Composite substrate and method for producing same, and surface acoustic wave element

Abstract

Provided is a composite substrate with which surface acoustic wave leakage is suppressed, damage is suppressed, and excellent reliability is achieved. The composite substrate according to an embodiment of the present invention comprises a piezoelectric layer, a first intermediate layer, a second intermediate layer, and a support substrate, in the indicated order. The first intermediate layer contains silicon oxide, and the second intermediate layer contains silicon oxynitride. The silicon content of the second intermediate layer is at least 25 atom%, and the nitrogen content of the second intermediate layer is larger than the oxygen content.

Description

Composite substrate and method for manufacturing the same, and surface acoustic wave element 【0001】 The present invention relates to a composite substrate, a method for manufacturing the same, and a surface acoustic wave element. 【0002】 Communication devices such as mobile phones use filters that utilize surface acoustic waves (SAW filters) to extract electrical signals of arbitrary frequencies. In recent years, there has been a growing demand for high-frequency (RF) communication devices, and SAW filters are required to be compatible with such RF devices. SAW filters use composite substrates, for example, having a piezoelectric layer and a support substrate. In recent years, there has been a growing demand for even higher performance RF devices. To meet these demands, composite substrates having a piezoelectric film / low-sonic film / high-sonic film / support substrate configuration and acoustic wave devices using said composite substrates have been proposed (for example, Patent Document 1). While the composite substrate in Patent Document 1 can achieve good performance by suppressing leakage of surface acoustic waves, it has the problem of being very easily damaged (for example, easily cracked) and lacking reliability. 【0003】 Patent No. 5713025 【0004】 The main objective of the present invention is to provide a composite substrate that has excellent reliability, with suppressed leakage of surface acoustic waves and reduced damage. 【0005】[1] A composite substrate according to an embodiment of the present invention comprises a piezoelectric layer, a first intermediate layer, a second intermediate layer, and a support substrate in this order, wherein the first intermediate layer contains silicon oxide, the second intermediate layer contains silicon oxynitride, the silicon content in the second intermediate layer is 25 atomic percent or more, and the nitrogen content in the second intermediate layer is greater than the oxygen content. [2] In [1] above, the second intermediate layer has a region in the thickness direction where the oxygen content decreases toward the support substrate. [3] In [1] or [2] above, the second intermediate layer has a region in the thickness direction where the atomic composition ratio N / O of nitrogen to oxygen increases toward the support substrate. [4] In any of [1] to [3] above, the silicon content of the second intermediate layer increases toward the support substrate in the thickness direction, and the rate of increase in silicon content is greater than 0 atomic percent / nm and less than 0.2 atomic percent / nm. [5] In any of [1] to [4] above, the second intermediate layer and the support substrate are directly bonded, and a first amorphous layer is formed on the support substrate side of the bonding interface. [6] In [5] above, the first amorphous layer contains an inert gas element. [7] In [5] or [6] above, the first amorphous layer contains elements that constitute the support substrate. [8] In any of [5] to [7] above, a second amorphous layer is formed on the second intermediate layer side of the bonding interface, and the second amorphous layer is an insulating layer containing silicon, nitrogen, and an inert gas element. [9] In [8] above, the inert gas element content of the first amorphous layer is greater than the inert gas element content of the second amorphous layer.

[10] In [8] or [9] above, the thickness of the first amorphous layer is greater than the thickness of the second amorphous layer.

[11] In any of [8] to

[10] above, the nitrogen content in the second amorphous layer is 20 atomic% to 60 atomic%.

[12] In any of [1] to

[11] above, the thickness of the first intermediate layer is greater than the thickness of the second intermediate layer.

[13] In any of [1] to [7] above, the composite substrate further has a third intermediate layer between the second intermediate layer and the support substrate, wherein the silicon content in the third intermediate layer is greater than the silicon content in the second intermediate layer.

[14] According to another aspect of the present invention, a method for manufacturing a composite substrate is provided. The manufacturing method includes, in this order, forming a first intermediate layer and a second intermediate layer on one side of a piezoelectric substrate; activating the surface of the second intermediate layer and the surface of the support substrate; joining the second intermediate layer and the support substrate such that the activated surfaces of the second intermediate layer and the support substrate face each other; and thinning the piezoelectric substrate to form a piezoelectric layer.

[15] In the above

[14] , the manufacturing method involves activating the surface of the support substrate to form a first amorphous layer near the surface of the support substrate, activating the surface of the second intermediate layer to form a second amorphous layer near the surface of the second intermediate layer, and joining the second intermediate layer and the support substrate via the first amorphous layer and the second amorphous layer.

[16] In the above

[14] or

[15] , the first intermediate layer and the second intermediate layer are formed by continuous sputtering.

[17] According to yet another aspect of the present invention, a surface acoustic wave element is provided, which has a composite substrate of any of the above [1] to

[13] . 【0006】 According to embodiments of the present invention, a composite substrate with excellent reliability can be realized in which leakage of surface acoustic waves is suppressed and damage is suppressed. 【0007】 This is a schematic cross-sectional view of a composite substrate according to one embodiment of the present invention. This is a schematic cross-sectional view of a composite substrate according to another embodiment of the present invention. This is a schematic cross-sectional view of a composite substrate according to yet another embodiment of the present invention. This is a schematic cross-sectional view illustrating one step in the manufacturing method of a composite substrate according to an embodiment of the present invention. This is a schematic cross-sectional view illustrating one step in the manufacturing method of a composite substrate according to an embodiment of the present invention. This is a graph showing the change in composition ratio from the vicinity of the surface on the second intermediate layer side of the first intermediate layer in the composite substrate of Example 1 to the vicinity of the surface on the second intermediate layer side of the support substrate. 【0008】 Embodiments of the present invention will be described below with reference to the drawings, but the present invention is not limited to these embodiments. Note that the drawings are schematic for clarity, and the thickness, length, width, shape, proportions, etc., do not accurately reflect the actual shape. 【0009】A. Composite Substrate A-1. Schematic Figure 1A of the composite substrate is a schematic cross-sectional view of a composite substrate according to one embodiment of the present invention. The composite substrate 100 in the illustrated example has a piezoelectric layer 10, a first intermediate layer 20, a second intermediate layer 30, and a support substrate 40 in this order. In the composite substrate in the illustrated example, the second intermediate layer 30 and the support substrate 40 are joined in any appropriate form. By integrating the support substrate by joining, a composite substrate with sufficient mechanical strength can be realized. As a result, damage is suppressed and a composite substrate with excellent reliability can be realized. Furthermore, it may be possible to thin the piezoelectric layer. A preferred joining method is, for example, direct joining. Direct joining allows for thinning of the composite substrate and prevents adverse effects from adhesives. Examples of direct joining methods include surface activation bonding, plasma activation bonding, and atomic diffusion bonding. Alternatively, particles of one of the constituent materials of the layers or substrates to be directly bonded may be ejected by sputtering, forming a sputtered layer on the surface of the other layer or substrate, and this sputtered layer may be used as the bonding layer. Typically, an amorphous layer may be formed at the bonding interface of direct bonding. In the illustrated example, a first amorphous layer 51 is formed on the support substrate 40 side of the bonding interface, and a second amorphous layer 52 is formed on the second intermediate layer 30 side. Here, the first amorphous layer 51 and the second amorphous layer 52 are collectively referred to as the amorphous layer. As the name suggests, the amorphous layer has an amorphous structure and is typically an insulating layer. Typically, the amorphous layer contains the elements that make up the second intermediate layer 30 and the elements that make up the support substrate 40. Furthermore, typically, the amorphous layer may contain atomic species (typically argon and nitrogen) that make up the neutral atomic beam (sometimes referred to as a high-speed atomic beam) used for direct bonding. Furthermore, the second intermediate layer, the first amorphous layer, the second amorphous layer, and the support substrate may each contain atomic species derived from the direct bonding equipment (for example, aluminum derived from the electrostatic chuck member). The components of the composite substrate will be described in detail in sections A-2 to A-7. 【0010】In this specification, "direct bonding" means that the components of the composite substrate (the second intermediate layer 30 and the support substrate 40 in the embodiment of Figure 1) are bonded together without the use of an adhesive. The form of direct bonding can be appropriately set depending on the configuration of the layers or substrates to be bonded together. For example, direct bonding by surface activation can be achieved by the following procedure: in a high vacuum chamber (e.g., 1 × 10⁻⁶) －６ At a pressure of approximately Pa, the neutralizing beam is irradiated onto the bonding surfaces of the components to be joined (in this case, the support substrate and the second intermediate layer). This activates the bonding surfaces of the support substrate and the second intermediate layer. Typically, by activating the surface of the support substrate 40, a first amorphous layer 51 may be formed near the surface of the support substrate, and by activating the surface of the second intermediate layer 30, a second amorphous layer 52 may be formed near the surface of the second intermediate layer. Next, in a vacuum atmosphere, the activated bonding surfaces (substantially the first amorphous layer 51 and the second amorphous layer 52) are brought into contact, and the second intermediate layer and the support substrate are joined at room temperature. The load during this joining can be, for example, 100 N to 20000 N. In one embodiment, when performing surface activation with a neutralizing beam, an inert gas is introduced into the chamber, and a high voltage is applied from a DC power supply to electrodes placed in the chamber. In this configuration, electrons move due to the electric field generated between the electrode (positive electrode) and the chamber (negative electrode), generating a beam of atoms and ions from the inert gas. Of the beam that reaches the grid, the ion beam is neutralized at the grid, so a beam of neutral atoms is emitted from the high-speed atomic beam source. The atomic species constituting the beam are preferably inert gas elements (e.g., argon (Ar), nitrogen (N)). The voltage during activation by beam irradiation is, for example, 0.5 kV to 2.0 kV, and the current is, for example, 50 mA to 200 mA. 【0011】In embodiments of the present invention, the first intermediate layer 20 contains silicon oxide, and the second intermediate layer 30 contains silicon oxynitride. Furthermore, the silicon content in the second intermediate layer is 25 atomic percent or more, and the nitrogen content in the second intermediate layer is greater than the oxygen content. In this specification, "silicon oxynitride" means a compound of silicon, oxygen, and nitrogen. With this configuration, the sound velocity of the bulk wave in the first intermediate layer is lower than the sound velocity of the surface acoustic wave propagating through the piezoelectric layer; and the sound velocity of the bulk wave in the second intermediate layer is higher than the sound velocity of the surface acoustic wave propagating through the piezoelectric layer. That is, the first intermediate layer can function as a low-sound-velocity film, and the second intermediate layer can function as a high-sound-velocity film. By arranging the low-sound-velocity film and the high-sound-velocity film in this order from the piezoelectric layer side, the high-sound-velocity film can effectively confine the surface acoustic wave in the laminated portion between the piezoelectric layer and the low-sound-velocity film. As a result, leakage of surface acoustic waves is suppressed, and a composite substrate capable of realizing a high-performance (e.g., high Q-factor) surface acoustic wave element can be obtained. More specifically, the following applies: By arranging a low-sonic-velocity film (first intermediate layer) between the piezoelectric layer and the high-sonic-velocity film (second intermediate layer), the speed of sound of the surface acoustic wave is reduced. Surface acoustic waves inherently concentrate energy in a low-sonic-velocity medium. Therefore, the confinement effect of the elastic wave energy in the piezoelectric layer can be enhanced. As a result, losses can be reduced and the Q-factor can be increased compared to the case where the low-sonic-velocity film (first intermediate layer) is not arranged. Due to the synergistic effect of this confinement effect and the effect of direct bonding between the second intermediate layer and the support substrate, according to the embodiment of the present invention, leakage of surface acoustic waves and damage can be suppressed, and a composite substrate with excellent reliability can be realized. 【0012】The second intermediate layer 30 typically has a region in the thickness direction where the oxygen content decreases toward the support substrate 40. The "region where the oxygen content decreases" may be a region where the oxygen content decreases continuously or stepwise over the entire thickness direction or a part thereof, and may also be a region having an oxygen content lower than the average oxygen content formed relatively toward the support substrate side of the second intermediate layer compared to the first intermediate layer side. Note that "continuous" means different from a clear stepwise change (a change in which flat parts appear discontinuously), and encompasses a linear or curvilinear change as a whole. Furthermore / or, the second intermediate layer 30 typically has a region in the thickness direction where the atomic composition ratio of nitrogen to oxygen N / O increases toward the support substrate 40. The second intermediate layer may have a region in the thickness direction where the nitrogen content increases toward the support substrate side. The "region where the nitrogen content increases" may be a region where the nitrogen content increases continuously or stepwise over the entire thickness direction or a part thereof, and may also be a region having a nitrogen content higher than the average nitrogen content formed relatively toward the support substrate side of the second intermediate layer compared to the first intermediate layer side. "Continuous" is as described above. With such a configuration, the advantages of the second intermediate layer as a high-sonic-velocity film can be maintained, while the effects of direct bonding between the second intermediate layer and the support substrate can be made even more pronounced. Specifically, while maintaining a good containment effect, damage to the composite substrate (e.g., cracking) can be significantly suppressed, and as a result, the reliability of the composite substrate can be significantly improved. The mechanism by which such effects are obtained can be presumed to be as follows. However, this mechanism is merely a presumption and does not limit the embodiments of the present invention, nor does it restrict the embodiments of the present invention by this mechanism. 【0013】Generally, low-sonic-velocity films (first intermediate layer) have a low Young's modulus, and high-sonic-velocity films (second intermediate layer) have a high Young's modulus, and the difference between these Young's moduli is also large. Therefore, composite substrates simply laminated with low-sonic-velocity and high-sonic-velocity films are prone to stress concentration at the interface between the low-sonic-velocity and high-sonic-velocity films in high-temperature environments, and cracking in subsequent processes due to such stress concentration is a problem. In contrast, according to this embodiment, by forming a concentration gradient in the second intermediate layer such that the oxygen content on the first intermediate layer side is relatively high (the nitrogen content is relatively low) and the oxygen content on the support substrate side is relatively low (the nitrogen content is relatively high), a Young's modulus gradient can be applied in the thickness direction of the second intermediate layer. As a result, on the first intermediate layer side of the second intermediate layer, the difference in Young's moduli between the second intermediate layer and the first intermediate layer can be reduced, suppressing stress concentration at the interface and preventing cracking. On the other hand, on the support substrate side, the nitrogen content can be increased to maintain good performance as a high-sonic-velocity film, and surface acoustic waves can be effectively confined in the laminated portion between the piezoelectric layer and the low-sonic-velocity film. As a result, leakage of surface acoustic waves can be suppressed. Furthermore, by relatively increasing the nitrogen content on the support substrate side in the second intermediate layer, it becomes easier to form dangling bonds on the support substrate side surface of the second intermediate layer. As a result, the bonding strength between the second intermediate layer and the support substrate can be increased. 【0014】Figure 1B is a schematic cross-sectional view of a composite substrate according to another embodiment of the present invention. The composite substrate 101 in the illustrated example further has a third intermediate layer 60 between the second intermediate layer 30 and the support substrate 40. The third intermediate layer 60 is typically an amorphous silicon film. The silicon content in the third intermediate layer is typically greater than that in the second intermediate layer. With this configuration, the third intermediate layer is more likely to form dangling bonds due to its higher silicon content than the second intermediate layer, thus significantly increasing the bonding strength between the third intermediate layer and the support substrate. Note that "silicon content in the third intermediate layer" refers to the average content in the entire third intermediate layer. In the illustrated example, the third intermediate layer 60 and the support substrate 40 are typically directly bonded. In this case, typically, a first amorphous layer 51 is formed on the support substrate 40 side of the bonding interface, and a second amorphous layer 52 is formed on the third intermediate layer 60 side, and the third intermediate layer 60 and the support substrate 40 can be bonded via these amorphous layers. In this embodiment, the third intermediate layer is typically formed by sputtering, and more specifically, the first intermediate layer, the second intermediate layer and the third intermediate layer can be formed by continuous sputtering. Details of direct bonding and amorphous layers are as described with respect to the embodiment in Figure 1A. 【0015】 Figure 1C is a schematic cross-sectional view of a composite substrate according to yet another embodiment of the present invention. In the illustrated example composite substrate 102, typically, particles of the constituent material of the support substrate are ejected by sputtering to form a sputtered layer on the surface of the second intermediate layer, and this sputtered layer is used as the third intermediate layer. In this case, the second amorphous layer 52 on the third intermediate layer 60 side of the bonding interface may be omitted, and the third intermediate layer 60 and the support substrate 40 can be bonded via the first amorphous layer 51. Even with such a configuration, the same effects as in the embodiment of Figure 1B can be obtained. 【0016】 The total thickness of the composite substrate can be, for example, 250 μm to 2500 μm, or for example, 400 μm to 700 μm. 【0017】The composite substrate according to an embodiment of the present invention can typically be manufactured in the form of a so-called wafer. The size of the composite substrate can be appropriately set according to the purpose. The diameter of the wafer can be, for example, 75 mm to 200 mm, and can also be, for example, 4 inches (about 100 mm). Usually, a plurality of devices (for example, SAW filters) can be manufactured from one composite substrate. Note that the composite substrate is not limited to the form of a wafer and may be manufactured and provided in various forms. 【0018】 Hereinafter, the components of the composite substrate will be specifically described. 【0019】 A-2. Piezoelectric layer As the material constituting the piezoelectric layer 10, any suitable piezoelectric material can be used. As the piezoelectric material, preferably, a single crystal having the composition of LiAO ３ is used. Here, A is one or more elements selected from the group consisting of niobium and tantalum. Specifically, LiAO ３ may be lithium niobate (LiNbO ３ ), may be lithium tantalate (LiTaO ３ ), or may be a lithium niobate-lithium tantalate solid solution. Another example of the piezoelectric material is potassium titanyl phosphate (KTiOPO ４ : KTP), potassium lithium niobate (KxLi(1-x)NbO ２ , 0≦x≦1: KLN), potassium niobate (KNbO ３ : KN), potassium tantalate niobate (KNbxTa(1-x)O ３ , 0≦x≦1: KTN), a solid solution of lithium niobate and lithium tantalate, barium titanate (BaTiO ３ ), silicon, quartz, quartz, silicon carbide, gallium nitride, indium phosphide, lead zirconate titanate (PZT). 【0020】 When the piezoelectric material is lithium tantalate, the cut angle can be appropriately set according to the purpose. The piezoelectric layer is, for example, the X-axis (crystal axis) of the piezoelectric material in the propagation direction of the elastic surface wave (X １When this is the case, the direction rotated 32° to 55° (for example, 46°) from the Y-axis toward the Z-axis is the direction perpendicular to the main surface of the piezoelectric layer (X ３ It is preferable that the Euler angle ranges from (180°, 58° to 35°, 180°). With such a configuration, propagation loss can be suppressed. 【0021】 When the piezoelectric material is lithium niobate, the cut angle can be appropriately set depending on the purpose. For example, the piezoelectric layer is aligned with the X-axis (crystal axis) of the piezoelectric material in the direction of surface wave propagation (X １ When this is the case, the direction rotated from the Z-axis toward the -Y-axis by 0° to 45° (for example, 37.8°) is the direction perpendicular to the main surface of the piezoelectric layer (X ３ It is preferable that the Euler angle range corresponds to (0°, 0° to 45°, 0°). With such a configuration, the electromechanical coupling coefficient can be increased. When the piezoelectric material is lithium niobate, the piezoelectric layer is also, for example, aligned with the X-axis (crystal axis) of the piezoelectric material in the direction of surface wave propagation (X １ When this is done, the direction rotated from the Y-axis to the Z-axis, preferably 45° to 90°, more preferably 52.2°, is the direction perpendicular to the main surface of the piezoelectric layer (X ３ This corresponds to (180°, 45° to 90°, 180°) in Euler angle notation, and more preferably (180°, 52.2°, 180°). 【0022】 The thickness of the piezoelectric layer is, for example, 30 μm (30,000 nm) or less, preferably 10 μm or less, more preferably 5 μm or less, even more preferably 3 μm or less, particularly preferably 1.5 μm or less, and especially preferably 1.0 μm or less. With such a thickness, for example, a high-performance (for example, having good temperature characteristics and a high Q value) surface acoustic wave element can be obtained. On the other hand, the thickness of the piezoelectric layer may be, for example, 0.05 μm (50 nm) or more, or for example 0.15 μm or more, or for example 0.20 μm or more. 【0023】The arithmetic mean roughness Ra of the surface on the first intermediate layer side of the piezoelectric layer may be, for example, 1.0 nm or less, 0.8 nm or less, 0.6 nm or less, or 0.4 nm or less. On the other hand, the arithmetic mean roughness Ra may be, for example, 0.1 nm or more, or 0.2 nm or more. With such a piezoelectric layer, for example, a high-performance (for example, having a high Q value) surface acoustic wave element can be obtained. Note that the arithmetic mean roughness Ra is a value measured in a 10 μm × 10 μm field of view using an atomic force microscope (AFM). 【0024】 A metal film and / or insulating film may be formed on the surface of the piezoelectric layer, depending on the purpose. The surface on which the metal film and / or insulating film is formed may be the surface on the first intermediate layer side or the surface opposite to the first intermediate layer, depending on the purpose. For example, when fabricating a Lamb wave element from a composite substrate, a metal film may be formed on the first intermediate layer side of the piezoelectric layer. By providing a metal film, the electromechanical coupling coefficient near the surface on the first intermediate layer side of the piezoelectric layer can be increased. Examples of constituent materials for the metal film include aluminum, aluminum alloy, copper, and gold. Also, for example, when fabricating a thin-film resonator from a composite substrate, a metal film and an insulating film may be formed on the first intermediate layer side of the piezoelectric layer. Examples of constituent materials for the metal film include molybdenum, ruthenium, tungsten, chromium, and aluminum. Examples of constituent materials for the insulating film include silicon dioxide, phosphate silica glass, and boron phosphate silica glass. 【0025】 A-3. First Intermediate Layer The first intermediate layer 20 contains silicon oxide as described above. As described above, the speed of sound of bulk waves in the first intermediate layer is slower than the speed of sound of surface acoustic waves propagating through the piezoelectric layer, and therefore the first intermediate layer can function as a low-speed sound film. Furthermore, the first intermediate layer 20 can improve the stability of the temperature characteristics of the composite substrate. Specifically, this is as follows: The materials constituting the piezoelectric layer typically have negative temperature characteristics, while silicon oxide has positive temperature characteristics. Therefore, in the composite substrate, the temperature characteristics of the piezoelectric layer and the first intermediate layer cancel each other out, so the absolute value of the frequency temperature coefficient (TCF) can be reduced. 【0026】 The thickness of the first intermediate layer may be, for example, 30 nm to 1000 nm, or for example, 150 nm to 800 nm, or for example, 200 nm to 600 nm, or for example, 250 nm to 500 nm. The thickness of the first intermediate layer may be greater than or less than the thickness of the second intermediate layer (described later), or it may be the same as the thickness of the second intermediate layer. 【0027】 A-4. The second intermediate layer 30 contains silicon oxynitride as described above. Furthermore, as described above, the silicon content in the silicon oxynitride is 25 atomic percent or more, and the nitrogen content in the silicon oxynitride is greater than the oxygen content. Therefore, the silicon oxynitride constituting the second intermediate layer is typically Si ｘ O ｙ N ｚ It can be expressed as (x + y + z = 1, x ≥ 0.25). Hereinafter, the content of each element in the silicon oxynitride constituting the second intermediate layer will be expressed only in "atomic %". The silicon content in the silicon oxynitride constituting the second intermediate layer is preferably 30 atomic % to 60 atomic %, and more preferably 35 atomic % to 45 atomic %. The oxygen content in the silicon oxynitride constituting the second intermediate layer is preferably greater than 0 atomic % and 30 atomic % or less, and more preferably 5 atomic % to 15 atomic %. The nitrogen content in the silicon oxynitride constituting the second intermediate layer is greater than the oxygen content as described above, preferably 30 atomic % to 60 atomic %, and more preferably 45 atomic % to 55 atomic %. Note that "content of each element in the silicon oxynitride constituting the second intermediate layer" means the average content in the entire second intermediate layer. 【0028】By constructing the second intermediate layer with silicon oxynitride as described above, the speed of sound of the bulk wave in the second intermediate layer is faster than the speed of sound of the surface acoustic wave propagating through the piezoelectric layer, as described above. Therefore, the first intermediate layer can function as a high-speed sound film. Furthermore, as described above, silicon oxynitride readily forms dangling bonds, which can increase the bonding strength between the second intermediate layer and the support substrate. In particular, when amorphous layers are formed on both the second intermediate layer and the support substrate, and the two are directly bonded via these amorphous layers, the bonding strength can be significantly increased. For example, when an amorphous layer (second amorphous layer) is formed by irradiation with a high-speed atomic beam in a surface activation method, many dangling bonds can be formed on the second amorphous layer. As a result, when directly bonded to the first amorphous layer formed on the support substrate, the bonding strength can be significantly increased. 【0029】 As described above, the second intermediate layer may have a region in the thickness direction where the oxygen content decreases toward the support substrate side. As a result, the second intermediate layer may have a region with a relatively high oxygen content toward the first intermediate layer side and a region with a relatively low oxygen content toward the support substrate side. The oxygen content of the second intermediate layer's surface toward the first intermediate layer may be, for example, 15 to 70 atoms, or for example, 20 to 40 atoms. The oxygen content of the second intermediate layer's surface toward the support substrate may be, for example, 0 to 20 atoms, or for example, 3 to 10 atoms. The difference between the oxygen content of the second intermediate layer's surface toward the first intermediate layer and the oxygen content of the surface toward the support substrate may be, for example, 5 to 65 atoms, or for example, 10 to 35 atoms. Furthermore, the decrease in oxygen content per unit length from the first intermediate layer side surface of the second intermediate layer to the support substrate side surface of the second intermediate layer can be greater than, for example, 0 atomic % / nm and less than or equal to 0.1 atomic % / nm, and can also be, for example, between 0.01 atomic % / nm and 0.05 atomic % / nm. With such a configuration, the effects of the embodiment of the present invention, which achieve both suppression of surface acoustic wave leakage, suppression of cracking of the composite substrate, and improvement of reliability, can be made more pronounced. The oxygen content can be obtained, for example, by energy-dispersive X-ray spectroscopy (TEM-EDX) using a transmission electron microscope. 【0030】 As also described above, the second intermediate layer has a region in the thickness direction where the atomic composition ratio N / O of nitrogen to oxygen increases toward the support substrate side. As a result, the second intermediate layer may have a region on the first intermediate layer side where the atomic composition ratio N / O (and therefore nitrogen content) is relatively low, and a region on the support substrate side where the atomic composition ratio N / O is relatively high. The atomic composition ratio N / O of the first intermediate layer side surface of the second intermediate layer may be, for example, 0.8 to 3.7, or for example, 1.1 to 2.8. The atomic composition ratio N / O of the support substrate side surface of the second intermediate layer may be, for example, 2.5 to 100, or for example, 4.5 to 20. The difference between the atomic composition ratio N / O of the first intermediate layer side surface and the atomic composition ratio N / O of the support substrate side surface of the second intermediate layer may be, for example, 1.0 to 100, or for example, 2.0 to 18. Furthermore, the nitrogen content of the second intermediate layer on the first intermediate layer side surface may be, for example, 10 atomic% to 65 atomic%, or for example, 25 atomic% to 60 atomic%. The nitrogen content of the second intermediate layer on the support substrate side surface may be, for example, 15 atomic% to 68 atomic%, or for example, 25 atomic% to 63 atomic%. The difference between the nitrogen content of the second intermediate layer on the first intermediate layer side surface and the nitrogen content of the support substrate side surface may be, for example, 0 atomic% to 20 atomic%, or for example, 1 atomic% to 10 atomic%. Furthermore, the increase in the atomic composition ratio N / O per unit length from the first intermediate layer side surface to the support substrate side surface of the second intermediate layer may be, for example, 0.001 / nm to 0.1 / nm, or for example, 0.01 / nm to 0.03 / nm. With such a configuration, the effects of the embodiment of the present invention, which achieve both suppression of surface acoustic wave leakage, suppression of cracking of the composite substrate, and improvement of reliability, can be made more pronounced, similar to the case of oxygen content described above. The composition ratio N / O can be calculated, for example, from the nitrogen and oxygen content obtained by TEM-EDX. 【0031】As also described above, the silicon content of the second intermediate layer may increase toward the support substrate side in the thickness direction. As a result, the second intermediate layer may have a region with a relatively low silicon content toward the first intermediate layer side and a region with a relatively high silicon content toward the support substrate side. The silicon content of the second intermediate layer's surface toward the first intermediate layer may be, for example, 20 atomic% to 45 atomic%, or for example, 25 atomic% to 40 atomic%. The silicon content of the second intermediate layer's surface toward the support substrate may be, for example, 30 atomic% to 60 atomic%, or for example, 35 atomic% to 50 atomic%. The difference between the silicon content of the second intermediate layer's surface toward the first intermediate layer and the silicon content of the support substrate side surface may be, for example, 2 atomic% to 25 atomic%, or for example, 4 atomic% to 15 atomic%. Furthermore, the increase in silicon content per unit length from the first intermediate layer side surface of the second intermediate layer to the support substrate side surface of the second intermediate layer can be, for example, greater than 0 atomic % / nm, less than 0.2 atomic % / nm, and for example, between 0.01 atomic % / nm and 0.05 atomic % / nm. With such a configuration, (due to the high silicon content) the leakage of surface acoustic waves is suppressed and the bonding strength is increased due to the high silicon content at the bonding interface, thus making the effects of the embodiment of the present invention, which achieves both suppression of cracking of the composite substrate and improvement of reliability, more pronounced. The silicon content can be obtained, for example, by energy-dispersive X-ray spectroscopy (TEM-EDX) using a transmission electron microscope. 【0032】 The thickness of the second intermediate layer is preferably 5 nm to 3000 nm (3 μm), more preferably 50 nm to 2000 nm, even more preferably 100 nm to 1000 nm, particularly preferably 100 nm to 800 nm, and especially preferably 150 nm to 600 nm. As described above, the thickness of the second intermediate layer may be greater than, less than, or the same as the thickness of the first intermediate layer. 【0033】The arithmetic mean roughness Ra of the surface on the support substrate side of the second intermediate layer can be, for example, from 0.1 nm to 1.0 nm, or can be, for example, from 0.2 nm to 0.8 nm, or can be, for example, from 0.3 nm to 0.7 nm, or can be, for example, from 0.4 nm to 0.6 nm. With such a configuration, the bonding strength between the second intermediate layer and the support substrate can be further improved. 【0034】 A-5. Support Substrate As the support substrate 40, any suitable substrate can be used. The support substrate may be composed of a single crystal, a polycrystal, or a combination thereof. Examples of the material constituting the support substrate include silicon, sapphire, sialon, cordierite, mullite, glass, quartz, crystal, alumina, germanium, silicon carbide, silicon nitride, gallium oxide, gallium nitride, indium phosphide, and aluminum nitride. The support substrate is preferably composed of silicon. With such a configuration, sufficient bonding strength can be obtained when directly bonding to the second intermediate layer. The support substrate is more preferably composed of single crystal silicon. In this case, the orientation of the support substrate is the (111) plane or the (100) plane. 【0035】 The above silicon is single crystal silicon, and a polycrystalline layer may be formed on its surface, or it may be high-resistance silicon. 【0036】 The thermal expansion coefficient of the material constituting the support substrate is preferably smaller than the thermal expansion coefficient of the material constituting the piezoelectric layer. With such a support substrate, changes in the shape and size of the piezoelectric layer when the temperature changes can be suppressed. For example, changes in the frequency characteristics of the obtained surface acoustic wave device can be suppressed. 【0037】 As the thickness of the support substrate, any suitable thickness can be adopted. The thickness of the support substrate can be, for example, from 100 μm to 1500 μm (1.5 mm), or can be, for example, from 100 μm to 1000 μm (1.0 mm). If the thickness of the support substrate is within such a range, sufficient mechanical strength is imparted to the composite substrate as an integral object, and thinning of the piezoelectric layer becomes possible. 【0038】The arithmetic mean roughness Ra of the surface on the second intermediate layer side of the support substrate can be, for example, from 0.1 nm to 1.0 nm, or can be, for example, from 0.2 nm to 0.8 nm, or can be, for example, from 0.3 nm to 0.7 nm, or can be, for example, from 0.4 nm to 0.6 nm. According to such a support substrate, for example, a high-performance (for example, having a high Q value) surface acoustic wave device can be obtained. 【0039】 A-6. Amorphous layer A-6-1. First amorphous layer The first amorphous layer 51 can be formed on the support substrate 40 side of the bonding interface between the second intermediate layer 30 and the support substrate 40. The first amorphous layer can typically be formed by activating the surface of the support substrate during direct bonding between the second intermediate layer and the support substrate. Since the second intermediate layer and the support substrate are bonded via the first amorphous layer and the second amorphous layer formed on the second intermediate layer, the first amorphous layer typically contains the elements constituting the support substrate and the elements constituting the second intermediate layer, and may further contain the atomic species (typically argon, nitrogen) constituting the neutral atom beam used for direct bonding. 【0040】 Specifically, the first amorphous layer can contain silicon, nitrogen, oxygen, and argon. The silicon content in the first amorphous layer can be, for example, from 45 atomic% to 95 atomic%, or can be, for example, from 70 atomic% to 90 atomic%; the nitrogen content can be, for example, from 3 atomic% to 40 atomic%, or can be, for example, from 5 atomic% to 15 atomic%; the oxygen content can be, for example, from 3 atomic% to 10 atomic%, or can be, for example, from 4 atomic% to 6 atomic%; the argon content can be, for example, from 2.5 atomic% to 5 atomic%, or can be, for example, from 2.6 atomic% to 3 atomic%. Note that the total content of silicon, nitrogen, oxygen, and argon can typically be 100 atomic%. When an inert gas element other than argon is used as the atomic species constituting the neutral atom beam used for direct bonding, the first amorphous layer can contain the inert gas element instead of argon. Note that in this specification, "100 atomic%" is described with the intention of allowing inevitable trace amounts of impurities to be included. 【0041】In one embodiment, the argon (inert gas element) content of the first amorphous layer is greater than the argon (inert gas element) content of the second amorphous layer (described later). With this configuration, more dangling bonds are more easily formed in the first amorphous layer than in the second amorphous layer, and impurity atoms such as inert gas elements diffuse more easily. As a result, superior bonding strength can be achieved. 【0042】 The thickness of the first amorphous layer can be, for example, 5 nm to 20 nm, or for example, 5 nm to 10 nm. Typically, the thickness of the first amorphous layer is greater than the thickness of the second amorphous layer. With this configuration, impurity atoms such as inert gas elements diffuse more easily into the first amorphous layer than into the second amorphous layer when activated by heating or other means. As a result, superior bonding strength can be achieved. 【0043】 A-6-2. Second amorphous layer The second amorphous layer 52 can be formed on the second intermediate layer 30 side of the bonding interface between the second intermediate layer 30 and the support substrate 40. Typically, the second amorphous layer can be formed by activating the surface of the second intermediate layer during direct bonding between the second intermediate layer and the support substrate. Since the second intermediate layer and the support substrate are bonded via the second amorphous layer and the first amorphous layer formed on the support substrate, the second amorphous layer typically contains the elements constituting the second intermediate layer and the elements constituting the support substrate, and may further contain atomic species constituting the neutral atomic beam used for direct bonding (typically argon and nitrogen). 【0044】Specifically, the second amorphous layer may contain silicon, nitrogen, oxygen, and argon. The silicon content in the second amorphous layer may be, for example, 35 to 80 atomic percent, or 40 to 50 atomic percent; the nitrogen content may be, for example, 5 to 60 atomic percent, or 10 to 50 atomic percent, or 15 to 40 atomic percent; the oxygen content may be, for example, 2 to 20 atomic percent, or 4 to 15 atomic percent; and the argon content may be, for example, 1 to 3 atomic percent, or 1.5 to 2.5 atomic percent. The total content of silicon, nitrogen, oxygen, and argon may typically be 100 atomic percent. When an inert gas element other than argon is used as the atomic species constituting the neutral atomic beam used for direct bonding, the second amorphous layer may contain that inert gas element instead of argon. 【0045】 The thickness of the second amorphous layer can be, for example, 0.3 nm to 5 nm, or for example, 2 nm to 4 nm. As described above, the thickness of the second amorphous layer is typically smaller than the thickness of the first amorphous layer. 【0046】 A-7. Third Intermediate Layer The third intermediate layer 60 is typically an amorphous silicon film, as described above. Furthermore, as described above, the silicon content in the third intermediate layer is greater than that in the second intermediate layer. The silicon content in the third intermediate layer may be, for example, 70 atomic percent or more, or for example, 75 atomic percent to 95 atomic percent. The difference between the silicon content in the third intermediate layer and the silicon content in the second intermediate layer may be, for example, 30 atomic percent to 70 atomic percent, or for example, 45 atomic percent to 65 atomic percent. With such a configuration, the effect of providing the third intermediate layer can become more pronounced. 【0047】 The thickness of the third intermediate layer may be, for example, 1 nm to 500 nm, or for example, 20 nm to 200 nm. 【0048】B. Method for Manufacturing a Composite Substrate B-1. Outline of the Method for Manufacturing a Composite Substrate The method for manufacturing a composite substrate according to an embodiment of the present invention includes, in this order: forming a first intermediate layer and a second intermediate layer on one side of a piezoelectric substrate; activating the surface of the second intermediate layer and the surface of the support substrate; joining the second intermediate layer and the support substrate so that the activated surfaces of the second intermediate layer and the support substrate face each other; and thinning the piezoelectric substrate to form a piezoelectric layer. 【0049】 The manufacturing method for the composite substrate shown in Figure 1A will be described below with reference to Figures 2A to 2C, as an example of the manufacturing method. Note that Figure 2C is identical to Figure 1A. 【0050】 B-2. Formation of the First and Second Intermediate Layers First, a piezoelectric substrate 10' is prepared. The piezoelectric substrate is thinned to form a piezoelectric layer, as described later. Therefore, the materials constituting the piezoelectric substrate are as described in section A-2 above with respect to the piezoelectric layer. The thickness of the piezoelectric substrate may be, for example, 100 μm to 1000 μm (1 mm), or for example, 200 μm to 500 μm. The surface on which the first intermediate layer of the piezoelectric substrate is formed can be polished to have an arithmetic mean roughness Ra of, for example, 0.1 nm to 1.0 nm, or for example, 0.2 nm to 0.4 nm. Any suitable polishing method can be used. Examples of polishing methods include lapping and chemical mechanical polishing (CMP). 【0051】 Next, as shown in Figure 2A, a first intermediate layer 20 and a second intermediate layer 30 are formed in this order on one side (the polished surface side) of the piezoelectric substrate 10'. First, the first intermediate layer 20 is directly formed on the polished surface of the piezoelectric substrate 10'. The first intermediate layer contains silicon oxide as described above. Such a first intermediate layer can be formed, for example, by reactive sputtering of silicon in the presence of oxygen. 【0052】Reactive sputtering of silicon can be performed, for example, using a carousel-type sputtering apparatus. Typically, the sputtering apparatus includes a radical source. By performing reactive sputtering of silicon while oxygen radicals are generated from the radical source, a silicon oxide film (first intermediate layer) can be formed. A sufficient amount of oxygen to obtain silicon oxide can be supplied. 【0053】 The thickness of the formed first intermediate layer may be, for example, 30 nm to 1000 nm, or for example, 250 nm to 500 nm, as explained in section A-3 above. 【0054】 Next, a second intermediate layer is directly formed on the surface of the first intermediate layer. The second intermediate layer contains silicon oxynitride as described above. Such a second intermediate layer can be formed, for example, by reactive sputtering of silicon. 【0055】 Reactive sputtering can be carried out in the same manner as the formation of the first intermediate layer, for example, using a carousel-type sputtering apparatus. A sufficient amount of nitrogen to obtain silicon oxynitride can be supplied. Reactive sputtering can typically be carried out in the presence of oxygen. The oxygen may be residual oxygen used during the formation of the first intermediate layer, or it may be supplied separately. Preferably, the amount and / or method of supplying oxygen and nitrogen can be controlled so that a region in the thickness direction of the second intermediate layer where the oxygen content decreases toward the support substrate side, and / or a region where the atomic composition ratio of nitrogen to oxygen (N / O) increases, is appropriately formed. 【0056】 The thickness of the formed second intermediate layer may preferably be 5 nm to 3000 nm, as described in section A-4 above, and may also be, for example, 200 nm to 600 nm. 【0057】The first and second intermediate layers can preferably be formed by continuous sputtering. If the first intermediate layer is formed by reactive sputtering using a carousel-type sputtering apparatus, preferably the second intermediate layer is also formed by reactive sputtering using a carousel-type sputtering apparatus. With such a configuration, continuous formation is easy. "Continuous sputtering" means performing sputtering without opening the chamber of the film formation apparatus. Since continuous sputtering does not open the chamber, foreign matter from outside the chamber does not adhere after the formation of the first intermediate layer and before the formation of the second intermediate layer. Therefore, adhesion of foreign matter to the surface of the first intermediate layer can be prevented. 【0058】 In one embodiment, during the formation of the second intermediate layer, regions with decreasing oxygen content may be formed as the deposition of the second intermediate layer progresses. As a result, the resulting second intermediate layer may appropriately have regions in which the oxygen content decreases toward the support substrate in the thickness direction, and / or regions in which the atomic composition ratio of nitrogen to oxygen (N / O) increases. 【0059】 B-3. ​​Smoothing treatment of the second intermediate layer If necessary, the exposed surface of the second intermediate layer 30 may be subjected to a smoothing treatment. Through the smoothing treatment, the surface of the second intermediate layer may be polished to have an arithmetic mean roughness Ra of, for example, 0.05 nm to 1.0 nm. 【0060】 B-4. Bonding the second intermediate layer to the support substrate Next, the support substrate 40 is prepared. The surface of the support substrate to be bonded to the second intermediate layer may be polished so that the arithmetic mean roughness Ra is, for example, 0.05 nm to 1.0 nm. 【0061】 Next, as shown in Figure 2B, the second intermediate layer 30 and the support substrate 40 are joined. Typically, the first amorphous layer 51 is formed near the surface of the support substrate 40 by activating the surface of the support substrate 40, and the second amorphous layer 52 is formed near the surface of the second intermediate layer 30 by activating the surface of the second intermediate layer 30, thereby joining the second intermediate layer 30 and the support substrate 40 via the first amorphous layer 51 and the second amorphous layer 52. 【0062】B-5. Thinning of the piezoelectric substrate (formation of the piezoelectric layer) Next, as shown in Figure 2C, the piezoelectric substrate 10' is thinned to form the piezoelectric layer 10. Specifically, as described above, a piezoelectric substrate with a thickness of about 100 μm to 1000 μm is polished to thin it and form the piezoelectric layer 10. The thickness after polishing (i.e., the thickness of the piezoelectric layer) can be, for example, 50 nm to 30000 nm, or for example, 200 nm to 1500 nm, as explained in section A-2 above. 【0063】 As described above, a composite substrate 100 as shown in Figure 2C (Figure 1A) can be obtained. 【0064】 In one embodiment, when forming the third intermediate layer, the first, second, and third intermediate layers can be formed by continuous sputtering. In this case, a composite substrate 101 as shown in Figure 1B can be obtained. When the first and second intermediate layers are formed continuously by reactive sputtering using a carousel-type sputtering apparatus, preferably the third intermediate layer is also formed by reactive sputtering using a carousel-type sputtering apparatus. With such a configuration, continuous formation is easy. Since continuous sputtering does not open the chamber, foreign matter from outside the chamber does not adhere after the formation of the first and second intermediate layers but before the formation of the third intermediate layer. Therefore, adhesion of foreign matter to the surface of the second intermediate layer before the formation of the third intermediate layer can be prevented. 【0065】 In another embodiment, the third intermediate layer may be formed by sputtering with a neutral atomic beam. Specifically, after forming the first and second intermediate layers, particles of the supporting substrate's constituent material may be ejected by sputtering to form a sputtered layer on the surface of the second intermediate layer, and this sputtered layer may be used as the third intermediate layer. In this case, a composite substrate 102 as shown in Figure 1C can be obtained. 【0066】C. Surface Acoustic Wave Element The composite substrate according to an embodiment of the present invention can be applied to a surface acoustic wave element. Therefore, a surface acoustic wave element including a composite substrate according to an embodiment of the present invention can also be included in the embodiments of the present invention. A surface acoustic wave element typically comprises the composite substrate, an input-side IDT (Interdigital Transducer) electrode (comb-shaped electrode or curtain-shaped electrode) that excites surface acoustic waves provided on the piezoelectric layer surface of the composite substrate, and an output-side IDT electrode that receives surface acoustic waves. With such a surface acoustic wave element, when a high-frequency signal is applied to the input-side IDT electrode, an electric field is generated between the electrodes, exciting the surface acoustic wave which propagates through the piezoelectric layer, and the propagated surface acoustic wave can be extracted as an electrical signal from the output-side IDT electrode provided in the propagation direction. Such a surface acoustic wave element is suitably used, for example, as a SAW filter in communication equipment such as mobile phones. 【0067】 Other specific examples of surface acoustic wave elements include Lamb wave elements and thin-film resonators (FBARs). A Lamb wave element has a structure in which comb-shaped electrodes are formed on the surface of a piezoelectric layer, a metal film is formed on the surface of the piezoelectric layer opposite to the surface where the comb-shaped electrodes are formed, and the metal film is exposed by a cavity provided in the support substrate. A thin-film resonator has a structure in which electrodes are formed on both sides of a piezoelectric layer, a metal film and an insulating film are formed on one side of the piezoelectric layer, and the metal film is exposed by forming a cavity in the insulating film. 【0068】 The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. 【0069】<Example 1> As the piezoelectric substrate, a lithium tantalate substrate (LT substrate) with an orientation flat (OF) portion, a diameter of 4 inches, and a thickness of 250 μm was used. The LT substrate was a 46° Y-cut X-propagation LT substrate, where the propagation direction of surface acoustic waves (SAW) was X and the cutting angle was a rotational Y-cut plate. The surface of the LT substrate was mirror-polished to an arithmetic mean roughness Ra of 0.3 nm. The arithmetic mean roughness Ra was evaluated using an atomic force microscope (AFM) in a square field of view of 10 μm x 10 μm. On the other hand, as the support substrate, a single-crystal silicon substrate with an OF portion, a diameter of 4 inches, and a thickness of 520 μm was prepared. The arithmetic mean roughness Ra of the support substrate surface was 0.5 nm. The orientation of the support substrate was (111) plane. 【0070】 A silicon oxide film of 400 nm was deposited on an LT substrate by reactive sputtering of silicon while oxygen radicals were generated from a radical source, thereby forming a first intermediate layer. A second intermediate layer was formed continuously from the formation of the first intermediate layer (i.e., without opening the chamber of the film deposition apparatus). More specifically, a silicon oxynitride film of 300 nm was deposited on the first intermediate layer by supplying nitrogen gas in an environment where oxygen from the formation of the first intermediate layer remained, and then performing reactive sputtering of silicon while nitrogen radicals were generated from a radical source, thereby forming a second intermediate layer. Next, the surface of the second intermediate layer was smoothed by chemical mechanical polishing (CMP). The arithmetic mean roughness Ra of the smoothed surface of the second intermediate layer was 0.5 nm. 【0071】 Next, the smoothed second intermediate layer and support substrate were cleaned, and the second intermediate layer and support substrate were directly bonded by a surface activation method. Specifically, the procedure was as follows: In a vacuum chamber where fast atomic beam (FAB) guns were each installed, the piezoelectric substrate / first intermediate layer / second intermediate layer laminate and the support substrate were positioned within the irradiation range of each FAB gun, and arranged so that the surface of the second intermediate layer and the surface of the support substrate faced each other. In this state, the vacuum chamber was heated for 10 minutes. －６The pressure was reduced to Pa, and Ar gas-based FAB (acceleration voltage 1kV, Ar flow rate 72sccm) was simultaneously irradiated from each FAB gun towards the surface of the second intermediate layer and the surface of the support substrate for 90 seconds, activating the surfaces of the second intermediate layer and the support substrate, respectively. This formed a first amorphous layer near the surface of the support substrate and a second amorphous layer near the surface of the second intermediate layer. The first amorphous layer and the second amorphous layer were brought into contact, and a load of 1000N was applied for 2 minutes to directly bond the second intermediate layer and the support substrate. 【0072】 Finally, the LT substrate was polished from its initial thickness of 250 μm to 400 nm to form a piezoelectric layer. Then, it was annealed at 100°C for 24 hours under atmospheric conditions to obtain a composite substrate. 【0073】 The elemental content (composition ratio: based on atomic percent) of the support substrate, first amorphous layer, second amorphous layer, and second intermediate layer in the obtained composite substrate was measured by energy-dispersive X-ray spectroscopy (TEM-EDX) using a transmission electron microscope. The results are shown in Table 1. 【0074】 【0075】 As is clear from Table 1, both the first amorphous layer and the second amorphous layer of the obtained composite substrate contain 40.5 atomic percent or more of silicon, which readily forms dangling bonds; 11.4 atomic percent or more of nitrogen, which readily forms dangling bonds; and 10.5 atomic percent or less of oxygen, which does not readily form dangling bonds. Therefore, it can be seen that the composite substrate of this embodiment has excellent bonding strength. 【0076】 Furthermore, the change in composition ratio from the vicinity of the second intermediate layer surface of the first intermediate layer of the obtained composite substrate to the vicinity of the second intermediate layer surface of the support substrate was measured by TEM-EDX. The results are shown in Figure 3. As is clear from Figure 3, the second intermediate layer has regions in the thickness direction toward the support substrate where the oxygen content decreases and regions where the atomic composition ratio of nitrogen to oxygen (N / O) increases. 【0077】Furthermore, the resulting composite substrates were subjected to evaluation of surface acoustic wave leakage (confinement effect) and failure (cracking / fissure). The measurement methods and evaluation criteria were as follows. 【0078】 (1) The relationship between the resonant frequency and Q value of the elastic wave apparatus using the composite substrate that achieved the confinement effect was determined by simulation using the finite element method and evaluated according to the following criteria. The length of the period λ of the IDT electrode was adjusted so that the resonant frequency was 2.1 GHz. ○ (Good): Q value of 2500 or more △ (Average): Q value of 2300 or more and less than 2500 × (Poor): Q value less than 2300 【0079】 (2) Cracks / Fixes The obtained composite substrate was diced to obtain 30 composite substrate pieces. These composite substrate pieces were subjected to a heat cycle test in which the process of heating at 300°C for 1 hour and cooling to 25°C was repeated 100 times. After that, the presence or absence of cracks and / or cracks of 0.1 mm or longer was observed in the composite substrate pieces and evaluated according to the following criteria: ○ (Good): No cracks or fractures were observed in any of the 30 composite substrate pieces. △ (Average): Cracks or fractures were observed in 3 or fewer composite substrate pieces. × (Poor): Cracks or fractures were observed in 4 or more composite substrate pieces. 【0080】 The resulting composite substrate had a Q-value of 2635, indicating good containment. Furthermore, no cracks or fractures were observed in any of the composite substrate pieces obtained from the composite substrate during the heat cycle test. The results are shown in Table 2. 【0081】<Example 2> A silicon oxide film of 400 nm thickness was deposited on an LT substrate similar to that in Example 1 by plasma chemical vapor deposition to form a first intermediate layer. Next, a silicon oxynitride film of 300 nm thickness was deposited on the first intermediate layer by plasma chemical vapor deposition to form a second intermediate layer. In the second intermediate layer, neither regions where the oxygen content decreases toward the support substrate side in the thickness direction nor regions where the atomic composition ratio of nitrogen to oxygen (N / O) increases were formed. A composite substrate was obtained by the same procedure as in Example 1. The obtained composite substrate was subjected to the same evaluation as in Example 1. The Q value of the obtained composite substrate was 2643, indicating a good confinement effect. On the other hand, cracks were observed in some of the composite substrate pieces (2 pieces) obtained from the composite substrate during a heat cycle test. The results are shown in Table 2. 【0082】 <Example 3> A silicon oxide film of 400 nm thickness was deposited on an LT substrate similar to that in Example 1 by plasma chemical vapor deposition to form a first intermediate layer. Next, a silicon oxynitride film of 300 nm thickness was deposited on the first intermediate layer by supplying nitrogen gas in an environment where oxygen from the formation of the first intermediate layer remained, and reactive sputtering of silicon was performed while nitrogen radicals were generated from a radical source, thereby forming a second intermediate layer. In the second intermediate layer, regions were formed in the thickness direction toward the support substrate where the oxygen content decreased and regions where the atomic composition ratio of nitrogen to oxygen (N / O) increased. Particles of the constituent material of the support substrate were ejected by sputtering, and a sputtered layer of 3 nm was formed on the surface of the second intermediate layer to form a third intermediate layer. The following steps were the same as in Example 1 to obtain a composite substrate. The obtained composite substrate was subjected to the same evaluation as in Example 1. The Q value of the obtained composite substrate was 2635, indicating a good confinement effect. Furthermore, no cracks or fissures were observed in any of the composite substrate pieces obtained from the composite substrate during the heat cycle test. The results are shown in Table 2. 【0083】<Comparative Example 1> A composite substrate was obtained in the same manner as in Example 1, except that a second intermediate layer was not formed. The obtained composite substrate was subjected to the same evaluation as in Example 1. The Q value of the obtained composite substrate was 2320, indicating poor containment. On the other hand, no cracks or fractures were observed in any of the composite substrate pieces obtained from the composite substrate during the heat cycle test. The results are shown in Table 2. 【0084】 <Comparative Example 2> On an LT substrate similar to that in Example 1, a silicon nitride film was formed at a thickness of 300 nm by reactive sputtering of silicon while nitrogen radicals were generated from a radical source, thereby forming a second intermediate layer. The subsequent procedure was the same as in Example 1 to obtain a composite substrate. That is, a composite substrate was obtained in the same manner as in Example 1, except that the first intermediate layer was not formed. The obtained composite substrate was subjected to the same evaluation as in Example 1. The Q value of the obtained composite substrate was 2203, indicating poor confinement effect. Furthermore, cracks or fissures were observed in some of the composite substrate pieces obtained from the composite substrate during a heat cycle test. The results are shown in Table 2. 【0085】 【0086】 <Evaluation> As is clear from Table 2, the composite substrates of the embodiments of the present invention yield acceptable results in terms of both confinement effect and cracking. In particular, the composite substrates of embodiments 1 and 3, in which a region with a decreasing oxygen content and a region with an increasing nitrogen-to-oxygen atomic composition ratio N / O (oxygen and nitrogen concentration gradient) are formed in the second intermediate layer, yield excellent results in terms of both confinement effect and cracking. In other words, the composite substrates of the embodiments of the present invention exhibit suppressed surface acoustic wave leakage and reduced damage, demonstrating excellent reliability. 【0087】 A composite substrate according to an embodiment of the present invention can be suitably used, for example, in a surface acoustic wave device. 【0088】10 Piezoelectric layer 10' Piezoelectric substrate 20 First intermediate layer 30 Second intermediate layer 40 Support substrate 51 First amorphous layer 52 Second amorphous layer 100 Composite substrate

Claims

1. A composite substrate comprising a piezoelectric layer, a first intermediate layer, a second intermediate layer, and a support substrate in this order, wherein the first intermediate layer contains silicon oxide, the second intermediate layer contains silicon oxynitride, the silicon content in the second intermediate layer is 25 atomic percent or more, and the nitrogen content in the second intermediate layer is greater than the oxygen content.

2. The composite substrate according to claim 1, wherein the second intermediate layer has a region in the thickness direction in which the oxygen content decreases toward the support substrate side.

3. The composite substrate according to claim 1, wherein the second intermediate layer has a region in the thickness direction in which the atomic composition ratio of nitrogen to oxygen N / O increases toward the support substrate side.

4. The composite substrate according to claim 1, wherein the silicon content of the second intermediate layer increases toward the support substrate side in the thickness direction, and the rate of increase of the silicon content is greater than 0 atoms % / nm and less than 0.2 atoms % / nm.

5. The composite substrate according to claim 1, wherein the second intermediate layer and the support substrate are directly bonded, and the first amorphous layer is formed on the support substrate side of the bonding interface.

6. The composite substrate according to claim 5, wherein the first amorphous layer contains an inert gas element.

7. The composite substrate according to claim 6, wherein the first amorphous layer contains elements that constitute the support substrate.

8. The composite substrate according to claim 6, wherein a second amorphous layer is formed on the second intermediate layer side of the bonding interface, and the second amorphous layer is an insulating layer containing silicon, nitrogen, and an inert gas element.

9. The composite substrate according to claim 8, wherein the inert gas element content of the first amorphous layer is greater than the inert gas element content of the second amorphous layer.

10. The composite substrate according to claim 8, wherein the thickness of the first amorphous layer is greater than the thickness of the second amorphous layer.

11. The composite substrate according to claim 8, wherein the nitrogen content in the second amorphous layer is 20 atomic% to 60 atomic%.

12. The composite substrate according to claim 1, wherein the thickness of the first intermediate layer is greater than the thickness of the second intermediate layer.

13. The composite substrate according to claim 1, further comprising a third intermediate layer between the second intermediate layer and the support substrate, wherein the silicon content in the third intermediate layer is greater than the silicon content in the second intermediate layer.

14. A method for manufacturing a composite substrate, comprising the steps in this order: forming a first intermediate layer and a second intermediate layer on one side of a piezoelectric substrate; activating the surface of the second intermediate layer and the surface of a support substrate; joining the second intermediate layer and the support substrate such that the activated surfaces of the second intermediate layer and the support substrate face each other; and thinning the piezoelectric substrate to form a piezoelectric layer.

15. A method for manufacturing a composite substrate according to claim 14, comprising: activating the surface of the support substrate to form a first amorphous layer near the surface of the support substrate; activating the surface of the second intermediate layer to form a second amorphous layer near the surface of the second intermediate layer; and joining the second intermediate layer and the support substrate via the first amorphous layer and the second amorphous layer.

16. The method for manufacturing a composite substrate according to claim 14, wherein the first intermediate layer and the second intermediate layer are formed by continuous sputtering.

17. A surface acoustic wave element having a composite substrate according to any one of claims 1 to 13.

Images