阅读说明：本技术 用于表面声波器件的复合基板及其制造方法 (Composite substrate for surface acoustic wave device and method of manufacturing the same ) 是由 丹野雅行 秋山昌次 于 2021-05-07 设计创作，主要内容包括：提供了一种具有小损耗的用于SAW器件的压电复合基板。根据本发明的一个实施方式的用于表面声波器件的复合基板具有：压电单晶薄膜,支撑基板,以及在压电单晶薄膜和支撑基板之间的第一中间层。在所述复合基板中,第一中间层与压电单晶薄膜接触,并且第一中间层中的横波的声速比压电单晶薄膜中的快横波的声速快。(A piezoelectric composite substrate for SAW devices with small loss is provided. A composite substrate for a surface acoustic wave device according to an embodiment of the present invention has: the piezoelectric single crystal display device includes a piezoelectric single crystal thin film, a support substrate, and a first intermediate layer between the piezoelectric single crystal thin film and the support substrate. In the composite substrate, the first intermediate layer is in contact with the piezoelectric single-crystal thin film, and the acoustic velocity of a transverse wave in the first intermediate layer is faster than the acoustic velocity of a fast transverse wave in the piezoelectric single-crystal thin film.)

1. A composite substrate for a surface acoustic wave device, comprising: a piezoelectric single crystal thin film, a support substrate, and a first intermediate layer between the piezoelectric single crystal thin film and the support substrate, wherein:

the first intermediate layer is in contact with the piezoelectric single crystal thin film; and is

The sound velocity of the transverse wave of the first intermediate layer is faster than the sound velocity of the fast transverse wave of the piezoelectric single crystal thin film.

2. The composite substrate of claim 1, wherein the first intermediate layer has a water vapor transmission rate of 10^-3(g/m²One day) below.

3. The composite substrate of claim 1, wherein the first intermediate layer is any one of silicon oxynitride, silicon nitride, amorphous aluminum nitride, or aluminum oxide.

4. The composite substrate of claim 1, wherein:

a second intermediate layer is provided between the first intermediate layer and the support substrate; and is

The sound velocity of the transverse wave of the second intermediate layer is slower than the sound velocity of the fast transverse wave of the piezoelectric single crystal film.

5. The composite substrate of claim 4, wherein the second interlayer comprises any one of silicon dioxide, titanium dioxide, tantalum pentoxide, niobium pentoxide, and zirconium dioxide.

6. The composite substrate according to claim 1, wherein an adhesion interface between the first intermediate layer and at least one layer adjacent to the first intermediate layer has a concavo-convex structure, and

the ratio of the average length RSm of the cells in the cross-sectional curve of the concavo-convex structure to the wavelength λ of the surface acoustic wave when used as the surface acoustic wave device is 0.2 or more and 5.0 or less.

7. The composite substrate of claim 1, wherein the volume resistivity of the composite substrate is 1 x 10¹²Omega cm or less.

8. The composite substrate according to claim 1, wherein the piezoelectric single crystal thin film is formed of lithium tantalate or lithium niobate.

9. The composite substrate of claim 1, wherein the support substrate is any one of a silicon wafer, a sapphire wafer, an alumina wafer, a glass wafer, a silicon carbide wafer, an aluminum nitride wafer, a silicon nitride wafer, and a crystalline quartz wafer.

10. The composite substrate of claim 1, wherein the piezoelectric single crystal thin film has a single polarization.

11. A method of manufacturing a composite substrate, comprising:

a step of depositing a diffusion preventing layer on one side of the piezoelectric material substrate;

a step of further depositing an intermediate layer on the diffusion preventing layer;

a step of bonding a support substrate on the diffusion preventing layer; and

a step of thinning the other side of the piezoelectric material substrate,

wherein the composite substrate is heat-treated in a reducing or inert gas atmosphere containing nitrogen or hydrogen.

12. The method of manufacturing a composite substrate according to claim 11, wherein the diffusion preventing layer is any one of silicon oxynitride, silicon nitride, amorphous aluminum nitride, or aluminum oxide.

13. The method of manufacturing a composite substrate according to claim 11, wherein in the step of depositing the diffusion preventing layer, the diffusion preventing layer is deposited by a PVD method or a CVD method.

14. The method of fabricating a composite substrate of claim 11, wherein the intermediate layer comprises any one of silicon dioxide, titanium dioxide, tantalum pentoxide, niobium pentoxide, and zirconium dioxide.

15. The method of manufacturing a composite substrate according to claim 11, further comprising a step of forming a concave-convex structure on one surface of the piezoelectric material substrate before the step of depositing the diffusion preventing layer.

Technical Field

The present invention relates to a composite substrate for a surface acoustic wave device in which a piezoelectric single crystal substrate is bonded to a supporting substrate, and a method for manufacturing the same.

Background

In recent years, data traffic has been rapidly increasing in mobile communication markets typified by smartphones. To cope with this, the number of communication frequency bands must be increased, and it is essential to miniaturize various devices (for example, surface acoustic wave devices) and achieve high performance of the devices.

Piezoelectric materials, such as Lithium Tantalate (LT) and Lithium Niobate (LN), are widely used as materials for Surface Acoustic Wave (SAW) devices, such as surface acoustic wave filters. Although these materials have a large electromechanical coupling coefficient and can expand the bandwidth of the device, there is a problem in that the temperature stability of the materials is low, and thus the applicable frequency shifts with temperature change. This is because lithium tantalate or lithium niobate has a very high coefficient of thermal expansion.

In order to solve this problem, a composite substrate obtained by bonding a material having a small thermal expansion coefficient to lithium tantalate or lithium niobate and thinning one side of a piezoelectric material to a thickness of several μm to several tens of μm has been proposed. In this composite substrate, thermal expansion of the piezoelectric material is suppressed by bonding a material having a small thermal expansion coefficient (e.g., sapphire or silicon), and therefore, the temperature characteristics are improved (non-patent documents 1 and 2). In addition, patent document 1 discloses an acoustic wave device having a piezoelectric film. The acoustic wave device includes: supporting a substrate; a high acoustic velocity film formed on the support substrate and having a bulk acoustic velocity higher than an acoustic velocity propagated through the piezoelectric film; the piezoelectric film includes a low-sound-velocity film that is laminated on the high-sound-velocity film and has a slower bulk-sound velocity than a bulk-sound velocity propagated through the piezoelectric film, a piezoelectric film that is laminated on the low-sound-velocity film, and an IDT electrode that is formed on one surface of the piezoelectric film.

Further, patent document 2 discloses an acoustic wave device including: supporting a substrate; a dielectric layer laminated on the support substrate; a piezoelectric body laminated on a dielectric layer for propagating a bulk wave; and an IDT electrode formed on one surface of the piezoelectric body. In the device, the dielectric layer includes a low-speed medium in which the propagation speed of a bulk wave, which is a main component of an acoustic wave, is slower than the acoustic speed of the acoustic wave propagating in the piezoelectric body; in a high-speed medium, the propagation speed of a bulk wave, which is a main component of an acoustic wave, is faster than the acoustic speed of an acoustic wave propagating inside a piezoelectric body. When the sound velocity of the main vibration mode when the medium layer is formed of a high-speed medium is VH and the sound velocity of the main vibration mode when the medium layer is formed of a low-speed medium is VL, the medium layer is formed such that the sound velocity of the main vibration mode in the acoustic wave device having the medium layer is VL < the sound velocity of the main vibration mode < VH, and when the period of the IDT is λ, the thickness of the medium layer is 1 λ or more.

Further, patent document 3 discloses a composite substrate for a surface acoustic wave device, which includes a piezoelectric single crystal thin film and a supporting substrate. In the device, at least one of the piezoelectric single-crystal substrate and the supporting substrate has a concave-convex structure at an adhesion interface between the piezoelectric single-crystal thin film and the supporting substrate, and a ratio of an average length RSm of a cell in a cross-sectional curve of the concave-convex structure to a wavelength λ of a surface acoustic wave when used as a surface acoustic wave device is 0.2 or more and 7.0 or less.

Reference to the prior art

Patent document

Patent document 1: japanese patent No.5713025

Patent document 2: japanese patent No.5861789

Patent document 3: japanese patent No.6250856

Non-patent document

Non-patent document 1: temperature Compensation Technology for SAW-duplex Used in RF Front End of Smartphone, Dempa Shimbun High Technology, 11/8/2012

Non-patent document 2: a study on Temperature-Compensated hybrids for Surface active Wave Filters, 2010IEEE International Ultrasonic Symposium Proceedings, page 637-640.

Disclosure of Invention

Problems to be solved by the invention

However, when the surface acoustic wave filter is manufactured using the composite substrate of patent document 1 or patent document 2, there are problems as follows: noise, so-called noise, or ripple, is generated in the passband of the surface acoustic wave filter or at higher frequencies because the energy of the acoustic wave leaks from the piezoelectric material to the low-speed medium. This noise is generated due to reflection at the adhesive interface between the piezoelectric crystal film and the support substrate and capture of acoustic waves in the intermediate layer between the piezoelectric crystal film and the support substrate. This noise is undesirable because it deteriorates the frequency characteristics of the surface acoustic wave filter and causes an increase in loss.

In the composite substrate for a surface acoustic wave device described in patent document 3, the piezoelectric single crystal thin film or the supporting substrate each has a concave-convex structure, which is desirable because the concave-convex structure scatters an unnecessary wave, thereby suppressing the influence of a reflected wave.

However, the inventors have studied carefully the possibility that the resistivity of the single crystal piezoelectric film and the thermoelectric property thereof increase when subjected to the heat treatment of the wafer process for manufacturing the composite substrate for a surface acoustic wave device and the device process using the composite substrate described in patent document 3. It was found that the reason for this is that a temperature change during this process causes an electric field exceeding the coercive electric field to be generated in the concave-convex region of the piezoelectric single crystal of the composite substrate, which in extreme cases leads to a problem that the polarization of the single crystal piezoelectric film is disturbed.

In view of the above, an object of the present invention is to provide a piezoelectric composite substrate for a SAW device having a small loss.

Means for solving the problems

A composite substrate for a surface acoustic wave device according to an embodiment of the present invention has: the piezoelectric single crystal display device includes a piezoelectric single crystal thin film, a support substrate, and a first intermediate layer between the piezoelectric single crystal thin film and the support substrate. In the composite substrate, the first intermediate layer is in contact with the piezoelectric single-crystal thin film, and the acoustic velocity of a transverse wave of the first intermediate layer is faster than the acoustic velocity of a fast transverse wave in the piezoelectric single-crystal thin film. Preferably, the volume resistivity of the composite substrate is 1 × 10¹²Omega cm or less.

Taking a piezoelectric single crystal film as LiTaO₃The case of (LT) is an example to illustrate the present invention. The intermediate layer is provided between the LT as a piezoelectric single crystal thin film and the support substrate. If the speed of the bulk wave (shear wave) of the intermediate layer is slower than that of the bulk wave (fast shear wave) of the LT, the acoustic wave is easily trapped in the intermediate layerAnd (4) obtaining. Therefore, if the acoustic velocity of the transverse wave of the intermediate layer is made faster than the acoustic velocity of the slow transverse wave of the piezoelectric single crystal thin film in the composite substrate, the loss of the passband of the surface acoustic wave filter obtained using such composite substrate 1 can be improved. Hereinafter, details will be described.

In a surface acoustic wave filter obtained by forming a periodic electrode structure on a composite substrate, for example, in a composite substrate in which LT and Si cut by a Y-axis rotated by 46 ° are joined and LT thickness is 1 or more wavelength and LT thickness excludes an odd point of a dispersion curve, when an electrode is electrically opened, a sound velocity of a main mode of a surface acoustic wave is 4060m/s (slowness as a reciprocal of the sound velocity is 2.46 × 10^-3s/m), and 3910m/s when the electrodes are electrically short-circuited (slowness of 2.56 × 10 as the reciprocal of the speed of sound)^-3s/m)。

Therefore, a surface acoustic wave (or a leakage wave or an SH wave) propagating from the electrode along the LT surface can be coupled with a specific bulk wave in the LT that can propagate inside the LT substrate. That is, as shown in the slowness surface (calculated value) of the Y-axis-cut LT rotated by 46 ° shown in fig. 1, the main mode of the composite substrate structure in which the above-described Y-axis-cut LT rotated by 46 ° and Si are bonded as explained above can be coupled with a phase-matched bulk wave (slow shear wave) capable of propagating by about 22 degrees in the depth direction from the X-axis.

FIG. 2 shows that when LT cut by Y-axis rotation of 46 ° is used as the piezoelectric single crystal thin film and Si is used₃N₄An example of a slowness surface when used as an intermediate layer. When adding Si₃N₄When used as the intermediate layer, the acoustic velocity of the transverse wave of the intermediate layer can be made faster than the acoustic velocity of the fast transverse wave of the piezoelectric single crystal thin film.

In the case where the acoustic velocity of the slow transverse wave of the intermediate layer is faster than that of the piezoelectric single crystal thin film as shown in fig. 2, the slow transverse wave emitted in the direction of about 22 ° from the X axis of LT cut from the Y axis rotated by 46 ° is completely reflected by the intermediate layer even when it reaches the intermediate layer. Therefore, the bulk wave leaking inward from the electrode along the surface of the LT surface of the surface acoustic wave (or the leaky wave or the SH wave) is totally reflected by the intermediate layer and cannot stay in the intermediate layer.

As a result, no clutter remains in the intermediate layer near the main mode frequency. Therefore, deterioration of characteristics (e.g., ripple and loss in the pass band of the filter) can be prevented.

Further, in one embodiment of the present invention, the first intermediate layer is characterized by a high acoustic velocity and 10^-3(g/m²A day) or less. This suppresses diffusion of oxygen from the support substrate side of the intermediate layer to the piezoelectric single crystal thin film side, which in turn suppresses an increase in pyroelectricity in the piezoelectric single crystal thin film of the composite substrate and generation of an electric field when the composite substrate is heat-treated. Here, the water vapor transmission rate is a value measured by the Mocon method at a temperature of 40 ℃ and a relative humidity of 90%.

The material of the first intermediate layer may be any of silicon oxynitride, silicon nitride, amorphous aluminum nitride, or aluminum oxide.

In the present invention, the composite substrate may have at least a first intermediate layer and a second intermediate layer between the piezoelectric single-crystal thin film and the support substrate. Here, the first intermediate layer may be in contact with the piezoelectric single crystal thin film. Preferably, the acoustic velocity of the transverse wave in the first intermediate layer is faster than the acoustic velocity of the fast transverse wave in the piezoelectric single crystal thin film, and the acoustic velocity of the transverse wave of the second intermediate layer is slower than the acoustic velocity of the fast transverse wave in the piezoelectric single crystal thin film.

In the present invention, the second intermediate layer may contain oxygen. The second intermediate layer may include any one of silicon dioxide, titanium dioxide, tantalum pentoxide, niobium pentoxide, and zirconium dioxide. Such an intermediate layer provides a composite substrate with high adhesive strength.

Here, the acoustic velocity of the transverse wave in the second intermediate layer is slower than that of the fast transverse wave in the piezoelectric single-crystal thin film, which allows better suppression of the spurious at frequencies higher than the main mode frequency. In other words, bulk waves are radiated into the piezoelectric single-crystal thin film at a frequency higher than the main mode frequency at an angle even deeper than 22 °. In this case, there will be bulk waves leaking into the first intermediate layer. Since the acoustic velocity of the transverse wave in the second intermediate layer is slower than that of the fast transverse wave in the piezoelectric single crystal thin film, the bulk wave can easily enter the second intermediate layer. However, the bulk wave re-reflected in the support substrate is re-reflected at the first intermediate layer and cannot easily return to the first intermediate layer. As a result, bulk waves are confined in the second intermediate layer and are difficult to return to the piezoelectric single-crystal thin film. Thus, ripples at frequencies within or above the pass band of the filter can be prevented.

In the present invention, any adhesion interface between the piezoelectric single-crystal thin film and the support substrate (for example, an adhesion interface between the first intermediate layer and a layer adjacent to the first intermediate layer) may have a structure having irregularities. A ratio of an average length RSm of cells in a cross-sectional curve of the concave-convex structure to a wavelength λ of a surface acoustic wave when used as a surface acoustic wave device may be between 0.2 and 5.0.

When the piezoelectric single-crystal thin film of the present application has the concave-convex structure at the interface with the intermediate layer, the bulk wave in the direction of about 22 ° is scattered by the concave-convex structure due to the main mode from LT, and the component returning to the electrode can be greatly reduced. If the frequency of the main mode is fo and the radiation angle of bulk waves from the electrodes of the SAW device to the inside of the LT is θ, a reflected wave is generated at a frequency higher than fo, which is expressed as fo/cos θ, but the concave-convex structure scatters the reflected wave.

In addition, the inventors studied the degree of the above-mentioned unevenness, found that when the ratio of the average length RSm of the cell in the cross-sectional curve of the uneven structure to the wavelength λ of the surface acoustic wave when used as a surface acoustic wave device is between 0.2 and 5.0, the composite substrate for a surface acoustic wave device of the present application can maintain single polarization without losing polarization of the piezoelectric single crystal, and thus completed the present invention.

In the present invention, the piezoelectric material forming the piezoelectric single crystal thin film may be lithium tantalate or lithium niobate.

In the present invention, the support substrate may be any one of a silicon wafer, a sapphire wafer, an alumina wafer, a glass wafer, a silicon carbide wafer, an aluminum nitride wafer, a silicon nitride wafer, and crystalline quartz.

The method of manufacturing a composite substrate according to an embodiment of the present invention is characterized by a step of depositing a diffusion preventing layer on one side of a piezoelectric material substrate; a step of further depositing an intermediate layer on the diffusion preventing layer; a step of bonding a support substrate on the diffusion preventing layer; and thinning the other side of the piezoelectric material substrate. The method for manufacturing a composite substrate is characterized in that the composite substrate is subjected to heat treatment under a reducing or inert gas atmosphere containing nitrogen or hydrogen.

In the present invention, it is preferable that the diffusion preventing layer is deposited by a PVD method or a CVD method.

In the present invention, the adhesive surface of the diffusion preventing layer and/or the adhesive surface of the support substrate may be subjected to a surface activation treatment and then adhered to each other. The surface activation treatment may include any one of a plasma activation method, an ion beam activation method, and an ozone water activation method.

Drawings

FIG. 1 shows a slowness surface of an LT cut with a Y axis rotated 46.

FIG. 2 shows LT and intermediate layer (Si) cut with Y axis rotated 46 in YX plane₃N₄) Is represented by slowness of (c).

Fig. 3 shows a cross-sectional view of a cross-sectional structure of a composite substrate.

Fig. 4 shows a process flow for manufacturing a composite substrate.

Fig. 5 shows a waveform indicative of the characteristics (S11 frequency response) of the SAW filter formed on the composite piezoelectric substrate in example 1.

Figure 6 shows a piezoelectric response force microscope (PFM) image of a cross-section of the composite substrate of example 1.

Fig. 7 shows waveforms representing characteristics (S11 frequency response) of the SAW filter formed on the composite piezoelectric substrate in comparative example 1 before thermal cycling.

Fig. 8 shows a piezoelectric response force microscope (PFM) image of a cross-section of the composite substrate of comparative example 2 after thermal cycling.

Fig. 9 shows a piezoelectric response force microscope (PFM) image of a cross-section of the composite substrate of comparative example 3 after thermal cycling.

Detailed Description

In fig. 3 a cross-sectional structure of a composite substrate 1 according to the invention is shown. The composite substrate 1 shown in fig. 3 has a piezoelectric single crystal thin film 2 on a support substrate 3. The piezoelectric single-crystal thin film 2 has a high acoustic velocity. The piezoelectric single crystal thin film 2 is bonded to the support substrate 3 via the diffusion preventing layer 4 that prevents oxygen from diffusing and the intermediate layer 5.

The piezoelectric single crystal thin film 2 is formed of Lithium Tantalate (LT) or Lithium Niobate (LN) as a piezoelectric material. The piezoelectric single-crystal thin film 2 preferably has a single polarization. The support substrate 3 may be any one of a silicon wafer, a sapphire wafer, an alumina wafer, a glass wafer, a silicon carbide wafer, an aluminum nitride wafer, a silicon nitride wafer, and a crystalline quartz wafer.

The diffusion preventing layer 4 is sometimes referred to as a first intermediate layer in the present invention. The diffusion preventing layer 4 is placed in contact with the piezoelectric single-crystal thin film 2. The diffusion preventing layer 4 is formed so that the acoustic velocity of the transverse wave in the diffusion preventing layer 4 is faster than the acoustic velocity of the fast transverse wave in the piezoelectric single crystal thin film 2. The diffusion preventing layer 4 has 10^-3(g/m²A day) or less. The diffusion preventing layer 4 may be formed of any one of silicon oxynitride, silicon nitride, amorphous aluminum nitride, or aluminum oxide.

The intermediate layer 5 is disposed between the diffusion preventing layer 4 and the support substrate 3. The intermediate layer 5 is sometimes referred to as a first intermediate layer, or a second intermediate layer in order to distinguish it from the diffusion-preventing layer 4. The intermediate layer 5 may be formed of a material containing oxygen. More specifically, the intermediate layer may comprise any of the following: silicon dioxide, titanium dioxide, tantalum pentoxide, niobium pentoxide and zirconium dioxide. The intermediate layer 5 is formed such that the acoustic velocity of the transverse wave in the intermediate layer 5 is slower than the acoustic velocity of the fast transverse wave in the piezoelectric single-crystal thin film 2.

The concavo-convex structure is formed at the adhesion interface between the diffusion-preventing layer 4 and the layer adjacent to the diffusion-preventing layer 4 (in this embodiment, at the interface with the piezoelectric single-crystal thin film 2 or the interface with the intermediate layer 5). The concave-convex structure is formed such that a ratio of an average length RSm of cells in a cross-sectional curve of the concave-convex structure to a wavelength λ of a surface acoustic wave when used as a surface acoustic wave device is 0.2 or more and 5.0 or less.

Fig. 4 shows a flow of a method for manufacturing the composite substrate 1. In the manufacturing method, the piezoelectric single crystal wafer 2 as a piezoelectric single crystal thin film is first prepared (S01 of fig. 4), and the concave-convex structure is formed on one of the surfaces thereof by grinding or sandblasting with loose abrasive grains (S02 of fig. 4). Further, the diffusion preventing layer 4 is formed on the uneven structure of the piezoelectric single crystal thin film, and then the intermediate layer 5 is formed on the diffusion preventing layer 4 (S03 in fig. 4). At this time, it is preferable to deposit the diffusion preventing layer 4 by a PVD method or a CVD method. Preferably, the intermediate layer 5 is also deposited by PVD or CVD. Then, the surface of the intermediate layer is mirrored by grinding (S04 in fig. 4).

In parallel with the processing of the piezoelectric single crystal wafer described above, the support substrate 3 is prepared (S11 in fig. 4). As the supporting substrate 3, any one of a silicon wafer, a sapphire wafer, an alumina wafer, a glass wafer, a silicon carbide wafer, an aluminum nitride wafer, a silicon nitride wafer, and a crystal quartz wafer can be used. The surface of the support substrate is mirrored by grinding. The support substrate 3 may be provided with a textured structure or an intermediate layer instead of the piezoelectric single-crystal thin film 2 or in addition to the piezoelectric single-crystal thin film 2. In this case, the concave-convex structure on the support substrate 3 may be formed by grinding with free abrasive grains, sandblasting, chemical etching, or the like, and then an intermediate layer may be formed on the concave-convex structure. The intermediate layer can then be mirrored by grinding to form the adhesive surface.

The ground surface of the intermediate layer 5 and the adhesive surface of the support substrate 3 are bonded together (S21 in fig. 4). Thereafter, the other side (i.e., the side opposite to the side in which the diffusion preventing layer 4 is formed) of the piezoelectric single crystal wafer 2 is ground and polished to be thinned to a desired thickness. Thereby, a composite substrate for a surface acoustic wave device can be manufactured (S22 of fig. 4). At this time, a surface activation treatment may be applied to the surface to be bonded in advance. In this way, the adhesive strength can be increased. A plasma activation method, an ion beam activation method, and an ozone water activation method may be used for the surface activation treatment. In the plasma activation method, a plasma gas is introduced into a reaction vessel in which a wafer is placed, and a high-frequency plasma of about 100W is formed under a reduced pressure of about 0.01 to 0.1Pa, thereby exposing the wafer bonding surface to the plasma for about 5 to 50 seconds. As the plasma gas, oxygen, hydrogen, nitrogen, argon, or a mixture of these gases can be used.

After thinning, heat treatment is preferably performed under a reducing or inert gas atmosphere containing nitrogen or hydrogen to further improve the adhesive strength. As the reducing atmosphere, for example, a hydrogen atmosphere can be used. As the inert gas atmosphere, for example, a nitrogen atmosphere can be used.

Examples

[ example 1]

Silicon nitride of about 800nm was deposited by PVD method on one side of a LT wafer cut by Y axis rotated by 46 ° with a diameter of 150mm to form an anti-diffusion layer. Then, a silicon oxide film having a thickness of about 3 μm was formed on the diffusion preventing layer by CVD. Using the silicon oxide film as an intermediate layer, the silicon oxide film was polished and bonded to a p-type silicon wafer having a resistivity of 2000 Ω · cm. LT wafer used has about 1 x 10¹⁰Volume resistivity of Ω · cm.

The surface of the LT wafer on which the silicon nitride layer was formed was processed by sandblasting into an uneven structure having an average length RSm of a unit of a cell in a cross-sectional curve of 3 μm, and Ra ═ 0.06 μm.

After bonding, a heat treatment was applied at 100 ℃ for 48 hours in a nitrogen atmosphere. The LT layer is then thinned to a thickness of 10 μm by grinding and lapping. Then, in order to further improve the adhesive strength, heat treatment was performed at 250 ℃ for 24 hours in a hydrogen atmosphere.

For the composite substrate manufactured as described above, the thickness of the composite substrate was measured by "JIS C2139: 2008 solid electrical insulating Material-measurement method of volume resistivity and surface resistivity¹⁰Omega cm. The voltage applied when measuring the volume resistivity was 500V.

Next, an Al film was sputtered onto the composite substrate manufactured as described above at a thickness of 0.14 μm, and a resist pattern having a line width of about 0.5 μm was formed by i-line exposure after coating the resist. Then, Al is etched by dry etching to form a ladder type SAW filter. In addition, a 50nm silicon nitride layer was formed by sputtering on the surface layer of the substrate on which the SAW filter was formed.

The characteristics of the SAW filter were evaluated by the RF probe and the network analyzer, as shown in fig. 5, and a proper waveform was obtained (frequency response of S11). SAW filters with wavelengths of approximately 2.2 μm, 2.1 μm and 1.9 μm were placed in the same wafer. Therefore, in this embodiment, if the wavelength λ is taken as the average length of the cells in the cross-sectional curve of the concavo-convex structure of the LT adhesive surface, RSm/λ is 1.6, 1.5, and 1.4 for the SAW filters at the respective wavelengths, respectively.

Next, the composite substrate having the SAW filter pattern fabricated as described above was passed through a reflow oven at 265 ℃ 6 times, and then thermally cycled from-40 ℃ to 125 ℃ 1000 times. Thereafter, the characteristics of the SAW filter were evaluated again by the RF probe and the network analyzer, and the results were the same as fig. 5.

Next, a piezoelectric response force microscope (PFM) image of a cross section of the composite substrate manufactured in the same manner as described above was measured. As a result, as shown in fig. 6, it was found that the LT portion was uniformly polarized.

PFM images of cross-sections of composite substrates with SAW filter patterns were measured after passing the substrates through a reflow oven at 265 ℃ six times and then thermally cycling them 1000 times from-40 ℃ to 125 ℃. As a result, similarly to the case shown in fig. 6, it was found that the LT portion was uniformly polarized.

In the above example 1, silicon nitride and SiO as intermediate layers were measured by nanoindentation and X-ray reflectance (Xrr) methods, respectively₂Young's modulus and density. Table 1 shows the calculated silicon nitride film and SiO obtained from the results of example 1₂The acoustic velocity of the transverse wave of the membrane and the young's modulus and density described above.

[ Table 1]

	Silicon nitride	SiO2	SiO1.5N0.5
				Young's modulus (Gpa)	320	62	130
Density (kg/m)3)	2800	2180	2260
				Speed of transverse wave (m/s)	6700	3700	4865

Calculated fast transverse wave at 46 degree Y-LiTaO₃The sound velocity in the x-axis direction of (2) is 4227 m/s.

[ example 2]

A composite substrate was produced in the same manner as in example 1, except that the heat treatment for further improving the adhesive strength was performed after the LT layer was thinned in a nitrogen atmosphere instead of a hydrogen atmosphere. The composite substrate was obtained by a method of "JIS C2139: 2008 solid electrical insulating Material-measurement method of volume resistivity and surface resistivity¹¹Omega cm. The voltage applied when measuring the volume resistivity was 500V.

Next, in the same manner as in example 1, a ladder type SAW filter was formed on the composite substrate manufactured by the above-described method, and a 50nm silicon nitride layer was formed by sputtering on the surface layer of the substrate on which the SAW filter was formed.

When the characteristics of the SAW filter were evaluated using an RF probe and a network analyzer, a waveform similar to the case in fig. 5 was obtained. SAW filters with wavelengths of approximately 2.2 μm, 2.1 μm and 1.9 μm were placed in the same wafer. Therefore, in this embodiment, if the wavelength λ is taken as the average length of the cells in the cross-sectional curve of the concavo-convex structure of the LT adhesive surface, RSm/λ is 1.6, 1.5, and 1.4 for the SAW filters at the respective wavelengths, respectively.

Next, the composite substrate having the SAW filter pattern fabricated as described above was passed through a reflow oven at 265 ℃ 6 times, and then thermally cycled from-40 ℃ to 125 ℃ 1000 times. Thereafter, the characteristics of the SAW filter were evaluated again by the RF probe and the network analyzer, and the results were the same as in the case of fig. 5.

Further, PFM images of cross sections of the composite substrates manufactured in the same manner as described above were measured. As a result, similarly to the case shown in fig. 6, it was found that the LT portion was uniformly polarized.

PFM images of cross-sections of composite substrates with SAW filter patterns were measured after passing the substrates through a reflow oven at 265 ℃ six times and then thermally cycling them 1000 times from-40 ℃ to 125 ℃. As a result, similarly to the case shown in fig. 6, it was found that the LT portion was uniformly polarized.

[ example 3]

Deposition of about 800nm of SiO by CVD method at a temperature of about 35 ℃ on one side of a LT wafer cut by Y-axis rotating by 46 DEG and having a diameter of 150mm_1.5N_0.5To form an anti-diffusion layer. Then, a silicon oxide film having a thickness of about 3 μm was formed on the diffusion preventing layer by CVD. Using the silicon oxide film as an intermediate layer, the silicon oxide film was polished and bonded to a p-type silicon wafer having a resistivity of 2000 Ω · cm. LT wafer used has about 5 x 10¹⁰Volume resistivity of Ω · cm.

Formation of SiO on LT wafer by sandblasting_1.5N_0.5The surface of the layer is processed into a concavo-convex structure having an average length RSm of cells in a cross-sectional curve of 3 μm toAnd Ra 0.06 μm.

After bonding, a heat treatment was applied at 100 ℃ for 48 hours in a nitrogen atmosphere. The LT layer is then thinned to a thickness of 10 μm by grinding and lapping. Then, in order to further improve the adhesive strength, heat treatment was performed at 250 ℃ for 24 hours in a hydrogen atmosphere.

For the composite substrate manufactured as described above, the thickness of the composite substrate was measured by "JIS C2139: 2008 solid electrical insulating Material-measurement method of volume resistivity and surface resistivity¹⁰Omega cm. The voltage applied when measuring the volume resistivity was 500V.

Next, an Al film was sputtered onto the composite substrate manufactured as described above at a thickness of 0.14 μm, and a resist pattern having a line width of about 0.5 μm was formed by i-line exposure after coating the resist. Then, Al is etched by dry etching to form a ladder type SAW filter. In addition, a 50nm silicon nitride layer was formed by sputtering on the surface layer of the substrate on which the SAW filter was formed.

When the characteristics of the SAW filter were evaluated using an RF probe and a network analyzer, a waveform similar to the case in fig. 5 was obtained.

Next, the composite substrate having the SAW filter pattern fabricated as described above was passed through a reflow oven at 265 ℃ 6 times, and then thermally cycled from-40 ℃ to 125 ℃ 1000 times. Thereafter, the characteristics of the SAW filter were evaluated again by the RF probe and the network analyzer, and were not changed from fig. 5.

Further, PFM images of cross sections of the composite substrates manufactured in the same manner as described above were measured. As a result, similarly to the case shown in fig. 6, it was found that the LT portion was uniformly polarized. PFM images of cross-sections of composite substrates with SAW filter patterns were measured after passing the substrates through a reflow oven at 265 ℃ six times and then thermally cycling them 1000 times from-40 ℃ to 125 ℃. As a result, similarly to the case shown in fig. 6, it was found that the LT portion was uniformly polarized.

[ example 4]

Except that LT layer subtraction was performed in a nitrogen atmosphere instead of a hydrogen atmosphereA composite substrate was produced in the same manner as in example 3, except that the heat treatment for further improving the adhesive strength after thinning was performed. The composite substrate was obtained by a method of "JIS C2139: 2008 solid electrical insulating Material-measurement method of volume resistivity and surface resistivity¹¹Omega cm. The voltage applied when measuring the volume resistivity was 500V.

Next, in the same manner as in example 3, a ladder type SAW filter was formed on the composite substrate manufactured by the above-described method, and a 50nm silicon nitride layer was formed by sputtering on the surface layer of the substrate on which the SAW filter was formed.

When the characteristics of the SAW filter were evaluated using an RF probe and a network analyzer, a waveform similar to the case in fig. 5 was obtained. SAW filters with wavelengths of approximately 2.2 μm, 2.1 μm and 1.9 μm were placed in the same wafer. Therefore, in this embodiment, if the wavelength λ is taken as the average length of the cells in the cross-sectional curve of the concavo-convex structure of the LT adhesive surface, RSm/λ is 1.6, 1.5, and 1.4 for the SAW filters at the respective wavelengths, respectively.

Next, the composite substrate having the SAW filter pattern fabricated as described above was passed through a reflow oven at 265 ℃ 6 times, and then thermally cycled from-40 ℃ to 125 ℃ 1000 times. Thereafter, the characteristics of the SAW filter were evaluated again by the RF probe and the network analyzer, and the results were the same as in the case of fig. 5.

Further, PFM images of cross sections of the composite substrates manufactured in the same manner as described above were measured. As a result, similarly to the case shown in fig. 6, it was found that the LT portion was uniformly polarized.

PFM images of cross-sections of composite substrates with SAW filter patterns were measured after passing the substrates through a reflow oven at 265 ℃ six times and then thermally cycling them 1000 times from-40 ℃ to 125 ℃. As a result, similarly to the case shown in fig. 6, it was found that the LT portion was uniformly polarized.

[ examples 5 and 6]

A composite substrate manufactured in the same manner as in example 1, except that the diffusion preventing layer of about 800nm shown in table 2 was deposited on one side of the LT wafer cut by the Y-axis rotated by 46 ° with a diameter of 150mm by the PVD or PLD method, was prepared. For the composite substrate manufactured as described above, the thickness of the composite substrate was measured by "JIS C2139: 2008 solid electrical insulation-measurement method of volume resistivity and surface resistivity ", the apparent volume resistivity of the composite substrate was measured, and the volume resistivity was the value shown in table 2. The voltage applied when measuring the volume resistivity was 500V.

Next, an Al film was sputtered onto the composite substrate manufactured as described above at a thickness of 0.14 μm, and a resist pattern having a line width of about 0.5 μm was formed by i-line exposure after coating the resist. Then, Al is etched by dry etching to form a ladder type SAW filter. In addition, a 50nm silicon nitride layer was formed by sputtering on the surface layer of the substrate on which the SAW filter was formed.

When the characteristics of the SAW filter were evaluated using an RF probe and a network analyzer, a waveform similar to the case in fig. 5 was obtained. SAW filters with wavelengths of approximately 2.2 μm, 2.1 μm and 1.9 μm were placed in the same wafer. Therefore, in this embodiment, if the wavelength λ is taken as the average length of the cells in the cross-sectional curve of the concavo-convex structure of the LT adhesive surface, RSm/λ is 1.6, 1.5, and 1.4 for the SAW filters at the respective wavelengths, respectively.

Next, the composite substrate having the SAW filter pattern fabricated as described above was passed through a reflow oven at 265 ℃ 6 times, and then thermally cycled from-40 ℃ to 125 ℃ 1000 times. Thereafter, the characteristics of the SAW filter were evaluated again by the RF probe and the network analyzer, and the results were the same as in the case of fig. 5.

Further, PFM images of cross sections of the composite substrates manufactured in the same manner as described above were measured. As a result, similarly to the case shown in fig. 6, it was found that the LT portion was uniformly polarized.

PFM images of cross-sections of composite substrates with SAW filter patterns were measured after passing the substrates through a reflow oven at 265 ℃ six times and then thermally cycling them 1000 times from-40 ℃ to 125 ℃. As a result, similarly to the case shown in fig. 6, it was found that the LT portion was uniformly polarized.

In examples 5 and 6 above, the young's modulus and density of the interlayer were measured by nanoindentation and X-ray reflectance (Xrr) methods, respectively. Table 2 shows the young's modulus and density of the above examples 5 and 6, and the calculated sound velocity of the transverse wave of the diffusion preventing layer obtained from the young's modulus and density.

[ Table 2]

Comparative example 1

Then, a silicon oxide film having a thickness of about 4 μm was formed by CVD on one side of a LT wafer cut by Y-axis rotation of 46 DEG with a diameter of 150 mm. Using the silicon oxide film as an intermediate layer, the silicon oxide film was polished and bonded to a p-type silicon wafer having a resistivity of 2000 Ω · cm. LT wafer used has about 5 x 10¹⁰Volume resistivity of Ω · cm.

The surface of the LT wafer on which the silicon oxide layer was formed was processed by sandblasting into an uneven structure having an average length RSm of cells in a cross-sectional curve of 3 μm, and Ra ═ 0.06 μm.

After bonding, a heat treatment was applied at 100 ℃ for 48 hours in a nitrogen atmosphere. The LT layer is then thinned to a thickness of 10 μm by grinding and lapping. Then, in order to further improve the adhesive strength, heat treatment was performed at 250 ℃ for 24 hours under a nitrogen atmosphere.

For the composite substrate manufactured as described above, the thickness of the composite substrate was measured by "JIS C2139: 2008 solid electrical insulating Material-measurement method of volume resistivity and surface resistivity¹²Omega cm. The voltage applied when measuring the volume resistivity was 500V.

Next, an Al film was sputtered onto the composite substrate manufactured as described above at a thickness of 0.14 μm, and a resist pattern having a line width of about 0.5 μm was formed by i-line exposure after coating the resist. Then, Al is etched by dry etching to form a ladder type SAW filter. In addition, a 50nm silicon nitride layer was formed by sputtering on the surface layer of the substrate on which the SAW filter was formed.

The characteristics of the SAW filter were evaluated by an RF probe and a network analyzer, and a waveform as shown in fig. 7 was obtained. SAW filters with wavelengths of approximately 2.2 μm, 2.1 μm and 1.9 μm were placed in the same wafer. Therefore, in this comparative example, if the wavelength λ is taken as the average length of the cells in the cross-sectional curve of the concavo-convex structure of the LT adhesive surface, RSm/λ is 1.6, 1.5, and 1.4 for the SAW filters at the respective wavelengths, respectively.

Next, the composite substrate having the SAW filter pattern fabricated as described above was passed through a reflow oven at 265 ℃ 6 times, and then thermally cycled from-40 ℃ to 125 ℃ 1000 times. Thereafter, the characteristics of the SAW filter were evaluated again by the RF probe and the network analyzer, and the insertion loss increased by about 5dB compared to the waveform in fig. 7.

Further, PFM images of cross sections of the composite substrates manufactured in the same manner as described above were measured. As a result, similarly to the case shown in fig. 6, it was found that the LT portion was uniformly polarized.

PFM images of cross-sections of composite substrates with SAW filter patterns were measured after passing the substrates through a reflow oven at 265 ℃ six times and then thermally cycling them 1000 times from-40 ℃ to 125 ℃. As a result, as shown in fig. 8, the polarization of the LT portion is disordered.

In the above comparative example 1, SiO as an intermediate layer was measured by the nanoindentation method and the X-ray reflectance (Xrr) method, respectively₂Young's modulus and density. Results of the above comparative example 1 and SiO obtained from Young's modulus and density₂The transverse sound velocity (calculated value) of the film was equal to the value shown in table 1.

Comparative example 2

A composite substrate was prepared in the same manner as in comparative example 1, except that: the LT layer was thinned to a thickness of 10 μm by grinding and lapping, and the composite substrate was heat-treated at 250 ℃ for 24 hours in an atmospheric atmosphere to further improve the adhesive strength.

For the composite substrate manufactured as described above, the thickness of the composite substrate was measured by "JIS C2139: 2008 solid electricityInsulation material-measurement method of volume resistivity and surface resistivity "the apparent volume resistivity of the composite substrate was measured, and the volume resistivity was 1 × 10¹⁴Omega cm. The voltage applied when measuring the volume resistivity was 500V.

Next, an Al film was sputtered onto the composite substrate manufactured as described above at a thickness of 0.14 μm, and a resist pattern having a line width of about 0.5 μm was formed by i-line exposure after coating the resist. Then, Al is etched by dry etching to form a ladder type SAW filter. In addition, a 50nm silicon nitride layer was formed by sputtering on the surface layer of the substrate on which the SAW filter was formed.

When the characteristics of the SAW filter were evaluated using an RF probe and a network analyzer, a waveform as in the case of fig. 7 was obtained. SAW filters with wavelengths of approximately 2.2 μm, 2.1 μm and 1.9 μm were placed in the same wafer. Therefore, in this comparative example, if the wavelength λ is taken as the average length of the cells in the cross-sectional curve of the concavo-convex structure of the LT adhesive surface, RSm/λ is 1.6, 1.5, and 1.4 for the SAW filters at the respective wavelengths, respectively.

Next, the composite substrate having the SAW filter pattern fabricated as described above was passed through a reflow oven at 265 ℃ 6 times, and then thermally cycled from-40 ℃ to 125 ℃ 1000 times. Thereafter, the characteristics of the SAW filter were evaluated again by the RF probe and the network analyzer, and the insertion loss increased by about 6dB compared to the waveform in fig. 7.

Further, PFM images of cross sections of the composite substrates manufactured in the same manner as described above were measured. As a result, similarly to the case shown in fig. 6, it was found that the LT portion was uniformly polarized.

PFM images of cross-sections of composite substrates with SAW filter patterns were measured after passing the substrates through a reflow oven at 265 ℃ six times and then thermally cycling them 1000 times from-40 ℃ to 125 ℃. As a result, similarly to the case shown in fig. 8, the polarization of the LT portion is disordered.

Comparative example 3

Then, an LT wafer cut by CVD at a Y axis rotated by 46 DEG and having a diameter of 150mm was formed on one side thereof to a thickness of about 4 μm silicon oxide film. Using the silicon oxide film as an intermediate layer, the silicon oxide film was polished and bonded to a p-type silicon wafer having a resistivity of 2000 Ω · cm. LT wafer used has about 1 x 10¹⁰Volume resistivity of Ω · cm.

The surface of the LT wafer on which the silicon oxide layer was formed was processed into an uneven structure having an average length RSm of cells in a cross-sectional curve of 12 μm by loose abrasive grains, and Ra ═ 0.3 μm.

After bonding, a heat treatment was applied at 100 ℃ for 48 hours in a nitrogen atmosphere. The LT layer is then thinned to a thickness of 10 μm by grinding and lapping. Then, in order to further improve the adhesive strength, heat treatment was performed at 250 ℃ for 24 hours under a nitrogen atmosphere.

For the composite substrate manufactured as described above, the thickness of the composite substrate was measured by "JIS C2139: 2008 solid electrical insulating Material-measurement method of volume resistivity and surface resistivity¹²Omega cm. The voltage applied when measuring the volume resistivity was 500V.

Next, an Al film was sputtered onto the composite substrate manufactured as described above at a thickness of 0.14 μm, and a resist pattern having a line width of about 0.5 μm was formed by i-line exposure after coating the resist. Then, Al is etched by dry etching to form a ladder type SAW filter. In addition, a 50nm silicon nitride layer was formed by sputtering on the surface layer of the substrate on which the SAW filter was formed.

When the characteristics of the SAW filter were evaluated using an RF probe and a network analyzer, a waveform as in the case of fig. 7 was obtained. SAW filters with wavelengths of approximately 2.2 μm, 2.1 μm and 1.9 μm were placed in the same wafer. Therefore, in this comparative example, if the wavelength λ is taken as the average length of the cells in the cross-sectional curve of the concavo-convex structure of the LT adhesive surface, RSm/λ is 5.4, 5.7, and 6.3 for the SAW filters at the respective wavelengths, respectively.

Next, the composite substrate having the SAW filter pattern fabricated as described above was passed through a reflow oven at 265 ℃ 6 times, and then thermally cycled from-40 ℃ to 125 ℃ 1000 times. Thereafter, the characteristics of the SAW filter were evaluated again by the RF probe and the network analyzer, and the insertion loss increased by about 8dB compared to the waveform in fig. 7.

Further, PFM images of cross sections of the composite substrates manufactured in the same manner as described above were measured. As a result, similarly to the case shown in fig. 6, it was found that the LT portion was uniformly polarized.

PFM images of cross-sections of composite substrates with SAW filter patterns were measured after passing the substrates through a reflow oven at 265 ℃ six times and then thermally cycling them 1000 times from-40 ℃ to 125 ℃. As a result, as shown in fig. 9, the polarization of the LT portion is disordered.

In the above comparative example 3, SiO as the intermediate layer was measured by the nanoindentation and X-ray reflectance (Xrr) methods, respectively₂Young's modulus and density. Results of the above comparative example 3 and SiO obtained from Young's modulus and density₂The transverse sound velocity (calculated value) of the film was equal to the value shown in table 1.

Comparative example 4

A composite substrate was produced in the same manner as in comparative example 3, except that the heat treatment for further improving the adhesive strength was performed after the LT layer was thinned in the air atmosphere instead of the nitrogen atmosphere.

For the composite substrate manufactured as described above, the thickness of the composite substrate was measured by "JIS C2139: 2008 solid electrical insulating Material-measurement method of volume resistivity and surface resistivity¹⁴Omega cm. The voltage applied when measuring the volume resistivity was 500V.

Next, an Al film was sputtered onto the composite substrate manufactured as described above at a thickness of 0.14 μm, and a resist pattern having a line width of about 0.5 μm was formed by i-line exposure after coating the resist. Then, Al is etched by dry etching to form a ladder type SAW filter. In addition, a 50nm silicon nitride layer was formed by sputtering on the surface layer of the substrate on which the SAW filter was formed.

When the characteristics of the SAW filter were evaluated using an RF probe and a network analyzer, a waveform similar to the case in fig. 7 was obtained. SAW filters with wavelengths of approximately 2.2 μm, 2.1 μm and 1.9 μm were placed in the same wafer. Therefore, in this comparative example, if the wavelength λ is taken as the average length of the cells in the cross-sectional curve of the concavo-convex structure of the LT adhesive surface, RSm/λ is 5.4, 5.7, and 6.3 for the SAW filters at the respective wavelengths, respectively.

Next, the composite substrate having the SAW filter pattern fabricated as described above was passed through a reflow oven at 265 ℃ 6 times, and then thermally cycled from-40 ℃ to 125 ℃ 1000 times. Thereafter, the characteristics of the SAW filter were evaluated again by the RF probe and the network analyzer, and the insertion loss increased by about 9dB compared to the waveform in fig. 7.

Further, PFM images of cross sections of the composite substrates manufactured in the same manner as described above were measured. As a result, similarly to the case shown in fig. 6, it was found that the LT portion was uniformly polarized.

PFM images of cross-sections of composite substrates with SAW filter patterns were measured after passing the substrates through a reflow oven at 265 ℃ six times and then thermally cycling them 1000 times from-40 ℃ to 125 ℃. As a result, similarly to the case shown in fig. 9, it was found that the LT portion was uniformly polarized.

As can be seen from the above-described examples 1 to 6, the sound velocity of the transverse wave of the anti-diffusion layer is preferably faster than the sound velocity of the fast transverse wave of the piezoelectric single crystal thin film (LT). Preferably, the acoustic velocity of the transverse wave of the intermediate layer between the diffusion preventing layer and the support substrate is lower than the acoustic velocity of the fast transverse wave of the piezoelectric single crystal thin film. It is preferable that a ratio of an average length RSm of cells in a cross-sectional curve of the concave-convex structure to a wavelength λ of the surface acoustic wave when used as the surface acoustic wave device is 0.2 or more and 5.0 or less.