Composite substrate, surface acoustic wave element, and method of manufacturing the composite substrate

Abstract

The objective of the present invention is to provide a composite substrate having excellent durability. A composite substrate according to an embodiment of the present invention includes, in the stated order, a piezoelectric layer, a reflective layer including a low-impedance layer and a high-impedance layer containing silicon oxide, and a support substrate, wherein a density of the low-impedance layer is at most equal to 2.4 g / cm3, and an amorphous region is formed in the high-impedance layer.

Description

[Technical Field]

[0001] The present invention relates to a composite substrate, a surface acoustic wave element, and a method for manufacturing a composite substrate. [Background technology]

[0002] In communication devices such as mobile phones, filters that utilize surface acoustic waves (SAW filters) are used to extract electrical signals of any frequency. These SAW filters have a structure in which electrodes and the like are formed on a composite substrate having a piezoelectric layer (see, for example, Patent Document 1).

[0003] In recent years, in the field of information and communication devices, there has been a demand for communication in high frequency bands, and in the SAW filter, there is a possibility that elastic waves may leak from the piezoelectric layer. On the other hand, durability (specifically, bonding strength) is also required for the composite substrate. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2020-150488 Summary of the Invention [Problem to be solved by the invention]

[0005] A primary object of the present invention is to provide a composite substrate that is highly durable while confining the energy of elastic waves in the piezoelectric layer. [Means for solving the problem]

[0006] 1. A composite substrate according to an embodiment of the present invention includes a piezoelectric layer, a reflective layer including a low-impedance layer and a high-impedance layer containing silicon oxide, and a support substrate, in this order, and the density of the low-impedance layer is 2.4 g / cm 3 and an amorphous region is formed in the high impedance layer. 2. In the composite substrate described in 1 above, the amorphous region may be formed at an end portion in the thickness direction of the high-impedance layer. 3. In the composite substrate described in 2 above, the amorphous region may be formed on the piezoelectric layer side of the high impedance layer. 4. In the composite substrate according to any one of 1 to 3 above, the reflective layer may include a plurality of high impedance layers, and the amorphous region may be formed in at least the high impedance layer located closest to the support substrate. 5. In the reflective layer of the composite substrate according to any one of 1 to 4 above, the high-impedance layers and the low-impedance layers may be laminated alternately. 6. In the composite substrate according to any one of the above items 1 to 5, the reflective layer and the support substrate may be disposed adjacent to each other. 7. In the composite substrate according to any one of 1 to 6 above, the high impedance layer may contain at least one selected from the group consisting of hafnium oxide, tantalum oxide, zirconium oxide, and aluminum oxide. 8. In the composite substrate according to any one of 1 to 7 above, the high-impedance layer and the low-impedance layer may each have a thickness of 0.01 μm to 1 μm. 9. In the composite substrate according to any one of 1 to 8 above, the amorphous region may have an average thickness of 10 nm or more. 10. A surface acoustic wave element according to another embodiment of the present invention comprises the composite substrate according to any one of 1 to 9 above.

[0007] 11. According to another aspect of the present invention, there is provided a method for producing a composite substrate, the method comprising the steps of: forming a piezoelectric substrate and a support substrate, at least one of which contains silicon oxide and has a density of 2.4 g / cm 3 ; 3The method includes depositing a low-impedance layer as follows: depositing a high-impedance layer having an amorphous region on the substrate on which the low-impedance layer has been deposited; bonding the piezoelectric substrate and the support substrate together, and forming a reflective layer including the low-impedance layer and the high-impedance layer between the piezoelectric substrate and the support substrate; and the bonding is performed while the piezoelectric substrate and the support substrate are placed in a vacuum atmosphere. [Effects of the Invention]

[0008] According to the embodiment of the present invention, a composite substrate having excellent durability can be obtained. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a schematic cross-sectional view showing an outline of the configuration of a composite substrate according to one embodiment of the present invention. [Figure 2A] 1A to 1C are diagrams illustrating an example of a manufacturing process for a composite substrate according to one embodiment. [Figure 2B] This is a continuation of Figure 2A. [Figure 2C] This is a continuation of Figure 2B. [Figure 3A] 1 is a photograph showing a cross section of a hafnium oxide layer in Example 1. [Figure 3B] FIG. 3B is a crystalline layer map of the cross-sectional observation photograph shown in FIG. 3A. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to these embodiments. In addition, in order to clarify the description, the drawings may show the width, thickness, shape, etc. of each part more schematically than in the embodiments, but these are merely examples and do not limit the interpretation of the present invention.

[0011] A. Composite substrate FIG. 1 is a schematic cross-sectional view showing the overall configuration of a composite substrate according to one embodiment of the present invention. The composite substrate 100 includes a piezoelectric layer 10, a reflective layer 20, and a support substrate 30, in this order. The reflective layer 20 includes a high-impedance layer with a relatively high acoustic impedance and a low-impedance layer with a relatively low acoustic impedance. The reflective layer 20 is a laminate of multiple impedance layers, for example, with the high-impedance layers and the low-impedance layers alternately stacked. In the illustrated example, the reflective layer 20 includes, from the piezoelectric layer 10 side, a low-impedance layer 21, a high-impedance layer 22, a low-impedance layer 23, a high-impedance layer 24, a low-impedance layer 25, a high-impedance layer 26, a low-impedance layer 27, and a high-impedance layer 28, in this order. In the illustrated example, of the layers of the reflective layer 20, the low-impedance layer 21 is positioned closest to the piezoelectric layer 10, and the high-impedance layer 28 is positioned closest to the support substrate 30. By arranging the reflective layer 20 with such a laminated structure, the energy of the acoustic wave can be effectively confined to the piezoelectric layer 10 side.

[0012] In the illustrated example, the reflective layer 20 is a laminate of eight layers, including four high-impedance layers and four low-impedance layers, but the number of impedance layers included in the reflective layer is not limited to this. Specifically, the reflective layer may include at least one high-impedance layer and one low-impedance layer with different acoustic impedances. Preferably, the reflective layer has a multilayer structure of four or more layers.

[0013] An amorphous region may be formed in at least one of the high-impedance layers included in the reflective layer 20. In the illustrated example, an amorphous region 28a is formed in the high-impedance layer 28 located closest to the support substrate 30. By including a high-impedance layer with an amorphous region formed therein, the composite substrate may have excellent bonding strength. This may also contribute to improving the reflection characteristics.

[0014] Although not shown, the composite substrate 100 may further include any layer. The type, function, number, combination, and arrangement of such layers may be appropriately determined depending on the purpose. For example, the composite substrate 100 may include a bonding layer disposed between the piezoelectric layer 10 or the reflective layer 20 and the support substrate 30.

[0015] The composite substrate 100 can be manufactured in any suitable shape. In one embodiment, it can be manufactured in the form of a so-called wafer. The size of the composite substrate 100 can be appropriately set depending on the purpose. For example, the diameter of the wafer is 50 mm to 150 mm.

[0016] A-1. Piezoelectric layer Any appropriate piezoelectric material can be used as the material constituting the piezoelectric layer. A single crystal having the composition LiAO3 is preferably used as the piezoelectric material. Here, A is one or more elements selected from the group consisting of niobium and tantalum. Specifically, LiAO3 may be lithium niobate (LiNbO3), lithium tantalate (LiTaO3), or a lithium niobate-lithium tantalate solid solution.

[0017] When the piezoelectric material is lithium tantalate, it is preferable to use a piezoelectric layer whose normal direction is rotated 123 to 133° (for example, 128°) from the Y axis to the Z axis around the X axis, which is the propagation direction of the surface acoustic wave, from the viewpoint of reducing propagation loss.When the piezoelectric material is lithium niobate, it is preferable to use a piezoelectric layer whose normal direction is rotated 96 to 114° (for example, 110°) from the Y axis to the Z axis around the X axis, which is the propagation direction of the surface acoustic wave, from the viewpoint of reducing propagation loss.

[0018] The thickness of the piezoelectric layer is, for example, 0.2 μm or more and 30 μm or less, and preferably 0.2 μm or more and 5 μm or less.

[0019] A-2.Reflective layer As described above, the reflective layer includes a high-impedance layer and a low-impedance layer having different acoustic impedances. The acoustic impedance of the high-impedance layer is relatively higher than the acoustic impedance of the low-impedance layer. Specifically, the acoustic impedance of the material constituting the high-impedance layer is higher than the acoustic impedance of the material constituting the low-impedance layer.

[0020] The plurality of low-impedance layers that may be included in the reflective layer may each have the same configuration (e.g., material, thickness, density) or may have different configurations. Similarly, the plurality of high-impedance layers that may be included in the reflective layer may each have the same configuration (e.g., material, thickness, density) or may have different configurations. For example, amorphous regions may be formed in all of the high-impedance layers that may be included in the reflective layer, or amorphous regions may be selectively formed in some of the high-impedance layers.

[0021] A representative example of a material constituting the low-impedance layer is silicon oxide. In one embodiment, the content of silicon oxide in the low-impedance layer is, for example, 97% by weight or more. The ratio of oxygen atoms to silicon atoms (O / Si) in the low-impedance layer is, for example, 1.80 or more and 2.05 or less. The composition of the low-impedance layer can be confirmed by Rutherford backscattering spectroscopy (RBS). Note that, for the analysis, a sample obtained by forming a low-impedance layer on a suitable substrate under the same conditions can be used.

[0022] The thickness of the low-impedance layer is, for example, 0.01 μm to 1 μm, preferably 20 nm to 500 nm, and more preferably 100 nm to 300 nm. When the reflective layer includes multiple low-impedance layers, the thickness of the low-impedance layer refers to the thickness of each low-impedance layer. The low-impedance layer typically has a region with a granular structure.

[0023] The density of the low impedance layer is 2.4 g / cm 3It is preferable that the concentration is 2.35 g / cm or less, and more preferably 2.35 g / cm 3 A low-impedance layer with such a density can further increase the difference in acoustic impedance with the high-impedance layer, and more effectively confine the energy of the elastic waves to the piezoelectric layer side. On the other hand, a low-impedance layer with such a density is prone to absorb moisture. Specifically, it tends to absorb moisture from the atmosphere during film formation. The density of the low-impedance layer is typically 2.1 g / cm 3 That's all.

[0024] It is sufficient that at least one low-impedance layer included in the reflective layer satisfies the above density, but it is preferable that all low-impedance layers included in the reflective layer satisfy the above density.

[0025] The density of the impedance layer can be determined by X-ray reflectometry (XRR).

[0026] Examples of materials constituting the high-impedance layer include hafnium oxide, tantalum oxide, zirconium oxide, and aluminum oxide. Among these, hafnium oxide is preferably used. By using hafnium oxide, the energy of the elastic wave can be more effectively trapped on the piezoelectric layer side. In one embodiment, the content of hafnium oxide in the high-impedance layer is, for example, 97% by weight or more.

[0027] The thickness of the high-impedance layer is, for example, 0.01 μm to 1 μm, preferably 20 nm to 500 nm, and more preferably 100 nm to 300 nm. When the reflective layer includes multiple high-impedance layers, the thickness of the high-impedance layer refers to the thickness of each high-impedance layer.

[0028] The thickness of the amorphous region that can be formed in the high-impedance layer is, for example, 5 nm or more, and preferably 10 nm or more. On the other hand, the thickness of the amorphous region is, for example, 70 nm or less. The amorphous region may be formed in at least a portion in a plan view from the main surface of the substrate. Specifically, the amorphous region may be divided or may have a non-existent region that is thinner than other portions. Preferably, the amorphous region is formed over the entire surface in a plan view from the main surface of the substrate.

[0029] The presence or absence of an amorphous region may be determined based on its average thickness. For example, if the average thickness is 10 nm or more, it can be determined that an amorphous region is formed. The method for calculating the average thickness will be described in detail later.

[0030] The position of the amorphous region in one high-impedance layer is not particularly limited, but typically, the amorphous region is formed at the end in the thickness direction. In the formation of the impedance layer described below, the high-impedance layer tends to be amorphous in the early stages of formation. For example, when forming a high-impedance layer on a piezoelectric layer (a piezoelectric substrate described below), the amorphous region may be formed at the end on the piezoelectric layer side.

[0031] In the region of the high-impedance layer other than the amorphous region, for example, a columnar structure or a granular structure is formed. Here, the columnar structure is composed of structures (columnar bodies) extending in a direction at an angle to the substrate surface (in-plane direction) of the composite substrate, and the diameter of the columns is, for example, 5 nm or more. The granular structure is composed of approximately spherical structures. Such structures can be confirmed, for example, by observation with a transmission electron microscope (TEM). Note that the column diameter does not have to be satisfied at all positions in the film thickness direction of the observed columns.

[0032] In one embodiment, the area of the columnar structure in one high-impedance layer is, for example, 70% or more, preferably 80% or more, and more preferably 90% or more. It is believed that moisture contained in the low-impedance layer can move between the structures that make up the high-impedance layer. It is believed that moisture can move easily between the columns, and that the effect of forming the amorphous region can be significantly obtained.

[0033] The impedance layer can be deposited by any suitable method, such as sputtering, physical vapor deposition such as ion beam assisted deposition (IAD), chemical vapor deposition, or atomic layer deposition (ALD).

[0034] A-3.Support board Any suitable substrate can be used as the support substrate. The support substrate may be composed of a single crystal or a polycrystalline body. The material constituting the support substrate is preferably selected from the group consisting of silicon, sialon, sapphire, cordierite, mullite, glass, quartz, quartz crystal, and alumina.

[0035] The silicon may be single crystal silicon, polycrystalline silicon, or high-resistivity silicon.

[0036] Typically, the sialon is a ceramic obtained by sintering a mixture of silicon nitride and alumina. For example, Si 6-w Al w O w N 8-w Specifically, sialon has a composition in which alumina is mixed into silicon nitride, and w in the formula indicates the mixing ratio of alumina. w is preferably 0.5 or more and 4.0 or less.

[0037] Typically, the sapphire is a single crystal having a composition of Al2O3, and the alumina is a polycrystalline material having a composition of Al2O3. The alumina is preferably translucent alumina.

[0038] Typically, the cordierite is a ceramic having a composition of 2MgO·2Al2O3·5SiO2, and the mullite is a ceramic having a composition in the range of 3Al2O3·2SiO2 to 2Al2O3·SiO2.

[0039] The thermal expansion coefficient of the material constituting the support substrate is preferably smaller than that of the material constituting the piezoelectric layer. Such a support substrate can suppress changes in the shape and size of the piezoelectric layer when the temperature changes, thereby suppressing changes in the frequency characteristics of the resulting surface acoustic wave element.

[0040] The thickness of the support substrate can be any appropriate thickness, and is, for example, 100 μm to 1000 μm.

[0041] A-4.Joining layer As described above, the composite substrate may have a bonding layer. By providing the bonding layer, the bonding strength of the composite substrate can be improved. Examples of materials that form the bonding layer include silicon oxide (Si (1-x) O x ), silicon, tantalum oxide, niobium oxide, aluminum oxide, titanium oxide, and hafnium oxide. (1-x) O x In the formula, x preferably satisfies the relationship 0.008≦x≦0.408. The thickness of the bonding layer is, for example, 0.005 μm to 1 μm.

[0042] The bonding layer can be formed by any suitable method, specifically, by the same method as the method for forming the impedance layer.

[0043] In one embodiment, the composite substrate does not have a bonding layer. For example, the reflective layer is disposed adjacent to the support substrate and the piezoelectric layer. Alternatively, for example, the reflective layer does not include a bonding layer. Even in such a configuration, the composite substrate can have excellent bonding strength by providing a high-impedance layer in which the amorphous region is formed. Furthermore, by not having a bonding layer, the process of forming the bonding layer and the cost of forming the layer can be reduced.

[0044] A-5. Manufacturing method A method for manufacturing a composite substrate according to one embodiment of the present invention includes forming a low-impedance layer on at least one of a piezoelectric substrate and a support substrate, forming a high-impedance layer having an amorphous region on the substrate on which the low-impedance layer has been formed, and bonding the piezoelectric substrate and the support substrate to form a reflective layer including the low-impedance layer and the high-impedance layer between the piezoelectric substrate and the support substrate.

[0045] 2A to 2C are diagrams illustrating an example of a manufacturing process for a composite substrate according to one embodiment. Fig. 2A illustrates a state in which low-impedance layers 21, 23, 25, and 27 and high-impedance layers 22, 24, 26, and 28 are deposited on first main surface 12a of piezoelectric substrate 12, which has first and second main surfaces 12a and 12b facing each other, thereby completing reflective layer 20. Amorphous region 28a is formed in high-impedance layer 28, which is located farthest from piezoelectric substrate 12 (preferably, on the outermost surface). Amorphous region 28a can be favorably formed in the initial stage of deposition of high-impedance layer 28, and therefore, in the illustrated example, it is located at the end on the piezoelectric substrate 12 side.

[0046] FIG. 2B shows a process of directly bonding the piezoelectric substrate 12 on which the reflective layer 20 is formed to the support substrate 30. For direct bonding, the bonding surfaces are preferably activated by any appropriate activation process. For example, the surface 20a of the reflective layer 20 is activated, and the surface 30a of the support substrate 30 is activated. Then, the activated surfaces of the reflective layer 20 and the support substrate 30 are brought into contact with each other and pressure is applied to directly bond them. In this manner, the composite substrate 110 shown in FIG. 2C is obtained. Typically, the second main surface 12b of the piezoelectric substrate 12 of the obtained composite substrate 110 is subjected to processing such as grinding and polishing to form a piezoelectric layer having the desired thickness.

[0047] The activation process is typically performed by irradiating a neutralizing beam. Preferably, a neutralizing beam is generated using an apparatus such as that described in JP 2014-086400 A, and the activation process is performed by irradiating this beam. Specifically, a saddlefield-type fast atom beam source is used as the beam source, an inert gas such as argon or nitrogen is introduced into a chamber, and a high voltage is applied to the electrodes from a DC power supply. This generates a saddlefield-type electric field between the electrode (positive electrode) and the housing (negative electrode), causing electrons to move, generating a beam of atoms and ions from the inert gas. Of the beams that reach the grid, the ion beam is neutralized by the grid, and a beam of neutral atoms is emitted from the fast atom beam source. The voltage during the activation process by beam irradiation is preferably 0.5 kV to 2.0 kV, and the current during the activation process by beam irradiation is preferably 50 mA to 200 mA.

[0048] The bonding is preferably carried out in a vacuum atmosphere from the viewpoint of obtaining sufficient bonding strength. Specifically, it is preferable to place the substrates to be bonded in a vacuum atmosphere during the activation treatment. The temperature during bonding is typically room temperature. Specifically, it is preferably 20°C or higher and 40°C or lower, more preferably 25°C or higher and 30°C or lower. The pressure applied during bonding is preferably 100N to 20,000N.

[0049] The above-mentioned vacuum atmosphere is, for example, a vacuum of 5×10 -6The degree of vacuum is preferably 3×10 Pa or less. -6 Pa or less. By forming a high-impedance layer with an amorphous region on the substrate (piezoelectric substrate in the illustrated example) on which the low-impedance layer is formed, the vacuum process (vacuuming) for placing the two substrates to be bonded in a vacuum atmosphere can be completed in a short time, thereby achieving sufficient bonding strength. As described above, low-density low-impedance layers tend to absorb moisture from the atmosphere during their formation. When a substrate on which such a low-impedance layer is formed is subjected to a vacuum process, moisture is released from the low-impedance layer, and it takes a long time to reduce the vacuum level, which can lead to reduced productivity. By forming a high-impedance layer with an amorphous region on the substrate on which such a low-impedance layer is formed, the moisture path is blocked, preventing moisture release (outgassing), and the vacuum process can be completed in a short time, which can also contribute to improved productivity.

[0050] Preferably, the surface of each layer (e.g., piezoelectric substrate, reflective layer, support substrate) is flat. Specifically, the surface roughness Ra of each layer is preferably 1 nm or less, more preferably 0.3 nm or less. Methods for flattening the surface of each layer include, for example, mirror polishing, lap polishing, and chemical mechanical polishing (CMP).

[0051] During the above film formation and bonding, it is preferable to clean the surface of each layer, for example, to remove abrasive residues, processing-affected layers, etc. Cleaning methods include, for example, wet cleaning, dry cleaning, and scrub cleaning. Among these, scrub cleaning is preferred because it allows for simple and efficient cleaning. A specific example of scrub cleaning is a method in which a cleaning agent (e.g., Sunwash series manufactured by Lion Corporation) is used, followed by cleaning in a scrub cleaning machine using a solvent (e.g., a mixed solution of acetone and isopropyl alcohol (IPA)).

[0052] In the illustrated example, the impedance layer constituting the reflective layer is formed on the piezoelectric substrate side, but it may also be formed on the support substrate side, and the support substrate on which the reflective layer is formed and the piezoelectric substrate are bonded to obtain a composite substrate. In this case, it is preferable that the amorphous region is formed in the high-impedance layer located farthest from the support substrate. Also, unlike the illustrated example, it is also possible to form a portion of the impedance layer constituting the reflective layer on the piezoelectric substrate side, and a portion of the impedance layer constituting the reflective layer on the support substrate side, and then bond these together to obtain a composite substrate. In this case, it is preferable that the amorphous region is formed in the high-impedance layer located farthest from each substrate.

[0053] In the illustrated example, a bonding layer is not formed from the viewpoint of film formation cost, but the bonding layer may be formed at any appropriate position (timing) before bonding the piezoelectric substrate and the support substrate. By forming a bonding layer on the substrate side on which the low-impedance layer is formed, the time for the vacuum step can be further shortened.

[0054] B. Surface acoustic wave element A surface acoustic wave element according to an embodiment of the present invention includes the composite substrate. The surface acoustic wave element typically includes the composite substrate and an electrode (comb-shaped electrode) provided on the piezoelectric layer side of the composite substrate. Such a surface acoustic wave element is suitable for use as a SAW filter in communication devices such as mobile phones. [Example]

[0055] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. The density of the silicon oxide layer is a value measured by the following measurement method. <Density measurement> The density of the silicon oxide layer was determined by X-ray reflectometry (XRR). The analysis was performed using a fully automated multipurpose X-ray diffractometer (Rigaku's "SmartLab") under the following conditions: incident X-ray wavelength 0.15418 nm (CuKα radiation), X-ray output 45 kV, 200 mA, measurement range (angle with the sample surface) 0.0° to 4.0°, and measurement step 0.01°.

[0056] [Example 1] A lithium tantalate (LT) substrate with a diameter of 4 inches and a thickness of 250 μm was prepared (a 128° Y-cut X-propagation LT substrate, where the propagation direction of the surface acoustic wave (SAW) is X and the cut angle is a rotated Y-cut plate). The surface of this LT substrate was mirror-polished to an arithmetic mean roughness Ra of 0.3 nm. The arithmetic mean roughness Ra was measured using an atomic force microscope (AFM) in a 10 μm × 10 μm field of view.

[0057] A silicon oxide layer (thickness 150 nm, density 2.32 g / cm) was applied to the polished surface of the LT substrate. 3 Specifically, a single-wafer sputtering system (RF magnetron sputtering) was used, with a φ10-inch silicon oxide target, under the conditions of a power supply of 2 kW, a TS distance of 65 mm, and a flow ratio of oxygen to argon (oxygen flow rate / (oxygen flow rate + argon flow rate)) of 7%. A hafnium oxide layer (150 nm thick) was then formed on the surface of the silicon oxide layer. Specifically, a single-wafer sputtering system (RF magnetron sputtering) was used with a φ10-inch hafnium oxide target under the conditions of a power supply of 2 kW, a TS distance of 65 mm, and an oxygen / argon flow ratio of 3%, so that an amorphous region would form early in the film formation. In this way, an evaluation sample was obtained.

[0058] [Example 2] A silicon oxide layer (thickness 150 nm, density 2.32 g / cm) was formed on the polished surface of the LT substrate. 3 A silicon oxide layer (100 nm thick) and a hafnium oxide layer (150 nm thick) were deposited in this order. Specifically, a single-wafer sputtering system (RF magnetron sputtering) was used, with a φ10-inch silicon oxide target and a hafnium oxide target, under the conditions of a power supply of 2 kW, a TS distance of 65 mm, and an oxygen to argon flow ratio of 7%. This deposition was then repeated twice. Next, counting from the LT substrate side, a silicon oxide layer (thickness 150 nm, density 2.32 g / cm) was formed on the surface of the third hafnium oxide layer under the same conditions as above. 3 After forming a film of silicon oxide, a hafnium oxide layer (thickness 150 nm) was formed on the surface of this silicon oxide layer under the same conditions as in Example 1 (so that an amorphous region was formed in the early stage of film formation) to obtain an evaluation sample.

[0059] [Example 3] A silicon oxide layer (thickness 150 nm, density 2.32 g / cm) was formed on the polished surface of the LT substrate. 3 A silicon oxide layer (100 nm thick) and a hafnium oxide layer (150 nm thick) were deposited in this order. Specifically, a single-wafer sputtering system (RF magnetron sputtering) was used, with a φ10-inch silicon oxide target and a hafnium oxide target, under the conditions of a power supply of 2 kW, a TS distance of 65 mm, and an oxygen to argon flow ratio of 7%. This deposition was then repeated once more. Next, counting from the LT substrate side, a silicon oxide layer (thickness 150 nm, density 2.32 g / cm) was formed on the surface of the second hafnium oxide layer under the same conditions as above. 3 ), a hafnium oxide layer (thickness 150 nm) was formed on the surface of this silicon oxide layer under the same conditions as in Example 1 (so that an amorphous region was formed in the initial stage of film formation). Next, counting from the LT substrate side, a silicon oxide layer (thickness 150 nm, density 2.32 g / cm) was formed on the surface of the third hafnium oxide layer under the same conditions as above. 3 ) was formed into a film to obtain an evaluation sample.

[0060] [Comparative Example 1] A silicon oxide layer (thickness 150 nm, density 2.32 g / cm) was formed on the polished surface of the LT substrate. 3 A silicon oxide layer (100 nm thick) and a hafnium oxide layer (150 nm thick) were deposited in this order. Specifically, a single-wafer sputtering system (RF magnetron sputtering) was used, with a φ10-inch silicon oxide target and a hafnium oxide target, under the conditions of a power supply of 2 kW, a transfer station distance of 65 mm, and an oxygen to argon flow ratio of 7%. In this way, an evaluation sample was obtained.

[0061] Comparative Example 2 A silicon oxide layer (thickness 150 nm, density 2.32 g / cm) was formed on the polished surface of the LT substrate. 3 ) and a hafnium oxide layer (150 nm thick) were deposited in that order. Specifically, a single-wafer sputtering system (RF magnetron sputtering) was used, with a φ10-inch silicon oxide target and a hafnium oxide target, under the conditions of a power supply of 2 kW, a TS distance of 65 mm, and an oxygen to argon flow ratio of 7%. This deposition was then repeated three times to obtain an evaluation sample.

[0062] <Evaluation> The obtained evaluation samples were evaluated as follows. The evaluation results are summarized in Table 1. 1. Determine whether or not amorphous regions are formed The presence or absence of amorphous regions in the hafnium oxide layer located furthest from the LT substrate was determined using the ACOM-TEM (Automated Crystal Orientation Mapping-TEM) method. Specifically, cross-sectional observations were performed using a Schottky emission transmission electron microscope (JEOL "JEM-2100F") at an accelerating voltage of 200 kV, a spot size of 1.0 nm, and a camera length of 50 cm. Data were collected using a Nanomegas "ASTAR2 (TOSPIN)" microscope with a precession angle of 0.5°, a measurement magnification of 40,000x, and an interval of 2 nm / step. The samples for cross-sectional observation were prepared using the FIB method. To more accurately determine the crystal structure attribution, the sample depth was set to 40 nm to 50 nm. From the obtained data, a map of the crystalline layer was created using "OIM Analysis" manufactured by EDAX-TSL Solutions. From the created crystalline layer map, the percentage P (%) of the crystalline layer that makes up the hafnium oxide layer was calculated, and the average thickness d of the amorphous region was calculated from the thickness d (nm) of the entire hafnium oxide layer. AThe thickness (nm) was calculated using the following formula: An amorphous region was determined to have been formed when the average thickness was 10 nm or more, and an amorphous region was determined to have not been formed when the average thickness was less than 10 nm. d A (nm) = d(nm) × P(%) As an example, a cross-sectional observation photograph of Example 1 is shown in Figure 3A, and its crystalline layer map is shown in Figure 3B. From Figure 3A, it can be seen that an amorphous region is formed at the upper end of the hafnium oxide layer in the thickness direction, and a columnar structure (polycrystalline) is formed in the other regions. In the crystalline layer map shown in Figure 3B, the proportion of the amorphous region formed at the upper end of the hafnium oxide layer is 7.5%, and the average thickness of the amorphous region is 150 × 0.075 = 11.25 nm, obtained from the overall thickness of the hafnium oxide layer of 150 nm.

[0063] 2. Time to reach vacuum After cleaning the surface of the obtained evaluation sample, it was placed in the vacuum chamber of the bonding device used to fabricate the composite substrate, and the vacuum level was adjusted to 5×10 -6 The time required to reach a vacuum pressure of 100 Pa (the time required to draw a vacuum) was measured.

[0064] [Table 1]

[0065] Example 1 has a shorter time to reach vacuum than Comparative Example 1, and Examples 2 and 3 have a shorter time to reach vacuum than Comparative Example 2. [Industrial Applicability]

[0066] A composite substrate according to an embodiment of the present invention can be suitably used in a surface acoustic wave element. [Explanation of symbols]

[0067] 10 Piezoelectric layer 20 reflective layer 21 Low impedance layer 22 High Impedance Layer 23 Low impedance layer 24 high impedance layers 25 Low Impedance Layer 26 High Impedance Layer 27 Low Impedance Layer 28 High Impedance Layer 30 Support substrate 100 Composite Board 110 Composite substrate

Claims

1. a piezoelectric layer; a reflective layer including a low impedance layer and a high impedance layer including silicon oxide; a support substrate, in this order; The density of the low impedance layer is 2.4 g / cm ３ is as follows: an amorphous region and a region having a columnar structure or a granular structure are formed in the high-impedance layer, and the amorphous region is formed at an end portion of the high-impedance layer in a thickness direction; Composite board.

2. A composite substrate as described in claim 1, wherein the area of the columnar structure in the high impedance layer is 70% or more.

3. The composite substrate according to claim 1 , wherein the amorphous region is formed on the piezoelectric layer side of the high impedance layer.

4. The composite substrate according to claim 1 , wherein the reflective layer includes a plurality of high-impedance layers, and the amorphous region is formed in at least one of the high-impedance layers located closest to the support substrate.

5. The composite substrate according to claim 1 , wherein the high-impedance layers and the low-impedance layers are alternately stacked in the reflective layer.

6. The composite substrate of claim 1 , wherein the reflective layer and the support substrate are disposed adjacent to each other.

7. The composite substrate of claim 1 , wherein the high impedance layer comprises at least one selected from the group consisting of hafnium oxide, tantalum oxide, zirconium oxide, and aluminum oxide.

8. 2. The composite substrate according to claim 1, wherein the high-impedance layer and the low-impedance layer each have a thickness of 0.01 μm to 1 μm.

9. The composite substrate according to claim 1 , wherein the amorphous region has an average thickness of 10 nm or more.

10. A surface acoustic wave element comprising the composite substrate according to claim 1 .

11. At least one of the piezoelectric substrate and the support substrate contains silicon oxide and has a density of 2.4 g / cm ３ Depositing a low impedance layer comprising: forming a high-impedance layer having an amorphous region and a region having a columnar structure or a granular structure on the piezoelectric substrate and / or the support substrate on which the low-impedance layer has been formed; and bonding the piezoelectric substrate and the support substrate together, and forming a reflective layer including the low-impedance layer and the high-impedance layer between the piezoelectric substrate and the support substrate; the amorphous region is formed at an end portion in a thickness direction of the high-impedance layer, The bonding is performed by placing the piezoelectric substrate and the support substrate in a vacuum atmosphere. A method for manufacturing a composite substrate.

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