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

Abstract

Provided is a composite substrate in which electrical loss is suppressed and which can be manufactured easily and inexpensively. 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 that order. The resistivity R2Ｓ of the support substrate side of the second intermediate layer is 1000 (Ωcm) or more. A method for manufacturing the composite substrate according to an embodiment of the present invention comprises, in the following order: forming the first intermediate layer and the second intermediate layer, in that order, on one side of a piezoelectric substrate; smoothing the second intermediate layer; bonding the second intermediate layer and the support substrate; and thinning the piezoelectric substrate into the 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 the presence of a predetermined amount of oxygen.

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 any desired frequency. 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, for example, a composite substrate having a piezoelectric layer and a support substrate. To ensure good performance of RF devices, SiO is layered between the piezoelectric layer and the support substrate in order from the piezoelectric layer side. ２ A composite substrate having layers and an intermediate layer such as a Si layer has been proposed (for example, Patent Document 1).

[0003] The intermediate layer in the technology described in Patent Document 1 is typically formed using chemical vapor deposition (CVD). However, since the CVD method requires heating at high temperatures (e.g., 400°C to 1000°C), it is difficult to directly form the intermediate layer on the piezoelectric layer. Furthermore, the CVD method uses highly active and reactive gases such as silane, requiring special equipment to ensure safety when using such gases, which results in increased costs.

[0004] To avoid the problems described above in the CVD method, a technique for forming the intermediate layer by sputtering is being considered. However, when using an intermediate layer formed by sputtering, the resistance of the Si layer adjacent to the support substrate may be low. In this case, current leakage may occur due to the low-resistance Si layer, which can cause energy loss in the composite substrate.

[0005] Patent No. 6612872

[0006] The main objective of the present invention is to provide a composite substrate that can be manufactured simply and inexpensively while suppressing electrical losses.

[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 of the second intermediate layer on the support substrate side Ｓ is 1000 (Ωcm) or more. [2] In the above [1], the resistivity R2 on the support substrate side of the second intermediate layer Ｓ [1] is higher than the resistivity RS of the support substrate. [3] In [1] or [2] above, the first intermediate layer is made of silicon oxide, and the second intermediate layer is made of amorphous silicon containing oxygen. [4] In any of [1] to [3] above, the first intermediate layer and the second intermediate layer are sputtering layers, respectively. [5] In any of [1] to [4] above, the first intermediate layer and the second intermediate layer are continuously formed sputtering layers. [6] In any of [1] to [5] above, the silicon to oxygen composition ratio O / Si on the support substrate side of the second intermediate layer is 0.05 to 0.15. [7] 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 to 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 the presence of a predetermined amount of oxygen. [8] In [7] above, the first intermediate layer and the second intermediate layer are formed continuously. [9] In [7] or [8] above, the manufacturing method adds 0.05 volume% to 5.0 volume% of oxygen relative to the total amount of introduced gas in the formation of the second intermediate layer.

[10] In [9] above, the manufacturing method introduces the oxygen intermittently.

[11] According to yet another aspect of the present invention, a surface acoustic wave element is provided. The surface acoustic wave element has a composite substrate of any of the above [1] to [6].

[0008] According to embodiments of the present invention, a composite substrate can be realized that has suppressed electrical losses and can be manufactured simply and inexpensively.

[0009] This is a schematic cross-sectional view of a composite substrate according to an embodiment of the present invention. This is a conceptual diagram illustrating the characteristics of the change in resistivity in the thickness direction of the first intermediate layer, the second intermediate layer, and the support substrate in 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 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.

[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 joined in any appropriate form. By integrating the support substrate through joining, a composite substrate with sufficient mechanical strength can be realized. Furthermore, it may be possible to thin the piezoelectric layer. Examples of joining methods include joining with an adhesive and direct joining without an adhesive. Direct joining is preferred. 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 joined 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 joining layer. Typically, an amorphous layer may be formed at the joining interface of direct joining. As the name suggests, the amorphous layer has an amorphous structure and is composed of the elements constituting the second intermediate layer 30 and the elements constituting the support substrate 40. The amorphous layer may also contain atomic species (typically argon and nitrogen) that make up the neutral atomic beam used in direct bonding.

[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 pressure of approximately Pa, a neutralizing beam is irradiated onto each bonding surface of the components (layers or substrates) to be joined. This activates each bonding surface. Next, the activated bonding surfaces are brought into contact with each other in a vacuum atmosphere and bonded at room temperature. The load during this bonding can be, for example, 100 N to 20,000 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 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 from the inert gas is generated. 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 (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 embodiments of the present invention, the resistivity R2 of the second intermediate layer on the support substrate side (substantially, the surface on the support substrate side) Ｓ The resistivity R² is 1000 (Ωcm) or more. Ｓ The resistivity is preferably 1000 (Ωcm) to 30000 (Ωcm), more preferably 1200 (Ωcm) to 20000 (Ωcm), even more preferably 1500 (Ωcm) to 10000 (Ωcm), and particularly preferably 1800 (Ωcm) to 5000 (Ωcm). With such a configuration, the following advantages can be obtained. In a composite substrate according to an embodiment of the present invention, the resistivity of the second intermediate layer (especially the resistivity on the support substrate side) may be the lowest due to the materials and manufacturing methods of each component, and the second intermediate layer may become the cause of current leakage (ultimately, energy loss in the composite substrate). In contrast, by controlling the resistivity of the second intermediate layer as described above, current leakage caused by the second intermediate layer can be suppressed, and as a result, energy loss (electrical loss) in the composite substrate can be suppressed. Such an effect is particularly noticeable in RF devices to which high-frequency power is input. Therefore, a composite substrate according to an embodiment of the present invention can be suitably applied to RF devices.

[0015] As described above, the resistivity R2 of the support substrate side (substantially, the surface on the support substrate side) of the second intermediate layer Ｓ may be 1000 (Ωcm) or more. The resistivity R2 Ｓ The difference (R2 Ｓ - RS) between the resistivity RS of the support substrate 40 can vary depending on the configuration of the support substrate used. Preferably, the resistivity R2 Ｓ can be equal to or higher than the resistivity RS. Therefore, the difference (R2 Ｓ - RS) can be, for example, 0 (Ωcm) or more, or can be, for example, 8000 (Ωcm) or more, or can be, for example, 10000 (Ωcm) or more. With such a configuration, the above-described effects according to the embodiments of the present invention can become more prominent. On the other hand, the difference (R2 Ｓ - RS) can be, for example, 30000 (Ωcm) or less, or can be, for example, 20000 (Ωcm) or less.

[0016] FIG. 2 is a conceptual diagram for explaining the characteristics of the change in resistivity in the thickness direction of the first intermediate layer, the second intermediate layer, and the support substrate in the composite substrate. The horizontal direction of the drawing corresponds to the thickness direction of the composite substrate, and the vertical direction corresponds to the resistivity of the first intermediate layer, the second intermediate layer, and the support substrate. The resistivity R2 of the surface of the second intermediate layer on the support substrate side ＳAs described above, the resistivity R2 is 1000 (Ωcm) or more, and preferably higher than the resistivity RS of the support substrate. This configuration essentially means that the value of the resistivity R2 of the second intermediate layer at the boundary between the second intermediate layer and the support substrate (the minimum value of dashed line II in Figure 2) is 1000 (Ωcm) or more, and this minimum value is preferably higher than the resistivity RS of the support substrate (the minimum value of dashed line II in Figure 2 is located above the dashed line where it is substantially constant in the support substrate). The resistivity R2 of the second intermediate layer can substantially vary within the range shown by the shaded area between dashed line I and dashed line II in Figure 2. The resistivity R2 of the second intermediate layer can vary in any manner as long as it can vary within the range of the shaded area. The resistivity R2 of the second intermediate layer may decrease monotonically from the first intermediate layer side toward the support substrate side, as shown by dashed line II; it may remain constant up to a predetermined position in the thickness direction of the second intermediate layer, as shown by dashed line I, and then begin to decrease from that position (i.e., it may decrease in a curved profile); it may decrease in a stepwise manner; or it may decrease in a waveform profile. The resistivity R2 of the second intermediate layer can be controlled by controlling the oxygen supply method during the formation process of the second intermediate layer, as will be explained in detail in section B-2 below. If oxygen is not supplied during the formation process of the second intermediate layer, the resistivity R2 of the second intermediate layer will fluctuate, for example, as shown by dashed line III, and R2 Ｓ This can be significantly lower than 1000 (Ωcm). Furthermore, as shown in Figure 2, the resistivity R1 of the first intermediate layer is typically the resistivity R2 of the second intermediate layer on the first intermediate layer side (essentially, the surface on the first intermediate layer side). １ It is significantly higher than the resistivity R1 of the first intermediate layer and the resistivity R2 of the surface of the second intermediate layer on the first intermediate layer side. １ The difference between (R1-R2) １ ) For example, 1 x 10 １４ It can be greater than (Ωcm), and for example, 5 × 10 １５ (Ωcm) ~ 1 × 10 １６ It can be (Ωcm). Note that the dashed line showing resistivity in Figure 2 does not reflect the actual value, but is drawn with the difference emphasized for clarity. It goes without saying that the actual resistivity profile is not a straight line as shown in the example.

[0017] The total thickness of the composite substrate can be, for example, 250 μm to 700 μm.

[0018] 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 a single 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.

[0019] <Modification Example> The composite substrate according to an embodiment of the present invention may, if necessary, have a silicon oxide layer and an amorphous silicon layer in this order from the support substrate side on the side opposite to the second intermediate layer of the support substrate 40 (both are not shown). The thickness of the silicon oxide layer can be, for example, 100 nm to 1000 nm; the thickness of the amorphous silicon layer can be, for example, 100 nm to 1000 nm. With such a configuration, the warpage of the composite substrate can be well suppressed. As a result, problems during the polishing process can be suppressed, and a composite substrate with excellent surface uniformity and suppressed thickness variation can be obtained. Note that the silicon oxide layer can have the same configuration as in the case where the first intermediate layer is made of silicon oxide (Item A-3), and the amorphous silicon layer can have the same configuration as the second intermediate layer (Item A-4) except that it is produced without supplying oxygen.

[0020] Hereinafter, the components of the composite substrate will be specifically described.

[0021] 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 a 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 (KTiOPO４ :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).

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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).

[0026] 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.

[0027] 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.

[0028] 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 (Ωcm) is possible. As a result, as described above, the resistivity R1 of the first intermediate layer and the resistivity R2 of the surface 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 It can be greater than (Ωcm), and for example, 5 × 10 １５ (Ωcm) ~ 1 × 10 １６ It could be (Ωcm).

[0029] The thickness of the first intermediate layer may be, for example, 100 nm to 1000 nm, or for example, 200 nm to 800 nm, or for example, 300 nm to 700 nm, or for example, 400 nm to 600 nm.

[0030] A-4. Second Intermediate Layer The second intermediate layer 30 is typically composed of silicon containing oxygen. With such a configuration, electrical losses are suppressed, and a composite substrate with excellent high-frequency characteristics 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 suppressed electrical losses compared to crystalline silicon 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 characteristics, but amorphous silicon does not have grain boundaries and has an isotropic atomic arrangement, so such adjustments are not necessary.

[0031] In one embodiment, the second intermediate layer may further contain hydrogen 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.

[0032] As described above, the resistivity R2 of the surface on the support substrate side of the second intermediate layer Ｓ The resistivity R2 is 1000 (Ωcm) or more, preferably 1000 (Ωcm) to 30000 (Ωcm), more preferably 1200 (Ωcm) to 20000 (Ωcm), even more preferably 1500 (Ωcm) to 10000 (Ωcm), and particularly preferably 1800 (Ωcm) to 5000 (Ωcm). Preferably, resistivity R2 Ｓ The resistivity R2 is higher than that of the support substrate. With such a configuration, electrical losses are suppressed, and a composite substrate with excellent high-frequency characteristics can be realized. The desired resistivity R2 Ｓ Insofar as this can be obtained, the resistivity R2 on the surface of the second intermediate layer on the first intermediate layer side １ The resistivity can be any appropriate value. Typically, the resistivity R² is used. １ The resistivity R2 Ｓ It is higher than that. Specifically, the resistivity R2 １ Preferably, the Ω value is 1500 (Ωcm) to 30000 (Ωcm), and more preferably 1700 (Ωcm) to 10000 (Ωcm).

[0033] The silicon-to-oxygen composition ratio O / Si (based on atomic percent) on the support substrate side of the second intermediate layer is preferably 0.05 to 0.15. If the composition ratio O / Si is within this range, the desired resistivity R2 Ｓ This can be achieved. As a result, electrical losses are suppressed, and a composite substrate with excellent high-frequency characteristics can be realized. 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.

[0034] The second intermediate layer may have a composition ratio of O / Si that is substantially constant in the thickness direction, or it may have a region in the thickness direction where the composition ratio of O / Si increases toward the piezoelectric layer (in other words, a region in the thickness direction where the oxygen content increases toward the piezoelectric layer). The "region where the oxygen content increases" may be a region in which 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.

[0035] 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.

[0036] 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).

[0037] 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.

[0038] 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 maintained within an acceptable range.

[0039] 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.

[0040] 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.

[0041] 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).

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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 the presence of a predetermined amount of oxygen.

[0046] An example of the manufacturing method will be explained below with reference to Figures 3A to 3C. Note that Figure 3C is identical to Figure 1.

[0047] 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).

[0048] Next, as shown in Figure 3A, 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. Oxygen is silicon oxide (SiO₂ ２ A sufficient amount can be supplied to obtain the first intermediate layer. The thickness of the first intermediate layer formed may be, for example, 100 nm to 1000 nm, or for example, 400 nm to 600 nm, as described in section A-3 above.

[0049] Next, a 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 the presence of a predetermined amount of oxygen. More specifically, the sputtering may be reactive sputtering or direct sputtering.

[0050] Reactive sputtering can be carried out in the same manner as the formation of the first intermediate layer, except that the amount of oxygen supplied is small. The amount of oxygen supplied can be any appropriate amount, as long as the resistivity R2 of the second intermediate layer can vary within the shaded area of ​​Figure 2. The amount of oxygen supplied is preferably 0.05 to 5.0 volume%, more preferably 0.10 to 4.0 volume%, and even more preferably 0.15 to 3.0 volume%, relative to the total amount of introduced gas. The mode of oxygen supply can be any appropriate mode, as long as the resistivity R2 of the second intermediate layer can vary within the shaded area of ​​Figure 2. For example, oxygen may be supplied in a constant amount continuously or intermittently, the supply may be continuously or gradually increased or decreased as the deposition of the second intermediate layer progresses, the supply may be randomly increased or decreased, or a combination of these modes may be used to achieve a desired variation profile of the resistivity R2 of the second intermediate layer.

[0051] Direct sputtering can be performed using a target in which the amount of oxygen is adjusted so that the resistivity R2 of the second intermediate layer can vary within the shaded area shown in Figure 2.

[0052] The thickness of the formed second intermediate layer is preferably 5 nm to 3000 nm, as described in section A-4 above, and may also be, for example, 400 nm to 600 nm. Note that if the thickness of the formed second intermediate layer is 150 nm or less, oxygen may not be supplied in the above formation method.

[0053] In one embodiment, the first intermediate layer and the second intermediate layer can be formed continuously. Such continuous formation can be achieved by using a carousel-type sputtering apparatus as described above.

[0054] 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 second intermediate layer is deposited. 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 reduced 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 reduced oxygen content" can be formed in a desired form (concentration or content profile in the thickness direction) by appropriately setting the amount and method of oxygen supply during the formation of the second intermediate layer according to the purpose.

[0055] 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.

[0056] 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.

[0057] Next, as shown in Figure 3B, 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.

[0058] B-5. Thinning of the piezoelectric substrate (formation of the piezoelectric layer) Next, as shown in Figure 3C, the piezoelectric substrate 40' 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.

[0059] As described above, a composite substrate 100 as shown in Figure 3C can be obtained.

[0060] 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.

[0061] 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.

[0062] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

[0063] <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 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 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.

[0064] SiO is applied to the LT substrate by sputtering. ２ A film with a thickness of 400 nm 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 ２ A 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 reactive sputtering in the same manner as the formation of the first intermediate layer, except that the oxygen supply amount was changed. The oxygen supply amount was 0.25 volume% of the total introduced gas. 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.

[0065] 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.

[0066] A composite substrate was obtained as described above. The resistivity (spreading resistance) of the second intermediate layer of the obtained composite substrate was measured using a spreading 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

[0067] In the resulting composite substrate, the resistivity R2 on the support substrate side of the second intermediate layer Ｓ The resistivity was 2500 (Ωcm), which was higher than the resistivity RS of the support substrate (2000 (Ωcm)).

[0068] <Comparative Example 1> A composite substrate was obtained in the same manner as in Example 1, except that oxygen was not supplied 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 The resistivity was 100 Ωcm, which was lower than the resistivity RS of the support substrate (2000 Ωcm).

[0069] A composite substrate according to an embodiment of the present invention can be suitably used, for example, in a surface acoustic wave device.

[0070] 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 support substrate side. Ｓ A composite substrate with a capacitance of 1000 (Ωcm) or more.

2. Resistivity R2 on the support substrate side of the second intermediate layer Ｓ The composite substrate according to claim 1, wherein the resistivity RS is higher than that of the support substrate.

3. 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 containing oxygen.

4. The composite substrate according to claim 3, wherein the first intermediate layer and the second intermediate layer are each sputtering layers.

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

6. The composite substrate according to claim 3, wherein the silicon-to-oxygen composition ratio O / Si on the support substrate side of the second intermediate layer is 0.05 to 0.

15.

7. 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 this 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 the presence of a predetermined amount of oxygen.

8. The method for manufacturing a composite substrate according to claim 7, wherein the first intermediate layer and the second intermediate layer are formed in a continuous manner.

9. The method for manufacturing a composite substrate according to claim 7 or 8, wherein in the formation of the second intermediate layer, 0.05 volume% to 5.0 volume% of oxygen is added relative to the total amount of introduced gas.

10. A method for manufacturing a composite substrate according to claim 9, wherein the oxygen is introduced intermittently.

11. A surface acoustic wave element having a composite substrate according to any one of claims 1 to 6.

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