Composite substrate for surface acoustic wave device and method for manufacturing same

Abstract

To provide a composite substrate for a surface acoustic wave element that includes a piezoelectric substrate and a support substrate bonded at high strength, and has reduced reflection of a bulk wave on a bonded surface.SOLUTION: A composite substrate for a surface acoustic wave element according to the present disclosure comprises: a piezoelectric substrate that has a first surface being an element formation surface and a second surface being a surface on its back; a sapphire substrate that has a third surface arranged opposite to the second surface and a fourth surface being a surface on its back; and a bonding layer that has a fifth surface opposite to the second surface and a sixth surface opposite to the third surface, and bonds the second surface and the third surface to each other. The bonding layer is formed of alumina, and the third surface is a crystalline surface of an r-surface. The arithmetic average roughness Ra of the third surface is 0.1 μm or more and 0.5 μm or less, and the arithmetic average roughness Ra of the fifth surface is 0.1 μm or less and smaller than the arithmetic average roughness Ra of the third surface.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a composite substrate for a surface acoustic wave device having a structure in which a piezoelectric substrate and a sapphire substrate are joined, and a method for manufacturing the composite substrate for a surface acoustic wave device.

Background Art

[0002] In recent years, miniaturization and high performance of piezoelectric devices such as surface acoustic wave devices used in communication devices such as mobile phones have been demanded. As a small and high-performance piezoelectric device, a piezoelectric device having a configuration in which an element electrode is formed on a piezoelectric substrate of a composite substrate in which a piezoelectric substrate and a support substrate are joined has been proposed. A sapphire substrate is excellent in mechanical strength, insulation property, and heat dissipation property, and is excellent as a support substrate.

[0003] In a composite substrate, spurious caused by reflection of bulk waves at the bonding interface between the piezoelectric substrate and the support substrate becomes a problem. To solve this problem, Patent Document 1 discloses a composite substrate in which the surface of the support substrate is roughened by lapping. Further, Patent Document 2 discloses a composite substrate using a support substrate in which a pyramidal concavo-convex structure is formed by wet etching. However, when the surface roughness of the support substrate is increased to reduce the reflection of bulk waves, there is a problem that the bonding strength between the support substrate and the piezoelectric substrate decreases.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Disclosure of the Invention

Problems to be Solved by the Invention

[0005] An object of the present disclosure is to provide a composite substrate for a surface acoustic wave device having high bonding strength between a piezoelectric substrate and a support substrate and reducing reflection of bulk waves at the bonding surface.

Means for Solving the Problems

[0006] The composite substrate for a surface acoustic wave device according to the present disclosure includes a piezoelectric substrate having a first surface which is an element formation surface and a second surface which is the back surface thereof, a sapphire substrate having a third surface disposed to face the second surface and a fourth surface which is the 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 is made of alumina, and the third surface is a crystal plane of the r-plane. The arithmetic mean roughness Ra of the third surface is 0.1 μm or more and 0.5 μm or less, and the arithmetic mean roughness Ra of the fifth surface is 0.1 μm or less and smaller than the arithmetic mean roughness Ra of the third surface.

[0007] Another composite substrate for a surface acoustic wave device according to the present disclosure includes a piezoelectric substrate having a first surface which is an element formation surface and a second surface which is the back surface thereof, a sapphire substrate having a third surface disposed to face the second surface and a fourth surface which is the 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 is made of alumina, and the third surface is a crystal plane of the r-plane. The arithmetic mean roughness Ra of the second surface is 0.1 μm or more and 0.5 μm or less, and the arithmetic mean roughness Ra of the sixth surface is 0.1 μm or less and smaller than the arithmetic mean roughness Ra of the second surface.

[0008] The manufacturing method of a composite substrate for a surface acoustic wave device according to the present disclosure includes a preparation step of preparing a piezoelectric substrate having a first surface that is an element formation surface and a second surface that is the back surface thereof, and a sapphire substrate having a third surface and a fourth surface that is the back surface thereof, a roughening step of processing the third surface so that the arithmetic mean roughness Ra is 0.1 μm or more and 0.5 μm or less, a bonding layer formation step of forming a bonding layer on the roughened third surface and processing the fifth surface, which is an exposed surface located on the side opposite to the sapphire substrate of the bonding layer, so that the arithmetic mean roughness Ra is 0.1 μm or less 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 is made of alumina, and the third surface is the crystal plane of the r-plane.

[0009] Another manufacturing method of a composite substrate for a surface acoustic wave device according to the present disclosure includes a preparation step of preparing a piezoelectric substrate having a first surface that is an element formation surface and a second surface that is the back surface thereof, and a sapphire substrate having a third surface and a fourth surface that is the back surface thereof, a roughening step of processing the second surface so that the arithmetic mean roughness Ra is 0.1 μm or more and 0.5 μm or less, a bonding layer formation step of forming a bonding layer on the roughened second surface and processing the sixth surface, which is an exposed surface located on the side opposite to the piezoelectric substrate of the bonding layer, so that the arithmetic mean roughness Ra is 0.1 μm or less 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 and the third surface of the sapphire substrate. The bonding layer is made of alumina, and the third surface is the crystal plane of the r-plane.

Advantages of the Invention

[0010] According to the present disclosure, it is possible to provide a composite substrate for a surface acoustic wave device having high bonding strength between a piezoelectric substrate and a sapphire substrate and reducing reflection of bulk waves at the bonding surface.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Figure 4

Mode for Carrying Out the Invention

[0012] <Composite Substrate and Piezoelectric Element> A composite substrate and a piezoelectric element according to an embodiment of the present disclosure will be described with reference to the drawings. FIG. 1 shows a schematic cross-sectional view of a composite substrate 1 according to an embodiment. The composite substrate 1 includes a piezoelectric substrate 2 having a first surface 2a which is an element formation surface and a second surface 2b which is the back surface thereof, a sapphire substrate 3 having a third surface 3a disposed to face the second surface 2b, and a bonding layer 4 that bonds the second surface 2b and the third surface 3a. The bonding layer 4 is made of the same material as the piezoelectric substrate 2, alumina, or an oxide having a coefficient of thermal expansion intermediate between that of the piezoelectric substrate 2 and alumina. In FIG. 1, the bonding layer 4 will be described as an "alumina layer 4" made of alumina. The arithmetic mean roughness Ra of the third surface 3a of the sapphire substrate 3 is 0.1 μm or more and 0.5 μm or less, and the arithmetic mean roughness Ra of the fifth surface 4a which is the surface of the alumina layer 4 on the piezoelectric substrate 2 side is 0.1 μm or less and smaller than the arithmetic mean roughness Ra of the third surface 3a.

[0013] A piezoelectric element according to an embodiment of the present disclosure includes the composite substrate 1 according to an embodiment. Examples of the piezoelectric element include an oscillator used in an oscillation circuit or the like, and elastic wave elements such as surface acoustic wave elements, edge acoustic wave elements, and bulk wave elements used in a filter circuit or the like. As described above, the second surface 2b and the third surface 3a face each other via the alumina layer 4.

[0014] Hereinafter, the details of the composite substrate 1 according to an embodiment will be described. The composite substrate 1 according to an embodiment includes a piezoelectric substrate, a sapphire substrate 3, and an alumina layer 4. The piezoelectric substrate 2 has a first surface 2a which is an element formation surface, and a second surface 2b which is the back surface thereof and is joined to the alumina layer 4. The sapphire substrate 3 has a third surface 3a which is disposed opposite to the second surface 2b of the piezoelectric substrate 2, and a fourth surface 3b which is the back surface thereof. The alumina layer 4 has a fifth surface 4a which is in contact with the second surface 2b of the piezoelectric substrate 2, and a sixth surface 4b which is in contact with the third surface 3a of the sapphire substrate 3. The alumina layer 4 joins the piezoelectric substrate 2 and the sapphire substrate 3 without using an adhesive or the like.

[0015] Element electrodes are formed on the first surface 2a of the piezoelectric substrate 2, and it is used as the composite substrate 1 for piezoelectric elements such as surface acoustic wave elements. In the following description, for the sake of convenience, an example in which the piezoelectric substrate 2 is a substrate for an elastic surface wave element is described. The piezoelectric substrate 2 is not limited to this, and may be a substrate for other uses and functions such as a sensor substrate for a vibration sensor or the like or a substrate for a transmitter.

[0016] In the composite substrate 1 according to an embodiment, the first surface 2a is an element formation surface such as an element electrode, the second surface 2b and the third surface 3a are joining surfaces, and the fourth surface 3b is the back surface. The element formation surface is a portion where functional portions such as element electrodes are located as described above. The element electrodes are, for example, comb-shaped electrodes located so as to mesh with each other. Filtering of signals transmitted between the comb electrodes and the like is performed by surface acoustic waves between the comb electrodes.

[0017] Conventionally, in a surface acoustic wave element including a composite substrate, there has been a problem that noise called spurious occurs at a frequency higher than the passband (the frequency band in which a bandpass filter allows a signal to pass without attenuation). This noise is caused by the reflection of bulk waves at the bonding interface between the piezoelectric substrate 2 and the sapphire substrate 3 which is a support substrate. Although it is known to increase the surface roughness of the bonding surface in order to reduce the reflection of bulk waves, there is a problem that the bonding strength decreases when the surface roughness of the bonding surface is increased.

[0018] In a composite substrate 1 according to an embodiment, the arithmetic mean roughness Ra of the third surface 3a, which is the bonding surface of the sapphire substrate 3, is 0.1 μm or more and 0.5 μm or less, particularly preferably 0.1 μm or more and 0.3 μm or less. Therefore, a part of the bulk reaching the third surface 3a is absorbed or diffusely reflected, and the reflected bulk wave traveling toward the element formation surface 2a (that is, a functional part such as an element electrode) is reduced. As a result, spurious can be reduced.

[0019] Furthermore, in the composite substrate 1 according to an embodiment, the arithmetic mean roughness Ra of the fifth surface 4a, which is the surface of the alumina layer 4 on the piezoelectric substrate 2 side, is 0.1 μm or less, preferably 0.01 μm or less. Thereby, the bonding strength between the sapphire substrate 3 and the alumina layer 4 and the piezoelectric substrate 2 can be increased. Therefore, it is possible to provide the composite substrate 1 in which the bonding strength between the piezoelectric substrate 2 and the sapphire substrate 3 is high and the reflection of the bulk wave at the third surface 3a, which is the bonding surface, is reduced.

[0020] The arithmetic mean roughness Ra can be measured, for example, by a laser microscope, a stylus type surface shape measuring device, an atomic force microscope (AFM), or observation of a cross section of the joint part by a scanning electron microscope (SEM) or a transmission electron microscope (TEM). The measurement length is 5 μm or more, and measurement is performed at 5 or more points in the plane, and the average value thereof is taken as the measured value.

[0021] The piezoelectric substrate 2 is made of a material having piezoelectricity 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 element characteristics can be obtained. Also, when the arithmetic mean roughness Ra of the second surface 2b is 0.01 μm or less, the bonding strength with the alumina layer 4 increases.

[0022] Sapphire is single crystal alumina. In the sapphire substrate 3, the third surface 3a and the fourth surface 3b are specific crystal planes such as the c-plane, a-plane, m-plane, r-plane, or crystal planes having a predetermined off-angle with respect to these crystal planes. When the arithmetic mean roughness Ra of the fourth surface 3b of the sapphire substrate 3 is 1 μm or more, the reflection of the bulk wave at the fourth surface 3b can be reduced, which is effective for improving element characteristics.

[0023] The alumina layer 4 is also made of alumina, like the sapphire substrate 3. Therefore, compared with the case of using a bonding layer with different materials, residual stresses during bonding and the like caused by differences in physical properties such as the coefficient of thermal expansion and elastic modulus due to differences in materials can be reduced. When the alumina layer 4 is polycrystalline or amorphous, the reflection of bulk waves can be reduced because the regularity of the atomic arrangement is lower than that of a single crystal. Whether the alumina layer 4 is a single crystal, polycrystalline, or amorphous can be determined by methods such as X-ray diffraction and electron beam diffraction. From the viewpoints of bonding strength and reduction of reflected waves at the bonding part, the thickness of the alumina layer 4 is preferably 0.5 μm or more and 5 μm or less.

[0024] For the reduction of reflection of bulk waves and the like, it is conceivable to use a composite substrate including a sapphire substrate (not shown) whose surface portion on the piezoelectric substrate side is processed. However, the composite substrate 1 according to one embodiment is different from this. For example, when the third surface 3a of the sapphire substrate 3 is machined using a lapping device or the like, a machined affected layer with a large number of crystal defects is formed. Also, when atoms (or ions) are implanted into the sapphire substrate 3 from the third surface 3a, an ion (atom) implantation layer is formed. The alumina layer 4 of the present disclosure is different from the machined affected layer and the ion (atom) implantation layer in that the third surface 3a of the sapphire substrate 3 has an arithmetic mean roughness Ra of 0.1 μm or more and 0.5 μm or less, the third surface 3a is bonded to the alumina layer 4 (the sixth surface 4b), and the relatively smooth fifth surface 4a of the alumina layer 4 is bonded to the piezoelectric substrate 2.

[0025] Next, a composite substrate and a piezoelectric element according to another embodiment of the present disclosure will be described with reference to the drawings. FIG. 2 shows a schematic cross-sectional view of a composite substrate 1' according to another embodiment. The composite substrate 1' according to another embodiment includes a piezoelectric substrate 2' having a first surface 2a' which is an element formation surface and a second surface 2b' which is its back surface, a sapphire substrate 3' having a third surface 3a' disposed opposite to the second surface 2b', and a bonding layer 4' made of either the same material as the piezoelectric substrate 2', alumina, or an oxide having a coefficient of thermal expansion intermediate between that of the piezoelectric substrate 2' and alumina, for bonding 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 0.1 μm or more and 0.5 μm or less, and the arithmetic mean roughness Ra of the sixth surface 4b', which is the surface of the bonding layer 4' on the sapphire substrate 3' side, is 0.1 μm or less and smaller than the arithmetic mean roughness Ra of the second surface 2b'.

[0026] The piezoelectric element according to another embodiment of the present disclosure includes the composite substrate 1' according to another embodiment. The piezoelectric element is as described above, and a detailed description thereof will be omitted. The second surface 2b' and the third surface 3a' face each other via the bonding layer 4' as described above.

[0027] Hereinafter, details of the composite substrate 1' according to another embodiment will be described. The composite substrate 1' according to another embodiment includes a piezoelectric substrate 2', a sapphire substrate 3', and a bonding layer 4'. The piezoelectric substrate 2' has a first surface 2a' which is an element formation surface and a second surface 2b' which is its back surface and is bonded to the bonding layer 4'. The sapphire substrate 3' has a third surface 3a' disposed opposite to the second surface 2b' of the piezoelectric substrate 2' and a fourth surface 3b' which is 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.

[0028] Element electrodes are formed on the first surface 2a' of the piezoelectric substrate 2', and it is used as a composite substrate 1' for piezoelectric elements such as surface acoustic wave elements. In the following description, for convenience, an example in which the piezoelectric substrate 2' is a substrate for an elastic surface wave element is given for explanation. The piezoelectric substrate 2' is not limited to this, and may be a substrate for other uses or functions such as a substrate for sensors such as vibration sensors or a substrate for transmitters.

[0029] In the composite substrate 1' according to another embodiment, the first surface 2a' is an element formation surface such as an element electrode, the second surface 2b' and the third surface 3a' are bonding surfaces, and the fourth surface 3b' is a back surface. The element formation surface is a portion where functional parts such as element electrodes are located as described above. The element electrodes are, for example, comb-shaped electrodes positioned to mesh with each other. Filtering of signals transmitted between the comb electrodes is performed by surface acoustic waves between the comb electrodes.

[0030] Conventionally, in a surface acoustic wave element provided with a composite substrate, there has been a problem that noise called spurious occurs at a frequency higher than the passband (the frequency band in which a band-pass filter allows a signal to pass without attenuation). This noise is caused by the reflection of bulk waves at the bonding interface between the piezoelectric substrate 2' and the sapphire substrate 3' which is a support substrate. Although it is known to increase the surface roughness of the bonding surface in order to reduce the reflection of bulk waves, there is a problem that the bonding strength decreases when the surface roughness of the bonding surface is increased.

[0031] In the composite substrate 1' according to another embodiment, the arithmetic mean roughness Ra of the second surface 2b' which is the bonding surface of the piezoelectric substrate 2' is 0.1 μm or more and 0.5 μm or less, particularly preferably 0.1 μm or more and 0.3 μm or less. Therefore, a part of the bulk that reaches the second surface 2b' is absorbed or diffusely reflected, and the reflected bulk wave traveling toward the element formation surface 2a' (that is, functional parts such as element electrodes) is reduced. Thereby, spurious can be reduced.

[0032] Furthermore, in the composite substrate 1' according to another embodiment, 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', is 0.1 μm or less, preferably 0.01 μm or less. Thereby, the bonding strength between the piezoelectric substrate 2', the bonding layer 4', and the sapphire substrate 3' can be increased. Therefore, it is possible to provide a composite substrate 1' having high bonding strength between the piezoelectric substrate 2' and the sapphire substrate 3' and reducing the reflection of bulk waves at the second surface 2b' which is the bonding surface. The method for measuring the arithmetic mean roughness Ra is as described above, and detailed description thereof is omitted.

[0033] The piezoelectric substrate 2' is made of 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.

[0034] Sapphire is single crystal alumina. In the sapphire substrate 3', the third surface 3a' and the fourth surface 3b' are specific crystal planes such as the c-plane, a-plane, m-plane, r-plane, or crystal planes having a predetermined off-angle with respect to these crystal planes. When the arithmetic mean roughness Ra of the third surface 3a' of the sapphire substrate 3' is 0.01 μm or less, the bonding strength with the bonding layer 4' increases. Also, when the arithmetic mean roughness Ra of the fourth surface 3b' of the sapphire substrate 3' is 1 μm or more, the reflection of bulk waves at the fourth surface 3b' can be reduced, which is effective for improving device characteristics.

[0035] The bonding layer 4' is made of either the same material as the piezoelectric substrate 2', alumina, or an oxide having a coefficient of thermal expansion intermediate between that of the piezoelectric substrate 2' and alumina. If the bonding layer 4' is the same material as the piezoelectric substrate 2', the thermal stress and thermal strain generated according to the formation temperature (for example, several hundred degrees Celsius) when forming the bonding layer 4' on the piezoelectric substrate 2' can be reduced. On the other hand, if the bonding layer 4' is alumina, like the sapphire substrate 3', the thermal stress and thermal strain generated according to the bonding temperature (for example, from several tens of degrees Celsius to about 150 degrees Celsius) when bonding the bonding layer 4' and the sapphire substrate 3' can be reduced. Further, if the bonding layer 4' is an oxide having a coefficient of thermal expansion intermediate between that of the piezoelectric substrate 2' and alumina, the thermal stress and thermal strain generated during the formation of the bonding layer 4' or during bonding with the sapphire substrate 3' can be reduced.

[0036] The coefficient of thermal expansion of a single crystal material varies depending on the crystal orientation. The coefficient of thermal expansion of lithium niobate is 7.5 - 15 ppm / °C, and that of lithium tantalate is 4 - 16 ppm / °C. For the X-axis of a lithium tantalate substrate rotated at an angle of 36° - 46° from the Y-axis around the X-axis, that is, in the direction of elastic surface wave propagation, it is about 16 ppm / °C. Also, the coefficient of thermal expansion of sapphire is 7.0 - 7.7 ppm / °C. For example, if the coefficients of thermal expansion of the piezoelectric substrate 2' and the sapphire substrate 3' are 16 ppm / °C and 7 ppm, respectively, a material having a coefficient of thermal expansion of 7 - 16 ppm / °C may be used as the bonding layer 4'. Examples of such materials having such a coefficient of thermal expansion include silica (the coefficient of thermal expansion of single crystal silica is 7.5 - 14 ppm / °C).

[0037] If the bonding layer 4' is polycrystalline or amorphous, the reflection of bulk waves can be reduced because the regularity of the atomic arrangement is lower compared to that of a single crystal. Whether the bonding layer 4' is a single crystal, polycrystalline, or amorphous can be determined by methods such as X-ray diffraction and electron beam diffraction. From the viewpoints of bonding strength and reduction of reflected waves at the bonding part, the thickness of the bonding layer 4' is preferably 0.5 μm or more and 5 μm or less.

[0038] In order to reduce the reflection of bulk waves, etc., it is conceivable to use a composite substrate including a sapphire substrate (not shown) whose surface portion on the piezoelectric substrate side is processed. However, the composite substrate 1' according to other embodiments is different from this. When atoms (or ions) are implanted into the sapphire substrate 3' from the third surface 3a', an ion (atom) implantation layer is formed. The bonding layer 4' of the present disclosure has the second surface 2b of the piezoelectric substrate 2 with an arithmetic mean roughness Ra of 0.1 μm or more and 0.5 μm or less, and the second surface 2b is bonded to the bonding layer 4' (the fifth surface 4a'). Further, the relatively smooth sixth surface 4b of the bonding layer 4' is bonded to the sapphire substrate 3'. In these respects, it is different from the processed altered layer and the ion (atom) implantation layer.

[0039] <Method for manufacturing a composite substrate> Next, a method for manufacturing a composite substrate according to an embodiment of the present disclosure will be described with reference to the drawings. FIG. 3 shows a schematic explanatory diagram of a method for manufacturing a composite substrate according to an embodiment. The method for manufacturing a composite substrate according to an embodiment includes the following steps (1) to (4). By the following steps (1) to (4), for example, a composite substrate 1 according to an embodiment as shown in FIG. 1 can be manufactured. The composite layer 4 described in steps (3) and (4) below is made of any one of the same material as the piezoelectric substrate 2, alumina, and an oxide having a coefficient of thermal expansion intermediate between the piezoelectric substrate 2 and alumina. (1) Preparation step of preparing a piezoelectric substrate 2 having a first surface 2a which is an element formation surface and a second surface 2b which is the back surface thereof, and a sapphire substrate 3 having a third surface 3a and a fourth surface 3b which is the back surface thereof. (2) Roughening step of processing the third surface 3a of the sapphire substrate 3 so that the arithmetic mean roughness Ra becomes 0.1 μm or more and 0.5 μm or less. (3) Bonding layer forming step of forming a bonding layer 4 on the roughened third surface 3a and processing the exposed surface 5a which is the surface of the bonding layer 4 located on the side opposite to the sapphire substrate 3 so that the arithmetic mean roughness Ra is 0.1 μm or less and smaller than the arithmetic mean roughness Ra of the third surface 3a. (4) Bonding step of directly bonding the fifth surface 4a of the bonding layer 4 and the second surface 2b of the piezoelectric substrate 2.

[0040] Hereinafter, the details of the method for manufacturing the composite substrate of the present disclosure will be described. In FIG. 3, the bonding layer 4 will be described as an "alumina layer 4" made of alumina. First, a piezoelectric substrate 2 having opposing first and second surfaces 2a and 2b, and a sapphire substrate 3 having opposing third and fourth surfaces 3a and 3b are prepared. The first surface 2a is the element formation surface, the second surface 2b and the third surface 3a are the bonding surfaces, and the fourth surface 3b is the back surface.

[0041] The sapphire substrate 3 is formed by cutting an ingot-shaped or ribbon-shaped sapphire crystal grown by an appropriate growth method such as the Czochralski method, so that the third surface 3a and the fourth surface 3b are specific crystal surfaces such as the c-plane, a-plane, m-plane, r-plane, or have a predetermined off-angle with respect to these crystal surfaces.

[0042] The second surface 2b of the piezoelectric substrate 2 and the third surface 3a of the sapphire substrate 3 are planarized by lapping or the like using a surface plate made of copper, tin, iron, etc. and abrasive grains such as diamond, silicon carbide, and boron carbide. When the arithmetic mean roughness Ra of the third surface 3a is processed to be 0.1 μm or more and 0.5 μm or less, particularly preferably 0.1 μm or more and 0.3 μm or less, the reflection of the bulk wave can be reduced and the bonding strength can also be increased. If the arithmetic mean roughness Ra of the second surface 2b is 0.01 μm or less, the bonding strength with the alumina layer 4 can be increased.

[0043] Next, an alumina layer 4 is formed on the third surface 3a of the sapphire substrate 3. The alumina layer 4 can be formed by, for example, PVD methods such as evaporation and sputtering, CVD methods such as metalorganic chemical vapor deposition. The arithmetic mean roughness Ra of the fifth surface 4a, which is the surface of the alumina layer 4, is polished to a desired value (for example, about 0.01 μm) of 0.1 μm or less by chemical mechanical polishing (CMP) using silica particles and an alkaline aqueous solution. Note that if the arithmetic mean roughness Ra of the fifth surface 4a of the as-grown alumina layer 4 after film formation is a desired value of 0.1 μm or less (for example, 0.01 μm or less) by optimizing the formation method and formation conditions of the alumina layer 4, the polishing process may be omitted.

[0044] Next, at least one of the second surface 2b of the piezoelectric substrate 2 and the fifth surface 4a of the alumina layer 4 is activated by a method such as plasma treatment. The alumina layer 4 and the piezoelectric substrate 2 are joined by direct bonding without using an adhesive material. For example, the piezoelectric substrate 2 and the sapphire substrate 3 on which the alumina layer 4 is formed are heated and / or pressurized in a vacuum, in the atmosphere, or in a predetermined atmosphere to diffuse the atoms at the bonding interface for diffusion bonding. The previous activation treatment can lower the temperature during bonding. Therefore, it is possible to reduce the causes of breakage and poor processing accuracy due to the difference in the coefficient of thermal expansion between the piezoelectric substrate 2 and the sapphire substrate 3.

[0045] In the direct bonding between the piezoelectric substrate 2 and the alumina layer 4, diffusion bonding by the diffusion of atoms between the piezoelectric substrate 2 and the alumina film 4 is used. When the arithmetic mean roughness Ra of the second surface 2b of the piezoelectric substrate 2 and the fifth surface 4a of the alumina layer 4 is 0.1 μm or less, the bonding strength is improved.

[0046] After joining the piezoelectric substrate 2 and the sapphire substrate 3, the thickness of the sapphire substrate 3 may be reduced using a lapping device or the like. In this case, the sapphire substrate 3 is removed by the above processing from the fourth surface 3b side. The thickness of the piezoelectric substrate 2 may be reduced using a lapping device or the like. It is preferable that 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 becomes 1 nm or less.

[0047] Hereinafter, a method for manufacturing a composite substrate according to another embodiment of the present disclosure will be described with reference to the drawings. FIG. 4 shows a schematic explanatory diagram of a method for manufacturing a composite substrate according to another embodiment. The method for manufacturing a composite substrate according to another embodiment includes the following steps (5) to (8). By the following steps (5) to (8), for example, a composite substrate 1' according to another embodiment as shown in FIG. 2 can be manufactured. (5) A preparation step of preparing a piezoelectric substrate 2' having a first surface 2a' which is an element formation surface and a second surface 2b' which is the back surface thereof, and a sapphire substrate 3' having a third surface 3a' and a fourth surface 3b' which is the back surface thereof. (6) A roughening step of processing the second surface 2b' of the piezoelectric substrate 2' so that the arithmetic mean roughness Ra is 0.1 μm or more and 0.5 μm or less. (7) A bonding layer forming step of forming a bonding layer 4' made of any one of the same material as the piezoelectric substrate 2', alumina, and an oxide having a thermal expansion coefficient intermediate between the piezoelectric substrate 2' and alumina on the roughened second surface 2b', and processing the sixth surface 4b', which is the exposed surface located on the side opposite to the piezoelectric substrate 2' of the bonding layer 4', so that the arithmetic mean roughness Ra is 0.1 μm or less and smaller than the arithmetic mean roughness Ra of the second surface 2b'. (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'.

[0048] Regarding the details of the method for manufacturing a composite substrate according to another embodiment, the case where the piezoelectric substrate 2' is lithium tantalate and the bonding layer 4' is alumina will be described. First, a piezoelectric substrate 2' made of a lithium tantalate single crystal having opposing first surface 2a' and second surface 2b', and a sapphire substrate 3' having opposing third surface 3a' and fourth surface 3b' are prepared. The first surface 2a' is the element formation surface, the second surface 2b' and the third surface 3a' are the bonding surfaces, and the fourth surface 3b' is the back surface.

[0049] The sapphire substrate 3' is produced by cutting an ingot-shaped or ribbon-shaped sapphire crystal grown by an appropriate growth method such as the Czochralski method so that the third surface 3a' and the fourth surface 3b' have a specific crystal plane such as a c-plane, a-plane, m-plane, r-plane, or a predetermined off-angle with respect to these crystal planes.

[0050] The second surface 2b' of the piezoelectric substrate 2' and the third surface 3a' of the sapphire substrate 3' are planarized by lapping using a surface plate made of copper, tin, iron, etc. and abrasive grains such as diamond, silicon carbide, and boron carbide. When the arithmetic mean roughness Ra of the second surface 2b' is processed to be 0.1 μm or more and 0.5 μm or less, particularly preferably 0.1 μm or more and 0.3 μm or less, the reflection of the bulk wave can be reduced and the bonding strength can also be increased. The arithmetic mean roughness Ra of the third surface 3a' may be 0.01 μm or less so that the bonding strength with the bonding layer 4' can be increased.

[0051] Next, a bonding layer 4' made of alumina is formed on the second surface 2b' of the piezoelectric substrate 2'. The bonding layer 4' can be formed, for example, by PVD methods such as vapor deposition and sputtering, CVD methods such as metalorganic chemical vapor deposition, etc. By chemical mechanical polishing (CMP) using silica particles and an alkaline aqueous solution, etc., the arithmetic mean roughness Ra of the sixth surface 4b' which is the surface of the bonding layer 4' is polished to a desired value of 0.1 μm or less (for example, about 0.01 μm). By optimizing the formation method and formation conditions of the bonding layer 4', if 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 0.1 μm or less (for example, 0.01 μm or less), the polishing process may be omitted.

[0052] 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 subjected to an activation treatment 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' on which the bonding layer 4' is formed are heated and / or pressurized in a vacuum, in the air, or in a predetermined atmosphere to diffuse the atoms at the bonding interface for diffusion bonding. Due to the previous activation treatment, the temperature during bonding can be lowered. Therefore, it is possible to reduce the causes of breakage and poor processing accuracy due to the difference in the thermal expansion coefficients between the piezoelectric substrate 2' and the sapphire substrate 3'.

[0053] In the direct bonding between the sapphire substrate 3' and the bonding layer 4', diffusion bonding due to the diffusion of atoms between the sapphire substrate 3' and the bonding layer 4' is used. When the arithmetic mean roughness Ra between 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.

[0054] After bonding the piezoelectric substrate 2' and the sapphire substrate 3', the thickness of the sapphire substrate 3' may be reduced using a lapping device or the like. In this case, the sapphire substrate 3' is removed by the above processing from the fourth surface 3b' side. The thickness of the piezoelectric substrate 2' may be reduced using a lapping device or the like. It is preferable that 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 1 nm or less. Even when the bonding layer 4' is made of lithium tantalate or silica, the bonding layer can be formed in the same manner as above.

[0055] As described above, the embodiments of the present disclosure have been described. However, the present disclosure is not limited to the above-described embodiments, and various improvements and modifications may be made without departing from the gist of the present disclosure.

Example

[0056] Hereinafter, the present disclosure will be described in detail with reference to examples. However, the present disclosure is not limited to the following examples.

[0057] A plurality of LT substrates 2 were prepared as piezoelectric substrates, and a plurality of sapphire substrates 3 were prepared as support substrates. Then, using a lapping device, the second surface 2b of the LT substrate 2 was processed so that the arithmetic mean roughness Ra was 0.01 μm, and the third surface 3a of the sapphire substrate 3 was processed to have four levels of arithmetic mean roughness Ra of 0.02 μm (condition 1), 0.1 μm (condition 2), 0.5 μm (condition 3), and 2.5 μm (condition 4). Then, an alumina layer 4 was formed to a thickness of about 1 μm by vapor deposition on the processed third surface 3a, and the fifth surface 4a of the alumina layer 4 was processed using a CMP device so that the arithmetic mean roughness Ra was 0.01 μm. The second surface 2b of the piezoelectric substrate 2 and the fifth surface 4a of the alumina layer 4 were activated and bonded by argon plasma to produce a composite substrate 1.

[0058] Among conditions 1 to 4, in condition 4, sufficient bonding strength (1.0 N / m 2The above could not be obtained. Furthermore, when a surface acoustic wave device was fabricated using the composite substrate 1 under Conditions 1 to 3, the spurious reduction effect due to the roughening of the third surface 3a was small under Condition 1, whereas good device characteristics were obtained under Conditions 2 and 3.

[0059] A plurality of LT substrates 2' were prepared as piezoelectric substrates, and a plurality of sapphire substrates 3' were prepared as support substrates. Then, using a lapping device, the second surface 2b' of the LT substrate 2' was processed to have four levels of arithmetic mean roughness Ra of 0.02 μm (Condition 1), 0.1 μm (Condition 2), 0.5 μm (Condition 3), and 2.5 μm (Condition 4), and the third surface 3a' of the sapphire substrate 3' was processed to have an arithmetic mean roughness Ra of 0.01 μm. Then, a bonding layer 4' made of alumina was formed on the processed second surface 2b' by vapor deposition to a thickness of about 1 μm, and the sixth surface 4b' of the bonding layer 4' was processed to have an arithmetic mean roughness Ra of 0.01 μm using a CMP device. Then, the third surface 3a' of the sapphire substrate 3' and the sixth surface 4b' of the bonding layer 4' were activated and bonded by argon plasma to fabricate a composite substrate 1'.

[0060] Among Conditions 1 to 4, under Condition 4, sufficient bonding strength (1.0 N / m 2 The above) could not be obtained. Furthermore, when a surface acoustic wave device was fabricated using the composite substrate 1' under Conditions 1 to 3, the spurious reduction effect due to the roughening of the second surface 2b' was small under Condition 1, whereas good device characteristics were obtained under Conditions 2 and 3.

Explanation of Reference Numerals

[0061] 1, 1': Composite substrate 2, 2': Piezoelectric substrate 2a, 2a': First surface (element formation surface) 2b, 2b': Second surface (back surface of piezoelectric substrate) 3, 3': Sapphire substrate 3a, 3a': Third surface (bonding surface of sapphire substrate) 3b, 3b': Fourth surface (back surface of composite substrate) 4: Bonding layer 4a: Fifth surface (surface of bonding layer on piezoelectric substrate side) 4b: The 6th surface (the sapphire substrate side surface of the bonding layer) 4’: Alumina layer (bonding layer) 4a’: The 5th surface (the piezoelectric substrate side surface of the alumina layer) 4b’: The 6th surface (the sapphire substrate side surface of the alumina layer)

Claims

1. A piezoelectric substrate having a first surface which is an element formation surface and a second surface which is the back surface thereof, A sapphire substrate having a third surface disposed to face the second surface and a fourth surface which is the back surface thereof, 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, Comprising: The bonding layer is made of alumina, The third surface is a crystal plane of the r-plane, The arithmetic mean roughness Ra of the third surface is 0.1 μm or more and 0.5 μm or less, and the arithmetic mean roughness Ra of the fifth surface is 0.1 μm or less and smaller than the arithmetic mean roughness Ra of the third surface. A composite substrate for a surface acoustic wave device.

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 0.5 μm or more and 5.0 μm or less.

4. A surface acoustic wave device comprising the composite substrate according to any one of claims 1 to 3 and a comb-shaped electrode located on the first surface.

5. A preparation step of preparing a piezoelectric substrate having a first surface which is an element formation surface and a second surface which is the back surface thereof, and a sapphire substrate having a third surface and a fourth surface which is the back surface thereof, A roughening step of processing the third surface so that the arithmetic mean roughness Ra is 0.1 μm or more and 0.5 μm or less, A bonding layer forming step of forming a bonding layer on the roughened third surface and processing the fifth surface, which is an exposed surface located on the side opposite to the sapphire substrate of the bonding layer, so that the arithmetic mean roughness Ra is 0.1 μm or less and smaller than the arithmetic mean roughness Ra of the third surface, A bonding step of directly bonding the fifth surface of the bonding layer and the second surface of the piezoelectric substrate, Comprising: The bonding layer is made of alumina, The third surface is a crystal plane of the r-plane, A method for manufacturing a composite substrate for a surface acoustic wave device.

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