reflection of a bulk wave at the bonding interface between a piezoelectric substrate 2 and a sapphire substrate 3 as a supporting substrate. It is known that the surface roughness of the bonding surface is increased in order to reduce the reflection of the bulk wave, but there is a problem that if the surface roughness of the bonding surface is increased, the bonding strength is reduced.

[0028] Regarding the composite substrate 1 involved in one embodiment, the arithmetic mean roughness Ra of the third surface 3a of the sapphire substrate 3 as the bonding surface is 0.1 μm or more and 0.5 μm or less, and particularly preferably 0.1 μm or more and 0.3 μm or less. Therefore, a part of the body wave reaching the third surface 3a is absorbed or diffusely reflected, so that the reflected body wave toward the element forming surface 2a (that is, the functional part such as the element electrode) is reduced. Thus, it is possible to reduce stray.

[0029] Furthermore, the arithmetic mean roughness Ra of the fifth surface 4a of the aluminum oxide layer 4 on the piezoelectric substrate 2 side of the composite substrate 1 according to one embodiment is 0.1 μm or less, preferably 0.01 μm or less. Thus, the bonding strength between the sapphire substrate 3 and the aluminum oxide layer 4 and the piezoelectric substrate 2 can be improved. Therefore, a composite substrate 1 can be provided in which the bonding strength between the piezoelectric substrate 2 and the sapphire substrate 3 is high and the reflection of the bulk wave in the third surface 3a as the bonding surface is reduced.

[0030] The arithmetic mean roughness Ra can be measured, for example, by a laser microscope, a stylus-type surface profile measuring device, an atomic force microscope (AFM), or a scanning electron microscope (SEM) observation of a cross section of a joint, a transmission electron microscope (TEM) observation, etc. The measurement length is set to 5 μm or more, and 5 or more points in the surface are measured and the average value is taken as the measured value.

[0031] The piezoelectric substrate 2 includes a piezoelectric material such as lithium tantalate (LT), lithium niobate (LN), zinc oxide, or quartz. If the arithmetic mean roughness Ra of the first surface 2a of the piezoelectric substrate 2 is 1 nm or less, good element characteristics can be obtained. In addition, if the arithmetic mean roughness Ra of the second surface 2b is 0.01 μm or less, the bonding strength with the aluminum oxide layer 4' becomes greater.

[0032] Sapphire is single crystal aluminum oxide. The third surface 3a and the fourth surface 3b' of the sapphire substrate 3 become specific crystal planes such as the c-plane, a-plane, m-plane, and r-plane, or crystal planes having a given deviation angle (OFF angle) relative to these crystal planes. If the arithmetic mean roughness Ra of the fourth surface 3b of the sapphire substrate 3 is greater than 1 μm, the reflection of the body wave in the fourth surface 3b can be reduced, which is effective for improving the characteristics of the element.

[0033] The aluminum oxide layer 4 contains aluminum oxide in the same manner as the sapphire substrate 3. Therefore, compared with the case of using a bonding layer of different materials, the residual stress during bonding caused by the difference in physical properties such as thermal expansion coefficient and elastic modulus due to the difference in materials can be reduced. If the aluminum oxide layer 4 is polycrystalline or amorphous, the regularity of the atomic arrangement is lower than that of a single crystal, thereby reducing the reflection of the body wave. Whether the aluminum oxide layer 4 is single crystal, polycrystalline, or amorphous can be determined by methods such as X-ray diffraction and electron beam diffraction. From the viewpoint of bonding strength and reduction of reflected waves in the bonding portion, the thickness of the aluminum oxide layer 4 can be greater than 0.5 μm and less than 5 μm.

[0034] In order to reduce the reflection of body waves, etc., it is considered to be a composite substrate including a sapphire substrate (not shown) whose surface portion on the piezoelectric substrate side is processed, but the composite substrate 1 involved in one embodiment is different from this. For example, if the third surface 3a of the sapphire substrate 3 is machined using a grinding device, etc., a machined-deformed layer into which multiple crystal defects are introduced is formed. In addition, if atoms (or ions) are driven into the sapphire substrate 3 from the third surface 3a, an ion (atom) injection layer is formed. Regarding the aluminum oxide layer 4 disclosed in the present invention, the arithmetic mean roughness Ra of the third surface 3a of the sapphire substrate 3 is greater than 0.1μm and less than 0.5μm, and its third surface 3a is bonded to the aluminum oxide layer 4 (the sixth surface 4b), and further, the relatively smooth fifth surface 4a of the aluminum oxide layer 4 is bonded to the piezoelectric substrate 2. In this respect, it is different from the machined-deformed layer and the ion (atom) injection layer.

[0035] Next, a composite substrate and a piezoelectric element according to another embodiment of the present disclosure will be described with reference to the accompanying drawings. Figure 2 , a schematic cross-sectional view of a composite substrate 1' according to another embodiment is shown. The composite substrate 1' according to another embodiment comprises: a piezoelectric substrate 2' having a first surface 2a' as an element forming surface and a second surface 2b' as the back surface thereof; a sapphire substrate 3' having a third surface 3a' arranged opposite to the second surface 2b'; and a bonding layer 4', which includes any one of the same material as the piezoelectric substrate 2', aluminum oxide, and an oxide having a thermal expansion coefficient intermediate between the piezoelectric substrate 2' and aluminum oxide, and bonds the second surface 2b' and the third surface 3a'. The arithmetic mean roughness Ra of the second surface 2b' of the piezoelectric substrate 2' is greater than or equal to 0.1 μm and less than or equal to 0.5 μm, and the arithmetic mean roughness Ra of the sixth surface 4b' of the bonding layer 4', which is the surface on the sapphire substrate 3' side, is less than or equal to 0.1 μm and is smaller than the arithmetic mean roughness Ra of the second surface 2b'.

[0036] A piezoelectric element according to another embodiment of the present disclosure includes a composite substrate 1' according to another embodiment. The piezoelectric element is as described above, and detailed description thereof is omitted. As described above, the second surface 2b' and the third surface 3a' face each other via the bonding layer 4'.

[0037] Hereinafter, a composite substrate 1' according to another embodiment will be described in detail. A composite substrate 1' according to another embodiment comprises a piezoelectric substrate 2', a sapphire substrate 3' and a bonding layer 4'. The piezoelectric substrate 2' has a first surface 2a' as an element forming surface, and a second surface 2b' as its back surface and bonded to the bonding layer 4'. The sapphire substrate 3' has a third surface 3a' arranged opposite to the second surface 2b' of the piezoelectric substrate 2', and a fourth surface 3b' as its back surface. The bonding layer 4' has a fifth surface 4a' in contact with the second surface 2b' of the piezoelectric substrate 2', and a sixth surface 4b' in contact with the third surface 3a' of the sapphire substrate 3'. The bonding layer 4' bonds the piezoelectric substrate 2' and the sapphire substrate 3' without using an adhesive or the like.

[0038] The element electrodes are formed on the first surface 2a' of the piezoelectric substrate 2', and the composite substrate 1' is used as a piezoelectric element such as a surface acoustic wave element. In the following description, for the sake of convenience, the piezoelectric substrate 2' is used as an example of a substrate for a surface acoustic wave element. The piezoelectric substrate 2' is not limited to this, and can also be a substrate for other purposes and functions such as a sensor substrate such as a vibration sensor or a substrate for a signal transmitter.

[0039] The first surface 2a' of the composite substrate 1' according to another embodiment is a component forming surface such as a component electrode, the second surface 2b' and the third surface 3a' are bonding surfaces, and the fourth surface 3b' is a back surface. The component forming surface is a portion where functional parts such as component electrodes are located as described above. The component electrodes are, for example, comb-shaped electrodes that are configured to mesh with each other. The signal transmitted between the comb-tooth electrodes is filtered by the surface elastic wave between the comb-tooth electrodes.

[0040] Conventionally, in a surface acoustic wave element having a composite substrate, there is a problem that a so-called stray noise is generated at a frequency higher than a passband (a frequency band through which a bandpass filter allows a signal to pass without attenuation). This noise is caused by the reflection of a bulk wave at the bonding interface between a piezoelectric substrate 2 and a sapphire substrate 3' as a supporting substrate. It is known that the surface roughness of the bonding surface is increased in order to reduce the reflection of the bulk wave, but there is a problem that if the surface roughness of the bonding surface is increased, the bonding strength is reduced.

[0041] The arithmetic mean roughness Ra of the second surface 2b' as the bonding surface of the piezoelectric substrate 2' of the composite substrate 1' involved in another embodiment is 0.1 μm or more and 0.5 μm or less, and particularly preferably 0.1 μm or more and 0.3 μm or less. Therefore, part of the body wave reaching the second surface 2b' is absorbed or diffusely reflected, and the reflected body wave toward the element forming surface 2a' (that is, the functional part such as the element electrode) can be reduced. As a result, stray waves can be reduced.

[0042] Furthermore, the arithmetic mean roughness Ra of the sixth surface 4b', which is the surface on the sapphire substrate 3' side, of the bonding layer 4' of the composite substrate 1' involved in another embodiment is less than 0.1 μm, preferably less than 0.01 μm. As a result, the strength of the bonding between the piezoelectric substrate 2' and the bonding layer 4' and the sapphire substrate 3' can be improved. Therefore, a composite substrate 1' can be provided in which the bonding strength between the piezoelectric substrate 2' and the sapphire substrate 3' is high and the reflection of the body wave in the second surface 2b', which is the bonding surface, is reduced. As for the method for measuring the arithmetic mean roughness Ra, as described above, a detailed description is omitted.

[0043] The piezoelectric substrate 2' includes a piezoelectric material such as lithium tantalate (LT), lithium niobate (LN), zinc oxide, or quartz. When the arithmetic mean roughness Ra of the first surface 2a' of the piezoelectric substrate 2 is 1 nm or less, good device characteristics can be obtained.

[0044] Sapphire is single crystal aluminum oxide. The third surface 3a' and the fourth surface 3b' of the sapphire substrate 3' become specific crystal planes such as the c-plane, a-plane, m-plane, and r-plane, or crystal planes having a given deviation angle (OFF angle) relative to these crystal planes. If the arithmetic mean roughness Ra of the third surface 3a' of the sapphire substrate 3' is less than 0.01 μm, the bonding strength with the bonding layer 4' becomes greater. In addition, if the arithmetic mean roughness Ra of the fourth surface 3b' of the sapphire substrate 3 is more than 1 μm, the reflection of the body wave in the fourth surface 3b' can be reduced, which is effective for improving the characteristics of the element.

[0045] The bonding layer 4' includes any one of the same material as the piezoelectric substrate 2', aluminum oxide, and an oxide having an intermediate thermal expansion coefficient between the piezoelectric substrate 2' and aluminum oxide. If the bonding layer 4' is the same material as the piezoelectric substrate 2', it is possible to reduce the thermal stress and thermal strain generated by the formation temperature (for example, hundreds of degrees) when the bonding layer 4' is formed on the piezoelectric substrate 2'. On the other hand, if the bonding layer 4' is aluminum oxide like the sapphire substrate 3', it is possible to reduce the thermal stress and thermal strain generated by the bonding temperature (for example, tens of degrees to about 150 degrees Celsius) when the bonding layer 4' and the sapphire substrate 3' are bonded. In addition, if the bonding layer 4' is an oxide having an intermediate thermal expansion coefficient between the piezoelectric substrate 2' and aluminum oxide, it is possible to reduce the thermal stress and thermal strain generated when the bonding layer 4' is formed or bonded to the sapphire substrate 3'.

[0046] The thermal expansion coefficient of single crystal materials varies depending on the crystal orientation. The thermal expansion coefficient of lithium niobate is 7.5 to 15 ppm / °C, and the thermal expansion coefficient of lithium tantalate is 4 to 16 ppm / °C. The thermal expansion coefficient of the lithium tantalate substrate rotated at an angle of 36° to 46° from the Y axis with the X axis as the center, that is, the propagation direction of the surface acoustic wave, is about 16 ppm / °C. In addition, the thermal expansion coefficient of sapphire is 7.0 to 7.7 ppm / °C. For example, if the thermal expansion coefficients of the piezoelectric substrate 2' and the sapphire substrate 3' are 16 ppm / °C and 7 ppm, a material with a thermal expansion coefficient of 7 to 16 ppm / °C can be used as the bonding layer 4'. As a material having such a thermal expansion coefficient, for example, silicon dioxide can be cited (the thermal expansion coefficient of single crystal silicon dioxide is 7.5 to 14 ppm / °C).

[0047] If the bonding layer 4' is polycrystalline or amorphous, the regularity of the atomic arrangement is lower than that of a single crystal, thereby reducing the reflection of the body wave. Whether the bonding layer 4' is single crystal, polycrystalline, or amorphous can be determined by methods such as X-ray diffraction and electron beam diffraction. From the perspective of bonding strength and reduction of reflected waves at the bonding portion, the thickness of the bonding layer 4' can be greater than 0.5 μm and less than 5 μm.

[0048] In order to reduce the reflection of body waves, etc., a composite substrate including a sapphire substrate (not shown) whose surface portion on the piezoelectric substrate side is processed can be considered. However, the composite substrate 1' involved in another embodiment is different from this. If atoms (or ions) are driven into the sapphire substrate 3 from the third surface 3a', an ion (atom) implantation layer is formed. The arithmetic mean roughness Ra of the second surface 2b' of the piezoelectric substrate 2' of the bonding layer 4' disclosed in the present invention is greater than 0.1μm and less than 0.5μm, and its second surface 2b' is bonded to the bonding layer 4 (fifth surface 4a). Furthermore, the relatively smooth sixth surface 4b' of the bonding layer 4' is bonded to the sapphire substrate 3'. In this respect, it is different from a processed metamorphic layer and an ion (atom) implantation layer.

[0049] <Method for Manufacturing Composite Substrate>

[0050] Next, a method for manufacturing a composite substrate according to an embodiment of the present disclosure will be described with reference to the accompanying drawings. Figure 3 , a schematic diagram of a method for manufacturing a composite substrate according to an embodiment is shown. A method for manufacturing a composite substrate according to an embodiment comprises the following steps (1) to (4). By the following steps (1) to (4), for example, Figure 1 The composite substrate 1 according to one embodiment as shown in the figure. The composite layer 4 described in the following steps (3) and (4) includes any one of the same material as the piezoelectric substrate 2, aluminum oxide, and an oxide having an intermediate thermal expansion coefficient between the piezoelectric substrate 2 and aluminum oxide.

[0051] (1) Preparation step: preparing a piezoelectric substrate 2 having a first surface 2a as an element formation surface and a second surface 2b as its back surface, and a sapphire substrate 3 having a third surface 3a and a fourth surface 3b as its back surface.

[0052] (2) Surface roughening step: The third surface 3 a of the sapphire substrate 3 is processed so that the arithmetic mean roughness Ra becomes 0.1 μm or more and 0.5 μm or less.

[0053] (3) A bonding layer forming step, forming a bonding layer 4 on the roughened third surface 3a, and processing the exposed surface of the bonding layer 4 located on the opposite side of the sapphire substrate 3, namely the fifth surface 4a, so that the arithmetic mean roughness Ra is less than 0.1 μm and smaller than the arithmetic mean roughness Ra of the third surface 3a.

[0054] (4) A bonding step of directly bonding the fifth surface 4 a of the composite layer 4 and the second surface 2 b of the piezoelectric substrate 2 .

[0055] The following is a detailed description of the method for manufacturing the composite substrate disclosed in the present invention. Figure 3 In the description, the bonding layer 4 is set as "aluminum oxide layer 4" containing aluminum oxide. First, a piezoelectric substrate 2 having a first surface 2a and a second surface 2b facing each other, and a sapphire substrate 3 having a third surface 3a and a fourth surface 3b facing each other are prepared. The first surface 2a becomes an element forming surface, the second surface 2b and the third surface 3a become bonding surfaces, and the fourth surface 3b becomes a back surface.

[0056] An ingot-shaped or ribbon-shaped sapphire crystal grown by an appropriate growth method such as the Czochralski method is cut so that the third face 3a and the fourth face 3b become specific crystal planes such as the c-plane, a-plane, m-plane, and r-plane, or have a given deviation angle relative to these crystal planes, thereby producing a sapphire substrate 3.

[0057] The second surface 2b of the piezoelectric substrate 2 and the third surface 3a of the sapphire substrate 3 are flattened by grinding using a plate containing copper, tin, iron, etc. and abrasive grains such as diamond, silicon carbide, and boron carbide. If the processing is performed so that the arithmetic mean roughness Ra of the third surface 3a is greater than 0.1 μm and less than 0.5 μm, and particularly preferably greater than 0.1 μm and less than 0.3 μm, the reflection of the body wave can be reduced and the bonding strength is also increased. If the arithmetic mean roughness Ra of the second surface 2b is less than 0.01 μm, the bonding strength with the aluminum oxide layer 4 can be improved, which is preferred.

[0058] Next, an aluminum oxide layer 4 is formed on the third surface 3a of the sapphire substrate 3. The aluminum oxide layer 4 can be formed, for example, by a PVD method such as evaporation or sputtering, a CVD method such as an organic metal vapor deposition method, or the like. A grinding process is performed by chemical mechanical polishing (CMP) using silica particles and an alkaline aqueous solution, so that the arithmetic mean roughness Ra of the fifth surface 4a, which is the surface of the aluminum oxide layer 4, becomes a desired value of less than 0.1 μm (for example, about 0.01 μm). In addition, if the formation method and formation conditions of the aluminum oxide layer 4 are optimized so that the arithmetic mean roughness Ra of the fifth surface 4a of the aluminum oxide layer 4 after film formation (as-grown) becomes a desired value of less than 0.1 μm (for example, less than 0.01 μm), the grinding process step can also be omitted.

[0059] Next, at least one of the second surface 2b of the piezoelectric substrate 2 and the fifth surface 4a of the aluminum oxide layer 4 is activated by a method such as plasma treatment. The aluminum oxide layer 4 and the piezoelectric substrate 2 are bonded by direct bonding without using an adhesive material. For example, the piezoelectric substrate 2 and the sapphire substrate 3 forming the aluminum oxide layer 4 are heated and (or) pressurized in a vacuum, in the atmosphere, or in a given environment, so that the atoms at the bonding interface diffuse, thereby performing diffusion bonding. The prior activation treatment can reduce the temperature during bonding. Therefore, the causes of breakage and poor processing accuracy caused by the difference in thermal expansion coefficients between the piezoelectric substrate 2 and the sapphire substrate 3 can be reduced.

[0060] In direct bonding of the piezoelectric substrate 2 and the aluminum oxide film 4, diffusion bonding based on diffusion of atoms between the piezoelectric substrate 2 and the aluminum oxide film 4 is used. If the arithmetic mean roughness Ra of the second surface 2b of the piezoelectric substrate 2 and the fifth surface 4a of the aluminum oxide layer 4 is 0.1 μm or less, the bonding strength is improved.

[0061] After the piezoelectric substrate 2 and the sapphire substrate 3 are bonded, the thickness of the sapphire substrate 3 may be reduced by using a grinding device or the like. In this case, the sapphire substrate 3 is removed from the fourth surface 3b side by the above-mentioned processing. The thickness of the piezoelectric substrate 2 may also be reduced by using a grinding device or the like. It is preferred that the first surface 2a of the piezoelectric substrate 2' is processed by using a CMP device or the like so that the arithmetic mean roughness Ra is 1 nm or less.

[0062] Hereinafter, a method for manufacturing a composite substrate according to another embodiment of the present disclosure will be described with reference to the accompanying drawings. Figure 4 , a schematic diagram for explaining a method for manufacturing a composite substrate according to another embodiment is shown. The method for manufacturing a composite substrate according to another embodiment comprises the following steps (5) to (8). By the following steps (5) to (8), for example, Figure 2A composite substrate 1 ′ according to another embodiment is shown.

[0063] (5) Preparation step: preparing a piezoelectric substrate 2' having a first surface 2a' as an element formation surface and a second surface 2b' as its back surface, and a sapphire substrate 3' having a third surface 3a' and a fourth surface 3b' as its back surface.

[0064] (6) Surface roughening step: The second surface 2b' of the piezoelectric substrate 2' is processed so that the arithmetic mean roughness Ra is 0.1 μm or more and 0.5 μm or less.

[0065] (7) A bonding layer forming step, wherein a bonding layer 4' is formed on the roughened second surface 2b', wherein the bonding layer 4' comprises any one of the same material as the piezoelectric substrate 2', aluminum oxide, and an oxide having a thermal expansion coefficient intermediate between the piezoelectric substrate 2' and aluminum oxide, and the exposed surface of the bonding layer 4', i.e., the sixth surface 4b', which is located on the opposite side of the piezoelectric substrate 2' is processed so that the arithmetic mean roughness Ra is less than 0.1 μm and smaller than the arithmetic mean roughness Ra of the second surface 2b'.

[0066] (8) A bonding step of directly bonding the sixth surface 4b' of the bonding layer 4' and the third surface 3a' of the sapphire substrate 3'.

[0067] The details of the manufacturing method of the composite substrate involved in another embodiment are described in the case where the piezoelectric substrate 2' is lithium tantalate and the bonding layer 4' is aluminum oxide. First, a piezoelectric substrate 2' containing a lithium tantalate single crystal having a first surface 2a' and a second surface 2b' facing each other, and a sapphire substrate 3' having a third surface 3a' and a fourth surface 3b' facing each other are prepared. The first surface 2a' becomes the element forming surface, the second surface 2b' and the third surface 3a' become the bonding surface, and the fourth surface 3b' becomes the back surface.

[0068] An ingot or ribbon-shaped sapphire crystal grown by an appropriate growth method such as the Czochralski method is cut so that the third face 3a' and the fourth face 3b' are specific crystal planes such as the c-plane, a-plane, m-plane, and r-plane, or have a given deviation angle relative to these crystal planes, thereby producing a sapphire substrate 3'.

[0069] The second surface 2b' of the piezoelectric substrate 2' and the third surface 3a' of the sapphire substrate 3' are smoothed by grinding using a plate containing copper, tin, iron, etc., and abrasive grains such as diamond, silicon carbide, and boron carbide. If the processing is performed so that the arithmetic mean roughness Ra of the second surface 2b' is greater than 0.1μm and less than 0.5μm, and particularly preferably greater than 0.1μm and less than 0.3μm, the reflection of the body wave can be reduced, and the bonding strength is also increased. If the arithmetic mean roughness Ra of the third surface 3a' is less than 0.01μm, the bonding strength with the bonding layer 4' can be improved, which is preferred.

[0070] Next, a bonding layer 4' containing aluminum oxide is formed on the second surface 2b' of the piezoelectric substrate 2'. The bonding layer 4' can be formed, for example, by a PVD method such as evaporation or sputtering, a CVD method such as an organic metal vapor deposition method, or the like. Grinding is performed by chemical mechanical polishing (CMP) using silica particles and an alkaline aqueous solution, so that the arithmetic mean roughness Ra of the surface of the bonding layer 4, i.e., the sixth surface 4b', is a desired value of less than 0.1 μm (for example, about 0.01 μm). If the formation method and formation conditions of the bonding layer 4' are optimized so that the arithmetic mean roughness Ra of the sixth surface 4b' of the bonding layer 4' after film formation (as-grown) is a desired value of less than 0.1 μm (for example, less than 0.01 μm), the grinding process can also be omitted.

[0071] Next, at least one of the third surface 3a' of the sapphire substrate 3' and the sixth surface 4b' of the bonding layer 4' is activated by a method such as plasma treatment. Then, the sapphire substrate 3 and the bonding layer 4' are bonded by direct bonding without using an adhesive material. For example, the sapphire substrate 3' and the piezoelectric substrate 2' formed with the bonding layer 4' are heated and / or pressurized in a vacuum, in the atmosphere, or in a given environment to diffuse the atoms at the bonding interface, thereby performing diffusion bonding. The temperature during bonding can be lowered by the prior activation treatment. Therefore, the causes of breakage and poor processing accuracy caused by the difference in thermal expansion coefficients between the piezoelectric substrate 2' and the sapphire substrate 3' can be reduced.

[0072] In direct bonding of the sapphire substrate 3' and the bonding layer 4', diffusion bonding based on diffusion of atoms between the sapphire substrate 3' and the bonding layer 4' is used. If the arithmetic mean roughness Ra of the third surface 3a' of the sapphire substrate 3' and the sixth surface 4b' of the bonding layer 4' is 0.1 μm or less, the bonding strength is improved.

[0073] After the piezoelectric substrate 2' and the sapphire substrate 3' are bonded, the thickness of the sapphire substrate 3' may be thinned using a grinding device or the like. In this case, the sapphire substrate 3' is removed from the fourth surface 3b' side by the above-mentioned processing. The thickness of the piezoelectric substrate 2' may also be thinned using a grinding device or the like. Preferably, the first surface 2a' of the piezoelectric substrate 2' is processed using a CMP device or the like so that the arithmetic mean roughness Ra is less than 1 nm. In the case where the bonding layer 4' contains lithium tantalate or silicon dioxide, the bonding layer can also be formed in the same manner as described above.

[0074] As mentioned above, although embodiment of this disclosure is described, this disclosure is not limited to the said embodiment, Various improvements and changes are possible within the range which does not deviate from the summary of this disclosure.

[0075] [Example]

[0076] Hereinafter, the present disclosure will be described in detail with reference to Examples, but the present disclosure is not limited to the following Examples.

[0077] A plurality of LT substrates 2 are prepared as piezoelectric substrates, and a plurality of sapphire substrates 3 are prepared as support substrates. Then, the second surface 2b of the LT substrate 2 is processed using a grinding device so that the arithmetic mean roughness Ra becomes 0.01 μm, and the third surface 3a of the sapphire substrate 3 is processed so that the arithmetic mean roughness Ra becomes 0.02 μm (condition 1), 0.1 μm (condition 2), 0.5 μm (condition 3), and 2.5 μm (condition 4). Then, an aluminum oxide layer 4 is formed on the processed third surface 3a by evaporation with a thickness of about 1 μm, and the fifth surface 4a of the aluminum oxide layer 4 is processed using a CMP device so that the arithmetic mean roughness Ra becomes 0.01 μm. The second surface 2b of the piezoelectric substrate 2 and the fifth surface 4a of the aluminum oxide layer 4' are activated by argon plasma to be bonded, thereby producing a composite substrate 1.

[0078] In condition 4 among conditions 1 to 4, sufficient bonding strength (1.0 N / m 2 Furthermore, a surface acoustic wave device was fabricated using the composite substrate 1 under conditions 1 to 3. Under condition 1, the effect of reducing the spurious emission due to the roughening of the third surface 3a was small, whereas under conditions 2 and 3, good device characteristics were obtained.

[0079] A plurality of LT substrates 2' are prepared as piezoelectric substrates, and a plurality of sapphire substrates 3' are prepared as support substrates. Then, the second surface 2b' of the LT substrate 2' is processed using a grinding device so that the arithmetic mean roughness Ra becomes 0.02μm (condition 1), 0.1μm (condition 2), 0.5μm (condition 3), and 2.5μm (condition 4) and the third surface 3a' of the sapphire substrate 3' is processed so that the arithmetic mean roughness Ra becomes 0.01μm. Then, a bonding layer 4' containing aluminum oxide is formed on the processed second surface 2b with a thickness of about 1μm by evaporation, and the sixth surface 4b' of the bonding layer 4' is processed using a CMP device so that the arithmetic mean roughness Ra becomes 0.01μm. Then, the third surface 3a' of the sapphire substrate 3' and the sixth surface 4b' of the bonding layer 4' are activated by argon plasma for bonding, thereby producing a composite substrate 1'.

[0080] In condition 4 among conditions 1 to 4, sufficient bonding strength (1.0 N / m 2 Furthermore, a surface acoustic wave device was fabricated using the composite substrate 1' under conditions 1 to 3. Under condition 1, the effect of reducing the spurious signal due to the roughening of the second surface 2b' was small, whereas under conditions 2 and 3, good device characteristics were obtained.

[0081] Explanation of symbols

[0082] 1, 1': Composite substrate

[0083] 2, 2': Piezoelectric substrate

[0084] 2a, 2a': Surface 1 (device formation surface)

[0085] 2b, 2b': Surface 2 (back side of piezoelectric substrate)

[0086] 3, 3': Sapphire substrate

[0087] 3a, 3a': Surface 3 (joining surface of sapphire substrate)

[0088] 3b, 3b': Surface 4 (back side of composite substrate)

[0089] 4: Bonding layer

[0090] 4a: Surface 5 (piezoelectric substrate side surface of bonding layer)

[0091] 4b: Surface 6 (sapphire substrate side surface of bonding layer)

[0092] 4': Alumina layer (bonding layer)

[0093] 4a': Surface 5 (surface on the piezoelectric substrate side of the aluminum oxide layer)

[0094] 4b': Surface 6 (the surface of the aluminum oxide layer on the sapphire substrate side).

Claims

1. A composite substrate comprising: A piezoelectric substrate having a first surface as an element forming surface and a second surface as a back surface thereof; a sapphire substrate having a third surface disposed opposite to the second surface and a fourth surface serving as a back surface thereof; and a bonding layer having a fifth surface facing the second surface and a sixth surface facing the third surface, and bonding the second surface and the third surface, The bonding layer comprises aluminum oxide, The third face is an r-face crystal face, The arithmetic mean roughness Ra of the third surface is not less than 0.1 μm and not more than 0.5 μm, and the arithmetic mean roughness Ra of the fifth surface is not more than 0.01 μm and is smaller than the arithmetic mean roughness Ra of the third surface.

2. The composite substrate according to claim 1, wherein: The bonding layer is polycrystalline or amorphous.

3. The composite substrate according to claim 1 or 2, wherein: The thickness of the bonding layer is greater than or equal to 0.5 μm and less than or equal to 5.0 μm. A piezoelectric element comprising the composite substrate according to any one of claims 1 to 3.

5. The piezoelectric element according to claim 4, wherein The piezoelectric element is a surface elastic wave element.

6. A method for manufacturing a composite substrate, comprising: A preparation step of preparing a piezoelectric substrate having a first surface as an element forming surface and a second surface as a back surface thereof, and a sapphire substrate having a third surface and a fourth surface as a back surface thereof; a roughening step of processing the third surface so that the arithmetic mean roughness Ra becomes greater than or equal to 0.1 μm and less than or equal to 0.5 μm; and A bonding layer forming step of forming a bonding layer on the roughened third surface and processing the exposed surface of the bonding layer, i.e., the fifth surface, located on the opposite side of the sapphire substrate, so that the arithmetic mean roughness Ra is less than 0.01 μm and smaller than the arithmetic mean roughness Ra of the third surface; and a bonding step of directly bonding the fifth surface of the bonding layer and the second surface of the piezoelectric substrate; The bonding layer comprises aluminum oxide, The third plane is an r-plane crystal plane.

7. A composite substrate comprising: A piezoelectric substrate having a first surface as an element forming surface and a second surface as a back surface thereof; a sapphire substrate having a third surface disposed opposite to the second surface and a fourth surface serving as a back surface thereof; and a bonding layer comprising aluminum oxide, having a fifth surface facing the second surface and a sixth surface facing the third surface, and bonding the second surface and the third surface, The third face is an r-face crystal face, The arithmetic mean roughness Ra of the second surface is not less than 0.1 μm and not more than 0.5 μm, and the arithmetic mean roughness Ra of the sixth surface is not more than 0.01 μm and is smaller than the arithmetic mean roughness Ra of the second surface.

8. The composite substrate according to claim 7, wherein: The bonding layer is polycrystalline or amorphous.

9. The composite substrate according to claim 7 or 8, wherein: The thickness of the bonding layer is greater than or equal to 0.5 μm and less than or equal to 5.0 μm. 10 . A piezoelectric element comprising the composite substrate according to claim 7 .

11. The piezoelectric element according to claim 10, wherein The piezoelectric element is a surface elastic wave element.

12. A method for manufacturing a composite substrate, comprising: A preparation step of preparing a piezoelectric substrate having a first surface as an element forming surface and a second surface as a back surface thereof, and a sapphire substrate having a third surface and a fourth surface as a back surface thereof; a roughening step of processing the second surface so that the arithmetic mean roughness Ra is greater than or equal to 0.1 μm and less than or equal to 0.5 μm; A bonding layer forming step of forming a bonding layer on the roughened second surface, the bonding layer comprising aluminum oxide, and processing an exposed surface of the bonding layer located on the opposite side of the piezoelectric substrate, i.e., a sixth surface, so that the arithmetic mean roughness Ra is not more than 0.01 μm and is smaller than the arithmetic mean roughness Ra of the second surface; and a bonding step of directly bonding the sixth surface of the bonding layer to the third surface of the sapphire substrate, The third plane is an r-plane crystal plane.

Images