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

Abstract

Provided is a composite substrate that has excellent bonding strength and can be manufactured easily and inexpensively. A composite substrate according to an embodiment of the present invention has a piezoelectric layer, a first intermediate layer, a second intermediate layer, and a support substrate in this order. The resistivity R21 of the second intermediate layer on the first intermediate layer side is higher than the resistivity RS of the support substrate, and the resistivity R2S of the second intermediate layer on the support substrate side is lower than the resistivity RS of the support substrate. A method for manufacturing a composite substrate according to an embodiment of the present invention includes, in the order given: forming a first intermediate layer and a second intermediate layer in this order on one side of a piezoelectric substrate; subjecting the second intermediate layer to smoothing processing; bonding the second intermediate layer and a support substrate; and thinning the piezoelectric substrate to form a piezoelectric layer. The first intermediate layer is formed by reactive sputtering of silicon in the presence of oxygen, and the second intermediate layer is formed by sputtering of silicon in an atmosphere in which the oxygen used for forming the first intermediate layer remains.

Description

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

[0001] The present invention relates to a composite substrate, a method for manufacturing the same, and a surface acoustic wave device.

[0002] In communication devices such as mobile phones, for example, a filter (SAW filter) using surface acoustic waves is used to extract electrical signals of arbitrary frequencies. In recent years, the demand for high-frequency (RF) communication devices has been increasing, and SAW filters are also required to be compatible with such RF devices. For example, a composite substrate having a piezoelectric layer and a support substrate is used for SAW filters. In order to ensure good performance of RF devices, an intermediate layer such as a SiO ,

[0007] , ,

[0006] layer and a Si layer is provided in this order from the piezoelectric layer side between the piezoelectric layer and the support substrate (for example, Patent Document 1).

[0003] In the technique of Patent Document 1, the intermediate layer is typically formed using a chemical vapor deposition (CVD) method. However, since the CVD method requires heating at a high temperature (for example, 400°C to 1000°C), it is difficult to directly form the intermediate layer on the piezoelectric layer. Further, in the CVD method, a highly active and reactive gas such as silane is used, so special equipment is required to ensure safety when using such a gas, resulting in an increase in cost.

[0004] In order to avoid the above problems in the CVD method, a technique for forming the intermediate layer by sputtering has been studied. However, when joining (integrating) the piezoelectric layer and the support substrate through the intermediate layer (typically, a Si layer), the joining strength may be insufficient when using the intermediate layer formed by sputtering.

[0005] Japanese Patent No. 6612872

[0006] A main object of the present invention is to provide a composite substrate having excellent joining strength and capable of being manufactured simply and at low cost.

[0007] [1] The composite substrate according to an embodiment of the present invention has a piezoelectric layer, a first intermediate layer, a second intermediate layer, and a support substrate in this order, and the resistivity R2 on the first intermediate layer side of the second intermediate layer１ The resistivity RS of the support substrate is higher than the resistivity R2 of the second intermediate layer on the support substrate side. Ｓ [1] is lower than the resistivity RS of the support substrate. [2] In [1] above, the resistivity R2 (Ωcm) of the second intermediate layer satisfies the following equation (1): 16000 - 60 × T ≤ R2 ≤ 24000 - 40 × T ... (1) In equation (1), T (nm) is the distance from the first intermediate layer in the thickness direction of the second intermediate layer, is zero (nm) on the surface of the second intermediate layer on the first intermediate layer side, and the maximum value is equal to the thickness of the second intermediate layer. [3] In [1] or [2] above, the resistivity R2 of the second intermediate layer on the support substrate side Ｓ The difference between the resistivity RS of the above-mentioned support substrate and (R2 Ｓ-(RS) is -8000 (Ωcm) or more. [4] In any one of the above [1] to [3], the first intermediate layer is made of silicon oxide, and the second intermediate layer is made of amorphous silicon. [5] In any one of the above [1] to [4], the first intermediate layer and the second intermediate layer are each a sputtering layer. [6] In any one of the above [1] to [5], the first intermediate layer and the second intermediate layer are continuously formed sputtering layers. [7] In any one of the above [1] to [6], the composition ratio O / Si of silicon and oxygen in the second intermediate layer is 0.2 or less, and the second intermediate layer has a region where the composition ratio O / Si increases toward the piezoelectric layer side in the thickness direction. [8] 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, smoothing the second intermediate layer, bonding the second intermediate layer and a support substrate, and thinning the piezoelectric substrate to form a piezoelectric layer. The first intermediate layer is formed by reactive sputtering of silicon in the presence of oxygen; the second intermediate layer is formed by sputtering of silicon in an atmosphere with an oxygen residual amount not exceeding that used for forming the first intermediate layer. [9] In the above [8], the first intermediate layer and the second intermediate layer are continuously formed.

[10] In the above [8] or [9], in the formation of the second intermediate layer, a region where the oxygen content decreases as the film formation of the second intermediate layer progresses is formed.

[11] According to yet another aspect of the present invention, an elastic surface wave device is provided. The elastic surface wave device has a composite substrate according to any one of the above [1] to [7].

[0008] According to an embodiment of the present invention, it is possible to realize a composite substrate having excellent bonding strength and capable of being manufactured simply and at low cost.

[0009] This is a schematic cross-sectional view 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

[0010] 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.

[0011] A. Composite Substrate A-1. Schematic Figure 1 of the composite substrate is a schematic cross-sectional view of a composite substrate according to an embodiment of the present invention. The illustrated example composite substrate 100 has a piezoelectric layer 10, a first intermediate layer 20, a second intermediate layer 30, and a support substrate 40 in this order. The first intermediate layer 20 is typically formed directly on the piezoelectric layer 10; the second intermediate layer 30 is also formed directly on the first intermediate layer 20. Typically, the first intermediate layer 20 and the second intermediate layer 30 are sputtering layers; more specifically, the first intermediate layer 20 and the second intermediate layer 30 are continuously formed sputtering layers. By making the first intermediate layer 20 and the second intermediate layer 30 sputtering layers, heating at high temperatures can be avoided, so these (substantially the first intermediate layer) can be formed directly on the piezoelectric layer. Furthermore, they can be formed more simply and inexpensively compared to the case where the first intermediate layer and the second intermediate layer are formed by the CVD method.

[0012] In the illustrated example of the composite substrate, the second intermediate layer 30 and the support substrate 40 are bonded together. By integrating the support substrate through bonding, a composite substrate with sufficient mechanical strength can be realized. Furthermore, it may be possible to thin the piezoelectric layer. A preferred bonding method is typically direct bonding without the use of adhesive. Direct bonding allows for thinning of the composite substrate and prevents adverse effects from adhesives. Examples of direct bonding 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. According to the embodiment of the present invention, the bonding strength between the second intermediate layer 30 and the support substrate 40 can be sufficiently increased, and as a result, a composite substrate with excellent strength as a single unit can be realized. When using, for example, surface activation bonding as the direct bonding method, an amorphous layer may typically be formed at the bonding interface. As its name suggests, the amorphous layer has an amorphous structure and is composed of elements that make up the second intermediate layer 30 and elements that make up the support substrate 40. The amorphous layer may also contain atomic species that make up the neutral atomic beam used for direct bonding (typically argon and nitrogen).

[0013] In this specification, "direct bonding" means, as described above, 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) －６At a Pa level, a neutralization beam is irradiated onto each bonding surface of the components (layers or substrates) to be joined. As a result, each bonding surface is activated. Next, in a vacuum atmosphere, the activated bonding surfaces are brought into contact with each other and joined at room temperature. The load during this joining can be, for example, 100 N to 20,000 N. In one embodiment, when performing surface activation by a neutralization beam, an inert gas is introduced into the chamber, and a high voltage is applied from a DC power source to an electrode disposed in the chamber. With such a configuration, electrons move due to the electric field generated between the electrode (positive electrode) and the chamber (negative electrode), and a beam of atoms and ions by the inert gas is generated. Among the beams reaching the grid, the ion beam is neutralized by the grid, so that 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 (for example, 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.

[0014] In an embodiment of the present invention, the resistivity R2 of the first intermediate layer side (substantially the surface on the first intermediate layer side) of the second intermediate layer １ is higher than the resistivity RS of the support substrate, and the resistivity R2 of the support substrate side of the second intermediate layer Ｓ is lower than the resistivity RS of the support substrate (substantially the surface of the support substrate). With such a configuration, the advantages of making the second intermediate layer a high resistance and the advantages of making the second intermediate layer a low resistance can be obtained in good balance. Due to the high resistivity R2 １ current leakage caused by the second intermediate layer (ultimately, energy loss in the composite substrate) is appropriately suppressed, and the bonding strength between the second intermediate layer and the support substrate can be made sufficiently large. As a result, a composite substrate excellent in the balance between suppression of energy loss (electrical loss) and bonding strength can be realized.

[0015] The resistivity R2 (Ωcm) of the second intermediate layer 30 preferably satisfies the following equation (1), more preferably satisfies the following equation (2), even more preferably satisfies the following equation (3), and particularly preferably satisfies the following equation (4). 16000 - 60 × T ≤ R2 ≤ 24000 - 40 × T ... (1) 17000 - 57 × T ≤ R2 ≤ 23000 - 43 × T ... (2) 18000 - 55 × T ≤ R2 ≤ 22000 - 45 × T ... (3) 19000 - 52 × T ≤ R2 ≤ 21000 - 48 × T ... (4) In equations (1) to (4), T (nm) is the distance from the first intermediate layer in the thickness direction of the second intermediate layer, is zero (nm) on the surface of the second intermediate layer on the first intermediate layer side, and the maximum value is equal to the thickness of the second intermediate layer. In other words, equations (1) to (4) represent the gradient of resistivity R2 in the thickness direction. If the gradient of resistivity R2 in the thickness direction is within this range, the balance between the advantages of having a high-resistance second intermediate layer and the advantages of having a low-resistance second intermediate layer can be better achieved. Such a gradient of resistivity R2 in the thickness direction can be determined, for example, by setting the residual oxygen concentration when forming the second intermediate layer to a predetermined amount (typically 6.6 × 10⁻⁶). －３ It can be controlled by adjusting it within a range of mg / L or less.

[0016] Resistivity R2 on the support substrate side of the second intermediate layer Ｓ The difference between the resistivity RS of the support substrate and (R2 Ｓ The difference (R2) is preferably -8000 (Ωcm) or higher, more preferably -5000 (Ωcm) or higher, and even more preferably -4000 (Ωcm) or higher. Ｓ If the difference (R2) is within this range, the bonding strength between the second intermediate layer and the support substrate can be maintained within an excellent range, while current leakage (ultimately, energy loss in the composite substrate) caused by the second intermediate layer can be suppressed more effectively. Ｓ A smaller absolute value of -RS is preferable. As described above, the resistivity R2 on the support substrate side of the second intermediate layer Ｓ Since the resistivity RS of the support substrate is lower, the difference (R2 Ｓ The difference (RS) can be a negative value close to zero. Ｓ-RS) may be, for example, -10 (Ωcm), or for example, -100 (Ωcm). Difference (R2 Ｓ -RS), effectively resistivity R2 Ｓ For example, the residual oxygen concentration when forming the second intermediate layer (e.g., 6.6 × 10⁻⁶) －３ This can be controlled by adjusting the thickness of the second intermediate layer (mg / L or less).

[0017] Resistivity R1 of the first intermediate layer and resistivity R2 of the second intermediate layer on the first intermediate layer side. １ The difference between (R1-R2) １ ) preferably 1 × 10 １４ (Ωcm) or greater, more preferably 1 × 10 １５ (Ωcm) ~ 5 × 10 １６ (Ωcm), and more preferably 5 × 10 １５ (Ωcm) ~ 1 × 10 １６ (Ωcm). The difference is (R1 - R2 １ ), effectively resistivity R2 １ This can be controlled, for example, by adjusting the residual oxygen concentration when forming the second intermediate layer.

[0018] The total thickness of the composite substrate can be, for example, 250 μm to 700 μm.

[0019] The composite substrate according to the 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 depending on the purpose. The diameter of the wafer may be, for example, 75 mm to 200 mm, or for example, 4 inches (about 100 mm). Typically, multiple devices (e.g., SAW filters) can be manufactured from a single composite substrate. However, the composite substrate is not limited to the form of a wafer and may be manufactured and provided in various forms.

[0020] <Modification> The composite substrate according to the embodiment of the present invention may optionally have a silicon oxide layer and an amorphous silicon layer on the side opposite the second intermediate layer of the support substrate 40, in order from the support substrate side (neither is shown). The thickness of the silicon oxide layer may be, for example, 100 nm to 1000 nm; the thickness of the amorphous silicon layer may be, for example, 100 nm to 1000 nm. With such a configuration, warping of the composite substrate can be well suppressed. As a result, defects during polishing are suppressed, and a composite substrate with excellent surface uniformity and suppressed thickness variation can be obtained. The silicon oxide layer may have the same configuration as when the first intermediate layer is composed of silicon oxide (Section A-3), and the amorphous silicon layer may have the same configuration as the second intermediate layer (Section A-4), except that it is manufactured without supplying oxygen.

[0021] The components of the composite substrate will be explained in detail below.

[0022] A-2. Piezoelectric layer Any suitable piezoelectric material can be used as the material constituting the piezoelectric layer 10. Preferably, LiAO is used as the piezoelectric material. ３ A single crystal having the following composition is used. Here, A is one or more elements selected from the group consisting of niobium and tantalum. Specifically, LiAO ３ Lithium niobate (LiNbO) ３ ) may also be lithium tantalate (LiTaO ３ ) may be a solid solution of lithium niobate and lithium tantalate. Another example of a piezoelectric material is potassium titanate 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 Examples include 0≦x≦1: KTN), silicon, quartz, silica, silicon carbide, gallium nitride, indium phosphide, and lead zirconate titanate (PZT).

[0023] When the piezoelectric material is lithium tantalate, 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 32° to 50° (for example, 43°) 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 40°, 180°). With such a configuration, propagation loss can be suppressed.

[0024] 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 40° (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 40°, 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 the case, the direction rotated 40° to 65° 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 angle corresponds to (180°, 50° to 25°, 180°) in Euler angle notation.

[0025] 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.

[0026] 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).

[0027] 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.

[0028] A-3. First Intermediate Layer The first intermediate layer 20 may be provided, for example, to improve the stability of the temperature characteristics of the composite substrate. Any suitable dielectric material can be used as the material constituting the first intermediate layer. Examples of dielectric materials include silicon oxide, silicon nitride, silicon oxynitride, aluminum oxide, aluminum nitride, and aluminum oxynitride. The first intermediate layer may be a single layer or may have a laminated structure consisting of multiple layers composed of different dielectric materials. The first intermediate layer may preferably be composed of silicon oxide. With such a configuration, the first intermediate layer and the second intermediate layer can be formed continuously by sputtering.

[0029] The resistivity R1 of the first intermediate layer is, for example, 1 × 10⁻⁶ １４ (Ωcm) ~ 1 × 10 １６ It can be (Ωcm), and also, for example, 5 × 10 １５ (Ωcm) ~ 1 × 10 １６ It can be (Ωcm). That is, the resistivity R1 of the first intermediate layer is typically the resistivity R2 of the surface of the second intermediate layer on the first intermediate layer side. １ It is significantly higher than the other. As a result, as described above, the resistivity R1 of the first intermediate layer and the resistivity R2 of the second intermediate layer on the first intermediate layer side are... １ The difference between (R1-R2) １ ) For example, 1 x 10 １４ (Ωcm) ~ 5 × 10 １６ It can be (Ωcm), and also, for example, 5 × 10 １５ (Ωcm) ~ 1 × 10 １６ It could be (Ωcm).

[0030] The thickness of the first intermediate layer may be, for example, 100 nm to 1000 nm, or 200 nm to 800 nm, or 300 nm to 700 nm, or 400 nm to 600 nm.

[0031] A-4. Second Intermediate Layer The second intermediate layer 30 is typically composed of silicon. With such a configuration, a composite substrate with excellent bonding strength can be realized. The silicon may be single-crystal silicon, polycrystalline silicon, or amorphous silicon. Preferably, it is amorphous silicon. By composing the second intermediate layer with amorphous silicon, a composite substrate with even better bonding strength can be realized. Furthermore, amorphous silicon may have the following advantages compared to polycrystalline silicon: Polycrystalline silicon requires adjustments such as optimizing the grain size of the columnar structure to obtain predetermined properties, but amorphous silicon does not have grain boundaries and has an isotropic atomic arrangement, so such adjustments are not necessary.

[0032] In one embodiment, the second intermediate layer may contain hydrogen atoms and oxygen atoms. Furthermore, if the second intermediate layer and the support substrate are directly bonded, for example, by a surface activation method as described above, the second intermediate layer may further contain argon atoms.

[0033] As described above, the resistivity R2 of the surface of the second intermediate layer on the first intermediate layer side. １ The resistivity R2 of the second intermediate layer is higher than the resistivity RS of the support substrate; the resistivity R2 of the second intermediate layer decreases as it approaches the support substrate; and the resistivity R2 of the second intermediate layer on the support substrate side is higher than the resistivity RS of the support substrate. Ｓ The resistivity R2 is lower than that of the support substrate. As mentioned above, with this configuration, the advantages of having a high-resistivity second intermediate layer and the advantages of having a low-resistivity second intermediate layer can be obtained in a good balance. Resistivity R2 １ Due to the high resistivity R2, current leakage (ultimately energy loss in the composite substrate) caused by the second intermediate layer is adequately suppressed, and the resistivity R2 Ｓ Because the resistivity R2 is lower, it becomes easier to activate, allowing the bonding strength between the second intermediate layer and the support substrate to be sufficiently increased. As a result, a composite substrate with an excellent balance between suppression of energy loss (electrical loss) and bonding strength can be realized. Specifically, the resistivity R2 on the surface of the second intermediate layer on the first intermediate layer side. １ Preferably 1.0 × 10 ３ (Ωcm) ~2.0×10 ４ (Ωcm) is more preferably 1.5 × 10 ３(Ωcm) ~2.0×10 ４ (Ωcm). Resistivity R2 on the surface of the second intermediate layer on the support substrate side. Ｓ Preferably, the impedance is 80 (Ωcm) to 1000 (Ωcm), and more preferably 200 (Ωcm) to 1000 (Ωcm).

[0034] The silicon-to-oxygen composition ratio O / Si (based on atomic percent) in the second intermediate layer is preferably 0.20 or less, more preferably 0.15 or less, even more preferably 0.10 or less, and particularly preferably below the detection limit. When the composition ratio O / Si is within this range, amorphous silicon is more easily activated when bonding to the support substrate via the second intermediate layer, so that a composite substrate with sufficient bonding strength can be obtained. The composition ratio O / Si can be calculated, for example, from the silicon concentration and oxygen concentration obtained by energy-dispersive X-ray spectroscopy (TEM-EDX) using a transmission electron microscope.

[0035] The second intermediate layer preferably has a region in the thickness direction where the composition ratio O / Si increases toward the piezoelectric layer side. In other words, the second intermediate layer has a region in the thickness direction where the oxygen content increases toward the piezoelectric layer side. The "region where the oxygen content increases" may be a region where the oxygen content increases continuously or stepwise over the entire thickness direction or a part thereof of the second intermediate layer, or it may be a region having an oxygen content higher than the average oxygen content formed on the piezoelectric layer side of the second intermediate layer. With such a configuration, when bonding to the support substrate via the second intermediate layer, the amount of hydrogen on the surface side of the second intermediate layer decreases, making it easier to maintain an activated state, and thus sufficient bonding strength can be obtained.

[0036] The average hydrogen content in the second intermediate layer is preferably 5.0 × 10⁻⁶. ２１ (atoms / cc) or less, more preferably 2.5 × 10 ２１ (atoms / cc) or less, and more preferably 1.0 × 10 ２１ The average hydrogen content is less than or equal to (atoms / cc), and particularly preferably below the detection limit. If the average hydrogen content is within this range, crystallization of the second intermediate layer can be suppressed.

[0037] The second intermediate layer preferably has a region in the thickness direction where the hydrogen content increases toward the piezoelectric layer side. The "region where the hydrogen content increases" may be a region where the hydrogen content increases continuously or stepwise over the entire thickness direction or a part thereof, or it may be a region having a hydrogen content higher than the average hydrogen content formed on the piezoelectric layer side of the second intermediate layer. With such a configuration, when bonding with the support substrate via the second intermediate layer, the amount of hydrogen on the surface side of the second intermediate layer decreases, resulting in fewer unbonded bonds and easier activation, thus enabling sufficient bonding strength to be obtained. The hydrogen content can be measured, for example, by secondary ion mass spectrometry (SIMS).

[0038] 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 200 nm to 800 nm, and especially preferably 400 nm to 600 nm.

[0039] The arithmetic mean roughness Ra of the surface of the second intermediate layer on the support substrate side can be, for example, 0.1 nm to 1.0 nm, or for example, 0.2 nm to 0.8 nm, or for example, 0.3 nm to 0.7 nm, or for example, 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 increased.

[0040] A-5. Support Substrate Any suitable substrate can be used as the support substrate 40. The support substrate may be made of a single crystal, a polycrystalline material, or a combination thereof. Examples of materials that make up the support substrate include silicon, sapphire, sialon, cordierite, mullite, glass, quartz, crystal, alumina, germanium, silicon carbide, gallium nitride, indium phosphide, and aluminum nitride. The support substrate is preferably made of silicon. With such a configuration, sufficient bonding strength can be obtained when directly bonded to the second intermediate layer. The support substrate is more preferably made of single-crystal silicon. In this case, the orientation of the support substrate is the (111) plane.

[0041] The silicon described above may be single-crystal silicon, which may have a polycrystalline layer formed on its surface, or it may be high-resistance silicon.

[0042] The resistivity RS of the support substrate may vary depending on the configuration of the support substrate. The resistivity RS is substantially constant in the thickness direction. The resistivity RS is preferably 1000 (Ωcm) to 8000 (Ωcm), and more preferably 2000 (Ωcm) to 5000 (Ωcm).

[0043] It is preferable that the thermal expansion coefficient of the material constituting the support substrate is smaller than that 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, and for example, changes in the frequency characteristics of the resulting surface acoustic wave element can be suppressed.

[0044] Any appropriate thickness can be used for the support substrate. For example, the thickness of the support substrate may be between 100 μm and 1000 μm (1 mm). If the thickness of the support substrate is within this range, sufficient mechanical strength is provided to the composite substrate as a single unit, and it becomes possible to thin the piezoelectric layer.

[0045] The arithmetic mean roughness Ra of the surface of the support substrate on the second intermediate layer side can be, for example, 0.1 nm to 1.0 nm, or for example, 0.2 nm to 0.8 nm, or for example, 0.3 nm to 0.7 nm, or for example, 0.4 nm to 0.6 nm. With such a support substrate, for example, a high-performance (for example, having a high Q factor) surface acoustic wave element can be obtained.

[0046] 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, smoothing the second intermediate layer, joining the second intermediate layer to a support substrate, and thinning the piezoelectric substrate to form a piezoelectric layer. In the embodiment of the present invention, the first intermediate layer is formed by reactive sputtering of silicon in the presence of oxygen; the second intermediate layer is formed by sputtering of silicon in an atmosphere with an amount of oxygen less than or equal to the residual amount of oxygen used to form the first intermediate layer.

[0047] An example of the manufacturing method will be explained below with reference to Figures 2A to 2C. Note that Figure 2C is identical to Figure 1.

[0048] 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).

[0049] 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 of the piezoelectric substrate 10'. First, the first intermediate layer 20 is directly formed on the polished surface of the piezoelectric substrate 10'. As described above, the first intermediate layer is formed by reactive sputtering of silicon in the presence of oxygen. In this case, the formed first intermediate layer may be composed of silicon oxide. More specifically, the first intermediate layer can typically be formed using a carousel-type sputtering apparatus. The sputtering apparatus typically includes a radical source. By performing reactive sputtering of silicon while oxygen radicals are generated from the radical source, a silicon oxide layer (first intermediate layer) can be formed. The thickness of the formed first intermediate layer may be, for example, 100 nm to 1000 nm, or for example, 400 nm to 600 nm, as described in section A-3 above.

[0050] Next, the second intermediate layer is directly formed on the surface of the first intermediate layer. As described above, the second intermediate layer is formed by sputtering silicon in an atmosphere with an oxygen concentration below that of the oxygen used to form the first intermediate layer. In other words, the second intermediate layer is formed by sputtering after the formation of the first intermediate layer without supplying any new oxygen. The concentration that is "below the amount of residual oxygen" is, for example, 6.6 × 10⁻⁶.－３ The concentration may be mg / L or less. The thickness of the formed second intermediate layer may preferably be 5 nm to 3000 nm, as described in item A-4 above, and may also be, for example, 400 nm to 600 nm.

[0051] In one embodiment, the first and second intermediate layers can be formed continuously. This continuous formation can be achieved by using a carousel-type sputtering apparatus as described above. For example, the first intermediate layer can be formed by performing reactive sputtering of silicon while generating oxygen radicals by turning on the radical source; and the second intermediate layer can be formed by turning off the radical source when the first intermediate layer is formed and continuing the sputtering of silicon. In this way, the first and second intermediate layers can be formed continuously.

[0052] In one embodiment, during the formation of the second intermediate layer, regions may be formed in which the hydrogen content and oxygen content decrease as the deposition of the second intermediate layer progresses. As a result, the second intermediate layer of the resulting composite substrate may have regions in the thickness direction in which the hydrogen content increases toward the piezoelectric layer and / or regions in which the composition ratio O / Si increases. It can be inferred that the "region with decreased hydrogen content" may be formed because residual moisture in the deposition chamber is incorporated into the deposited second intermediate layer, and the residual moisture in the deposition chamber decreases as the deposition progresses. The "region with decreased oxygen content" may be formed by the decrease in residual oxygen as the deposition of the second intermediate layer progresses, by turning "OFF" the radical source of the sputtering apparatus during the formation of the second intermediate layer.

[0053] B-3. ​​Smoothing treatment of the second intermediate layer Next, the exposed surface of the second intermediate layer 30 is subjected to a smoothing treatment. Through the smoothing treatment, the surface of the second intermediate layer can be polished to have an arithmetic mean roughness Ra of, for example, 0.1 nm to 1.0 nm.

[0054] 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, 0.1 nm to 1.0 nm.

[0055] Next, as shown in Figure 2B, the second intermediate layer 30 and the support substrate 40 are joined. As for the joining method, direct joining is preferred, as explained in section A-1 above. The second intermediate layer and the support substrate can be directly joined, for example, by a surface activation method.

[0056] 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.

[0057] As described above, a composite substrate 100 as shown in Figure 2C can be obtained.

[0058] 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.

[0059] 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.

[0060] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[0061] <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 200 μm was used. The LT substrate used was a 43° 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 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. The resistivity RS of the support substrate was 2000 (Ωcm).

[0062] SiO is applied to the LT substrate by sputtering. ２ A 400 nm thick film was deposited to form the first intermediate layer. More specifically, a carousel-type sputtering apparatus equipped with a radical source was used to deposit SiO ２A first intermediate layer was formed by reactive sputtering of silicon while generating a sufficient amount of oxygen radicals to obtain the desired result. Following the formation of the first intermediate layer, a second intermediate layer with a thickness of 500 nm was deposited on the first intermediate layer by sputtering. More specifically, the second intermediate layer was formed by continuing silicon sputtering with the radical generation source turned "OFF" at the time the first intermediate layer was formed. Next, the surface of the second intermediate layer was smoothed by lapping. The arithmetic mean roughness Ra of the smoothed second intermediate layer surface was 0.5 nm.

[0063] 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 smoothed surfaces of the second intermediate layer and support substrate were activated, the activated surfaces were brought into contact with each other, and a load of 10,000 N was applied for 2 minutes to directly bond the second intermediate layer and support substrate. Finally, the LT substrate was polished from its initial thickness of 200 μm to 1,000 nm to form a piezoelectric layer.

[0064] A composite substrate was obtained as described above. The resistivity (spread resistance) of the second intermediate layer of the obtained composite substrate was measured using a spread resistance measuring device (SEMILAB, "SRP-2100"). The measurement conditions were as follows: Bevel angle: 0.005 [deg] Measurement interval: 3 μm Probe load: 10 g Applied voltage: DC 10 mV Probe spacing: approximately 70 μm Probe diameter: approximately 6 μm

[0065] In the resulting composite substrate, the resistivity R2 on the support substrate side of the second intermediate layer Ｓ It is 100 (Ωcm), and the difference from the resistivity RS of the support substrate (R2 Ｓ The resistivity R2 of the second intermediate layer on the first intermediate layer side was -1900 (Ωcm). １ The coefficient of force was 5000 (Ωcm). The bonding strength of the resulting composite substrate was 4 J / m ２ (Bulk fracture strength) was [value missing].

[0066] <Comparative Example 1> A composite substrate was obtained in the same manner as in Example 1, except that 1% of the total amount of oxygen was supplied relative to the total amount of introduced gas during the formation of the second intermediate layer. In the obtained composite substrate, the resistivity R2 on the support substrate side of the second intermediate layer Ｓ is 1 x 10 ４ (Ωcm) is the difference from the resistivity RS of the support substrate (R2 Ｓ The resistivity R2 of the second intermediate layer on the first intermediate layer side was also calculated. １ The coefficient of force was 5000 (Ωcm). The bonding strength of the resulting composite substrate was 4 J / m ２ (Bulk fracture strength) was [value missing].

[0067] A composite substrate according to an embodiment of the present invention can be suitably used, for example, in a surface acoustic wave element.

[0068] 10 Piezoelectric layer 10' Piezoelectric substrate 20 First intermediate layer 30 Second intermediate layer 40 Support substrate 100 Composite substrate

Claims

1. The piezoelectric layer, the first intermediate layer, the second intermediate layer, and the support substrate are arranged in this order, and the resistivity R2 of the second intermediate layer on the first intermediate layer side. １ The resistivity RS of the support substrate is higher than the resistivity R2 of the second intermediate layer on the support substrate side. Ｓ A composite substrate in which the resistivity RS of the support substrate is lower than that of the support substrate.

2. The composite substrate according to claim 1, wherein the resistivity R2 (Ωcm) of the second intermediate layer satisfies the following equation (1): 16000 - 60 × T ≤ R2 ≤ 24000 - 40 × T ... (1) In equation (1), T (nm) is the distance from the first intermediate layer in the thickness direction of the second intermediate layer, is zero (nm) on the surface of the second intermediate layer on the first intermediate layer side, and the maximum value is equal to the thickness of the second intermediate layer.

3. Resistivity R2 on the support substrate side of the second intermediate layer Ｓ The difference between the resistivity RS of the support substrate and (R2 Ｓ The composite substrate according to claim 2, wherein -RS) is -8000 (Ωcm) or greater.

4. The composite substrate according to claim 1, wherein the first intermediate layer is composed of silicon oxide and the second intermediate layer is composed of amorphous silicon.

5. The composite substrate according to claim 4, wherein the first intermediate layer and the second intermediate layer are each sputtering layers.

6. The composite substrate according to claim 5, wherein the first intermediate layer and the second intermediate layer are continuously formed sputtering layers.

7. The composite substrate according to claim 1, wherein the silicon-to-oxygen composition ratio O / Si of the second intermediate layer is 0.2 or less, and the second intermediate layer has a region in the thickness direction where the composition ratio O / Si increases toward the piezoelectric layer.

8. A method for manufacturing a composite substrate, comprising the steps of forming a first intermediate layer and a second intermediate layer on one side of a piezoelectric substrate in that order, smoothing the second intermediate layer, joining the second intermediate layer to a support substrate, and thinning the piezoelectric substrate to form a piezoelectric layer, wherein the first intermediate layer is formed by reactive sputtering of silicon in the presence of oxygen, and the second intermediate layer is formed by sputtering of silicon in an atmosphere with an amount of oxygen less than or equal to the residual amount of oxygen used to form the first intermediate layer.

9. The method for manufacturing a composite substrate according to claim 8, wherein the first intermediate layer and the second intermediate layer are formed continuously.

10. The method for manufacturing a composite substrate according to claim 8 or 9, wherein, in the formation of the second intermediate layer, a region is formed in which the oxygen content decreases as the deposition of the second intermediate layer progresses.

11. A surface acoustic wave element having a composite substrate according to any one of claims 1 to 7.

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