Composite substrate for elastic wave device

Abstract

The objective of this invention is to improve the bonding strength between the piezoelectric material substrate and the support substrate, and to effectively reduce the reflection of bulk waves and suppress parasitic signals. The composite substrate 7A for elastic wave devices comprises: a piezoelectric material layer PZ; a support substrate S; and intermediate layers 1, 2, and X located between the piezoelectric material layer and the support substrate, each layer being an integer x (x being 3 or more). The piezoelectric material layer, the support substrate, and the intermediate layers satisfy equation (1); when x is even, equation (2) is satisfied; and when x is odd, equation (3) is satisfied. n <R n+1 …(1)(n represents all integers from 1 to x, R n R is the arithmetic mean roughness of the surface of the piezoelectric material layer side of the nth intermediate layer, as observed from the piezoelectric material layer. x+1 This refers to the arithmetic mean roughness of the surface of the piezoelectric material layer supporting the substrate. n‑1 <V n …(2)(n represents all even numbers above 2 and below x, V n Let V be the sound velocity of the intermediate layer of the nth layer, as observed from the piezoelectric material layer. n‑1 >V n …(3)(n represents all odd numbers above 1 and below x, V n V0 represents the sound velocity of the intermediate layer of the nth layer as observed from the piezoelectric material layer.

Description

Technical Field

[0001] This invention relates to composite substrates for elastic wave devices. Background Technology

[0002] It is known that surface elastic wave filters, which bond lithium tantalate and sapphire together using a silicon oxide layer, generate bulk waves at their bonding interface, resulting in unwanted responses in the transmission and high-frequency regions. To prevent these problems, a method has been proposed that introduce a rough surface at the bonding interface to scatter the bulk waves and suppress unwanted responses (Patent Document 1, Patent Document 2).

[0003] In Patent Document 1, when the mating surface is roughened, the ratio of the average length RSm of the elements of the cross-sectional curve constituting the rough surface to the wavelength λ of the surface elastic wave is set to 0.2 or more and 7.0 or less, and the arithmetic mean roughness Ra of the cross-sectional curve of the rough surface is set to 100 nm or more. On the other hand, Patent Document 2 specifies the height difference of the rough surface.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent No. 6250856

[0007] Patent Document 2: U.S. Publication No. 2017-063333 Summary of the Invention

[0008] However, to achieve sufficiently high parasitic signal suppression, the back side of the piezoelectric material substrate needs to be significantly roughened. However, if a piezoelectric material substrate with this back side roughening is used to fabricate the bond, a processing-modified layer can easily form on the surface of the piezoelectric material during thinning, leading to performance degradation. Furthermore, when bonding the piezoelectric material substrate to the intermediate layer on the support substrate, a high roughness on the back side of the piezoelectric material substrate makes it difficult to improve the bonding strength.

[0009] The objective of this invention is to improve the bonding strength between the piezoelectric material substrate and the support substrate, and to effectively reduce the reflection of bulk waves and suppress parasitic signals, in relation to a composite substrate for an elastic wave device comprising a piezoelectric material substrate and a support substrate.

[0010] The present invention relates to a composite substrate for elastic wave devices, characterized in that it comprises: a piezoelectric material layer; a support substrate; and an intermediate layer of an x ​​layer (x is an integer of 3 or more) located between the piezoelectric material layer and the support substrate, wherein the piezoelectric material layer, the support substrate and the intermediate layer satisfy the following formula (1), and when x is an even number, it satisfies the following formula (2), and when x is an odd number, it satisfies the following formula (3).

[0011] R n <R n+1 …(1)

[0012] (In formula (1),)

[0013] n represents all integers from 1 to x.

[0014] R n The roughness is the arithmetic mean of the surface roughness of the piezoelectric material layer side of the intermediate layer of the nth layer, as viewed from the piezoelectric material layer.

[0015] R x+1 (This refers to the arithmetic mean roughness of the surface of the piezoelectric material layer side of the supporting substrate.)

[0016] V n-1 <V n …(2)

[0017] (In formula (2),)

[0018] n represents all even numbers greater than 2 and less than x.

[0019] V n (This refers to the sound velocity of the intermediate layer in the nth layer, as observed from the piezoelectric material layer.)

[0020] V n-1 >V n …(3)

[0021] (In formula (3),)

[0022] n represents all odd numbers greater than 1 and less than x.

[0023] V n The sound velocity of the intermediate layer in the nth layer as observed from the piezoelectric material layer.

[0024] V0 represents the sound velocity of the piezoelectric material layer.

[0025] Invention Effects

[0026] According to the present invention, in a composite substrate for an elastic wave device comprising a piezoelectric material substrate and a support substrate, the arithmetic mean roughness of the intermediate layer is progressively reduced from the support substrate toward the piezoelectric material layer, thereby improving the bonding strength with respect to the piezoelectric material substrate. Simultaneously, it has been found that by sequentially and adjacently arranging intermediate layers with high sound velocity and slow sound velocity from the piezoelectric material layer toward the support substrate, the reflection of bulk waves is effectively reduced and parasitic signal waves are significantly suppressed, thus realizing the present invention. Attached Figure Description

[0027] Figure 1 (a) is a schematic cross-sectional view showing the state in which intermediate layers X and 2 and bonding layer M are provided on a support substrate. Figure 1 (b) is a cross-sectional view showing the state in which the bonding layer Y is provided on the piezoelectric material substrate. Figure 1 (c) is a cross-sectional view of the joint 7A between the support substrate and the piezoelectric material substrate.

[0028] Figure 2 (a) is a schematic cross-sectional view showing the state in which intermediate layers X, 3, 2 and bonding layer M are provided on a support substrate. Figure 2 (b) is a cross-sectional view showing the state in which the bonding layer Y is provided on the piezoelectric material substrate. Figure 2 (c) is a cross-sectional view of the joint 7B of the support substrate and the piezoelectric material substrate.

[0029] Figure 3 (a) indicates the state of the piezoelectric material substrate of the bonding body that has been thinned through processing. Figure 3 (b) represents elastic wave element 8.

[0030] Figure 4 This is an example of a graph showing the sound velocity of the intermediate layer on a supporting substrate.

[0031] Figure 5 This is an example of a graph showing the sound velocity of the intermediate layer on a supporting substrate.

[0032] Figure 6 S represents the elastic wave element of the embodiment. 11 The frequency response curve.

[0033] Figure 7 S represents the elastic wave element of the comparative example. 11 The frequency response curve. Detailed Implementation

[0034] The invention will now be described in more detail with appropriate reference to the accompanying drawings.

[0035] First, such as Figure 1As shown in (a), intermediate layers X and 2, and bonding layer M are sequentially formed on the surface Sa of the support substrate S. Next, the surface Ma of bonding layer M is precision ground, for example, by chemical mechanical polishing. Next, surface activation is performed on the surface Ma of bonding layer M.

[0036] On the other hand, such as Figure 1 As shown in (b), a bonding layer Y is formed on the main surface PZa of the piezoelectric material layer PZ. Surface activation is performed on the surface Ya of the bonding layer Y. Next, the surface Ma of the bonding layer M and the surface Ya of the bonding layer Y are brought into contact to form a direct bond, thereby obtaining... Figure 1 (c) shows the joint 7A. When the joint layer Y and the joint layer M are made of the same material, they are essentially integrated to form the intermediate layer 1.

[0037] in addition, Figure 2 This is an embodiment in which four intermediate layers are provided between the piezoelectric material layer and the supporting substrate.

[0038] First, such as Figure 2 As shown in (a), intermediate layers X, 3, 2 and bonding layer M are sequentially formed on the surface Sa of the support substrate S. Next, the surface Ma of the uppermost bonding layer M is precision ground, for example, by chemical mechanical polishing. Next, surface activation is performed on the surface Ma of the bonding layer M.

[0039] On the other hand, such as Figure 2 As shown in (b), the surface PZa of the piezoelectric material substrate PZ is roughened. A bonding layer Y is formed on the main surface PZa of the piezoelectric material substrate PZ. Surface activation is performed on the surface Ya of the bonding layer Y. Next, the surface Ma of the bonding layer M and the surface Ya of the bonding layer Y are brought into contact for direct bonding, thereby obtaining... Figure 2 (c) shows the joint 7B.

[0040] Next, as Figure 3 As shown in (a), a piezoelectric material substrate PZ is thinned by processing to form a piezoelectric material substrate PZC, thereby obtaining a bond 7C. Electrodes can be disposed on the piezoelectric material substrate PZC in this state. However, preferably, as... Figure 3 As shown in (b), a specified electrode 9 can be formed on the processing surface of the piezoelectric material substrate PZC to obtain an elastic wave element 8.

[0041] Here, the arithmetic mean roughness Ra and sound velocity of each of the multiple intermediate layers between the piezoelectric material layer and the support substrate are adjusted, thereby improving the bonding strength between the support substrate and the piezoelectric material substrate and suppressing parasitic signal waves. This structure will be further explained.

[0042] First, an intermediate layer of x layers (x being 3 or more) is provided between the piezoelectric material layer PZ (PZC) and the surface Sa of the supporting substrate S. Three or more intermediate layers are required to allow the arithmetic mean roughness of the intermediate layers to decrease stepwise and to effectively reduce bulk wave reflection, thereby suppressing parasitic signals. From this perspective, the number of intermediate layers is more preferably four or more. Furthermore, there is no particular upper limit to the number of intermediate layers; however, increasing the number of intermediate layers increases manufacturing costs. Therefore, from this perspective, the number of intermediate layers is preferably ten or less.

[0043] The conjoint satisfies equation (1), and when x is even, it satisfies equation (2), and when x is odd, it satisfies equation (3).

[0044] First, Equation (1) specifies the arithmetic mean roughness of each surface of the supporting substrate and each intermediate layer.

[0045] R n <R n+1 …(1)

[0046] (In formula (1),)

[0047] n represents all integers from 1 to x.

[0048] R n The roughness is the arithmetic mean of the surface roughness of the piezoelectric material layer side of the intermediate layer of the nth layer, as viewed from the piezoelectric material layer.

[0049] R x+1 (This refers to the arithmetic mean roughness of the surface of the piezoelectric material layer side of the supporting substrate.)

[0050] That is, observing from the piezoelectric material layer PZ, the arithmetic mean roughness R1 of the piezoelectric material layer side surface 1a of the intermediate layer 1 of the first layer is lower than the arithmetic mean roughness R2 of the piezoelectric material layer side surface 2a of the intermediate layer 2 of the second layer. Similarly, observing from the piezoelectric material layer PZ, the arithmetic mean roughness R2 of the piezoelectric material layer side surface 2a of the intermediate layer 2 of the second layer is lower than the arithmetic mean roughness R3 of the piezoelectric material layer side surface 3a of the intermediate layer 3 of the third layer. Thereafter, the arithmetic mean roughness of the piezoelectric material layer side surfaces of the intermediate layers also tends to increase sequentially towards the supporting substrate. Furthermore, the arithmetic mean roughness R1 of the piezoelectric material layer side surface Sa of the supporting substrate S... x+1 The arithmetic mean roughness Rx of the piezoelectric material layer side surface Xa of the intermediate layer X closest to the support substrate S.

[0051] That is, from the supporting substrate toward the piezoelectric material layer, the arithmetic mean roughness of each surface on the piezoelectric material layer side of each intermediate layer decreases sequentially.

[0052] For example, Figure 1 In example (c), there are three intermediate layers 1, 2, and X (x = 3) between the piezoelectric material layer PZ and the supporting substrate S. In this case, when viewed from the piezoelectric material layer PZ, the arithmetic mean roughness R1 of the piezoelectric material layer side surface 1a of the first intermediate layer 1 is lower than the arithmetic mean roughness R2 of the piezoelectric material layer side surface 2a of the second intermediate layer 2, and the arithmetic mean roughness R2 of the surface 2a of the second intermediate layer 2 is lower than the arithmetic mean roughness R1 of the surface Xa of the third (xth) intermediate layer X. x The arithmetic mean roughness R of the surface Xa of the intermediate layer X in the third layer x The arithmetic mean roughness R of the surface Sa below the support substrate S x+1 That is, the arithmetic mean roughness of the intermediate layer surface increases stepwise as you move away from the piezoelectric material layer.

[0053] in addition, Figure 2 In example (c), there are four intermediate layers 1, 2, 3, and X (x = 4) between the piezoelectric material layer PZ and the supporting substrate S. In this case, observing from the piezoelectric material layer PZ, the arithmetic mean roughness R1 of the piezoelectric material layer side surface 1a of the first intermediate layer 1 is lower than the arithmetic mean roughness R2 of the piezoelectric material layer side surface 2a of the second intermediate layer 2; the arithmetic mean roughness R2 of the surface 2a of the second intermediate layer 2 is lower than the arithmetic mean roughness R3 of the surface 3a of the third intermediate layer 3; and the arithmetic mean roughness R3 of the surface 3a of the third intermediate layer 3 is lower than the arithmetic mean roughness R of the surface Xa of the fourth (xth) intermediate layer X. x The arithmetic mean roughness R of the surface Xa of the middle layer X of the fourth layer x The arithmetic mean roughness R of the surface Sa below the support substrate S x+1 That is, the arithmetic mean roughness of the intermediate layer surface increases stepwise as you move away from the piezoelectric material layer.

[0054] In this way, as the intermediate layer moves away from the piezoelectric material layer, the surface becomes rougher, thereby improving the bonding strength with the piezoelectric material substrate.

[0055] According to this viewpoint, R x+1 With R x The difference is preferably 0.2 nm or more, and more preferably 0.5 nm or more. Furthermore, from a practical point of view, R... x+1 With R x The difference is mostly less than 1nm.

[0056] Furthermore, according to the viewpoint of the present invention, R n-1 With R nThe difference is preferably 0.2 nm or more, and more preferably 0.5 nm or more. Furthermore, from a practical point of view, R... n-1 With R n The difference is mostly less than 1nm.

[0057] In addition, the arithmetic mean roughness R of the surface Sa of the supporting substrate S x+1 Preferably, the roughness is 0.5 to 5 nm, more preferably 1.5 to 4.0 nm. In addition, from the viewpoint of bonding strength, the arithmetic mean roughness R1 of the surface 1a of the intermediate layer 1 (the first layer) closest to the piezoelectric material layer is preferably 1 nm or less, more preferably 0.3 nm or less.

[0058] Furthermore, regarding the joint of the present invention, the sound velocity of each layer needs to satisfy a specified relationship. That is, when x is an even number, the following equation (2) is satisfied, and when x is an odd number, the following equation (3) is satisfied.

[0059] V n-1 <V n …(2)

[0060] (In formula (2),)

[0061] n represents all even numbers greater than 2 and less than x.

[0062] V n (This refers to the sound velocity of the intermediate layer in the nth layer, as observed from the piezoelectric material layer.)

[0063] V n-1 >V n …(3)

[0064] (In formula (3),)

[0065] n represents all odd numbers greater than 1 and less than x.

[0066] V n The sound velocity of the intermediate layer in the nth layer as observed from the piezoelectric material layer.

[0067] V0 represents the sound velocity of the piezoelectric material layer.

[0068] For example, Figure 1 In example (c), three intermediate layers 1, 2, and X are formed between the piezoelectric material layer and the supporting substrate. Since x = 3, equation (3) is satisfied.

[0069] Therefore, equation (3) is as follows Figure 4 As shown. It should be noted that V x+1 The sound velocity is to support the substrate S.

[0070] Typically, when x is an odd number, n is an odd number between 1 and x. Therefore, the layers are arranged from the supporting substrate toward the piezoelectric material layer in the following manner.

[0071] V x-1 >V x V x-3 >V x-2 ...V2 > V3; V0 > V1

[0072] In this case, V n-1 With V n-2 The size relationship between them is not limited, however, it is particularly preferred that the following relationship be satisfied:

[0073] V n-2 <V n-1 .

[0074] When x is even, the following conditions are met:

[0075] V n-1 <V n …(2).

[0076] Therefore, the sound velocity of each intermediate layer is as follows, moving from the piezoelectric material layer toward the supporting substrate.

[0077] V x-1 <V x V x-3 <V x-2 ...V3 < V4; V1 < V2

[0078] For example, Figure 2 In example (c), four intermediate layers 1, 2, 3, and X are formed between the piezoelectric material layer and the supporting substrate. Since x = 4, equation (2) is as follows.

[0079] V3<V x V1 < V2

[0080] Therefore, as Figure 5 As shown. It should be noted that V x+1 The sound velocity is to support the substrate S.

[0081] In this case, V n-1 With V n-2 The size relationship between them is not limited, but the following relationship is preferred:

[0082] V n-2 >V n-1 .

[0083] According to the viewpoint of the present invention, in equations (2) and (3), V n-1 With V n The difference is preferably 200 m / sec or more, and more preferably 500 m / sec or more. Furthermore, from a practical point of view, in equations (2) and (3), V n-1 With V nThe differences are mostly below 3000m / sec.

[0084] According to the present invention, the difference between V0 and V1 is preferably 200 m / sec or more, and more preferably 500 m / sec or more. Furthermore, the upper limit of the difference between V0 and V1 is not particularly limited; from a practical point of view, it can be 3000 m / sec or less.

[0085] Additionally, V n-2 and V n-1 In different situations, V n-2 With V n-1 The difference is preferably 1000 m / sec or more, and more preferably 3000 m / sec or more. Furthermore, from a practical point of view, V n-2 With V n-1 The difference is mostly below 10,000 m / sec.

[0086] The material of the support substrate is not particularly limited, but it is preferably made of a material selected from the group consisting of silicon, crystal, and sapphire. This allows for further improvement in the temperature characteristics of the elastic wave element's frequency.

[0087] Alternatively, the surface of the piezoelectric material layer on the support substrate can be roughened by grinding with a grinding stone or by sandblasting.

[0088] In addition, sandblasting refers to the process of using compressed air to spray abrasive materials onto a surface.

[0089] The method for forming each intermediate layer on the piezoelectric material substrate is not limited, and examples include sputtering, chemical vapor deposition (CVD), and evaporation.

[0090] The material of each intermediate layer is not particularly limited as long as it can undergo surface activation treatment, but metal oxide films are preferred, and materials selected from the group consisting of silicon, silicon oxide, aluminum oxide, tantalum pentoxide, niobium pentoxide, and titanium oxide are particularly preferred. Furthermore, the surface activation treatment method can be selected appropriately depending on the material of the bonding layer used. Examples of such surface activation methods include plasma activation and FAB (Ar atom beam).

[0091] According to the present invention, the thickness of each intermediate layer is preferably 0.02 μm or more, more preferably 0.05 μm or more, and particularly preferably 0.1 μm or more. Furthermore, the thickness of each intermediate layer is preferably 3 μm or less, more preferably 2 μm or less, and even more preferably 1 μm or less.

[0092] Furthermore, according to the present invention, the total thickness of the plurality of intermediate layers is preferably 0.1 to 5 μm, more preferably 0.2 to 2 μm.

[0093] The piezoelectric material substrate used in this invention is preferably lithium tantalate (LT) single crystal, lithium niobate (LN) single crystal, or lithium niobate-lithium tantalate solid solution. These materials have fast elastic wave propagation speed and large electromechanical coupling coefficient, making them suitable as elastic surface wave devices for high-frequency and wide-bandwidth applications.

[0094] Furthermore, the normal direction of the main surface of the piezoelectric material substrate is not particularly limited. For example, when the piezoelectric material substrate is made of LT, a piezoelectric material substrate with a direction of rotation of 32 to 55° from the Y axis to the Z axis as the center of the X axis, which is the direction of propagation of the elastic surface wave, and expressed in Euler angles as (180°, 58 to 35°, 180°) is preferred because the propagation loss is small. When the piezoelectric material substrate is composed of LN, (a) a piezoelectric material substrate is used with a direction centered on the X-axis (which is the direction of propagation of the elastic surface wave) and rotated 37.8° from the Z-axis to the Y-axis, with Euler angles of (0°, 37.8°, 0°). This method is preferred because it has a large electromechanical coupling coefficient. Alternatively, (i) a piezoelectric material substrate is used with a direction centered on the X-axis (which is the direction of propagation of the elastic surface wave) and rotated 40 to 65° from the Y-axis to the Z-axis, with Euler angles of (180°, 50 to 25°, 180°). This method is preferred because it can achieve high sound speeds. Furthermore, the size of the piezoelectric material substrate is not particularly limited, for example, with a diameter of 100 to 200 mm and a thickness of 0.15 to 1 μm.

[0095] For example, direct bonding can be achieved by surface activation of the surface of the bonding layer M on the outermost surface of the support substrate, the support substrate side surface PZa of the piezoelectric material substrate PZ, or the surface Ya of the bonding layer Y on the piezoelectric material substrate. For example, the bonding surfaces can be activated by irradiating each surface with plasma at a temperature of 150°C or below. According to the present invention, irradiation with nitrogen plasma is preferred; however, the bonded body of the present invention can also be obtained by irradiating with oxygen plasma.

[0096] The pressure during surface activation is preferably 100 Pa or less, more preferably 80 Pa or less. Furthermore, the atmosphere can be nitrogen only, oxygen only, or a mixture of nitrogen and oxygen.

[0097] The temperature during plasma irradiation is preferably below 150°C. This allows for the production of a bond with high bonding strength and undegraded crystallinity. Based on this viewpoint, the temperature during plasma irradiation is set to below 150°C, and more preferably below 100°C.

[0098] Furthermore, the energy used for plasma irradiation is preferably 30–150 W. Additionally, the product of the energy used for plasma irradiation and the irradiation time is preferably 0.12–1.0 Wh.

[0099] The bonding surfaces of the plasma-treated piezoelectric material substrate and the bonding layer are brought into contact with each other at room temperature. This process can be performed in a vacuum, but is more preferably carried out in the atmosphere.

[0100] When performing surface activation using an argon atomic beam, the apparatus described in Japanese Patent Application Publication No. 2014-086400 is preferably used to generate and irradiate an argon atomic beam. Specifically, a saddle-type high-speed atomic beam source is used as the beam source. Then, an inert gas is introduced into the chamber, and a high voltage is applied to the electrodes from a DC power supply. This utilizes the saddle-type electric field generated between the electrodes (positive electrode) and the shell (negative electrode) to cause electrons (e) to move, generating a beam of argon atoms and ions. In the beam reaching the grid, the ion beam is neutralized at the grid, thus the argon atomic beam is emitted from the high-speed atomic beam source. The voltage for activation by irradiating the beam is preferably 0.5–2.0 kV, and the current is preferably 50–200 mA.

[0101] Next, the surface Ma of the bonding layer M on the outermost surface of the support substrate and the surface PZa of the piezoelectric material substrate PZ, or the surface Ya of the bonding layer Y on the piezoelectric material substrate, are brought into contact to form a bond. Then, an annealing process is preferably performed to improve the bond strength. The annealing temperature is preferably 100°C to 300°C.

[0102] The bonding element of the present invention is preferably used in an elastic wave element. That is, the elastic wave element includes the bonding element of the present invention and an electrode disposed on a piezoelectric material substrate.

[0103] Specifically, surface wave devices, Lamb wave devices, and thin-film resonators (FBARs) are known as elastic wave elements. For example, an elastic surface wave device has an IDT (Interdigital Transducer) electrode (also called a comb electrode or interdigitated electrode) on the input side of a piezoelectric material substrate to excite the elastic surface wave, and an IDT electrode on the output side to receive the elastic surface wave. If a high-frequency signal is applied to the IDT electrode on the input side, an electric field is generated between the electrodes, thereby exciting the elastic surface wave and causing it to propagate on the piezoelectric material substrate. Furthermore, the propagating elastic surface wave can be extracted as an electrical signal from the IDT electrode on the output side, which is positioned in the propagation direction.

[0104] The preferred material for the electrodes constituting the piezoelectric material substrate is aluminum, aluminum alloy, copper, or gold, with aluminum or aluminum alloy being more preferred. The aluminum alloy is preferably an aluminum alloy containing 0.3 to 5% by weight of Cu in Al. In this case, Ti, Mg, Ni, Mo, or Ta can be used instead of Cu.

[0105] Example

[0106] (Example 1)

[0107] Based on reference Figure 1 , Figure 3 Made according to the method described Figure 3 (b) shows the elastic wave element 8.

[0108] Specifically, a 42Y-cut X-ray propagation LiTaO3 substrate (piezoelectric material substrate) PZ with a thickness of 350 μm and mirror finish on both sides was prepared. Additionally, a high-resistivity (>2 kΩ·cm) Si(100) substrate (support substrate) S with a thickness of 675 μm was prepared. The substrate size for both substrates was 150 mm.

[0109] Next, the surface Sa of the support substrate S was ground using a grinding stone equivalent to GC#6000. The machining depth was set to 5 μm to ensure uniform machining across the entire surface. The surface roughness of the machined support substrate Sa was measured using an optical interferometer, and the arithmetic mean roughness R was obtained. x+1 It is 3.2nm.

[0110] After cleaning the surface Sa of the support substrate S, an intermediate layer X containing Ta2O5 is formed using a sputtering apparatus. The thickness of the intermediate layer X is 1200 nm. The wafer after film deposition is temporarily removed, and the arithmetic mean roughness Rx of surface Xa is measured, showing a significant reduction to 1.9 nm. An 800 nm thick intermediate layer 2 made of silicon is continuously formed on the intermediate layer X. The arithmetic mean roughness R2 of the surface 2a of the intermediate layer 2 is 1.3 nm. Next, a 400 nm thick bonding layer M made of silicon oxide is formed, ultimately creating a 3-layer stacked structure. The arithmetic mean roughness Ma of the bonding layer M is 1.0 nm, achieving a significantly smoother surface compared to the initial 3.2 nm. The surface of the outermost bonding layer is processed by CMP to remove approximately 30 nm. As a result, the arithmetic mean roughness of the surface reaches 0.6 nm.

[0111] Furthermore, a bonding layer Y composed of silicon oxide with a thickness of 100 nm is formed on the surface PZa of the piezoelectric material substrate PZ. At this time, the arithmetic mean roughness of the surface Ya of the bonding layer Y is 1.2 nm. By performing CMP processing on this surface to remove approximately 50 nm, the arithmetic mean roughness reaches 0.3 nm.

[0112] The bonding surfaces of the bonding layers on the obtained piezoelectric material substrate and the outermost surface of the bonding layer on the support substrate were cleaned and surface activated, respectively. Specifically, ultrasonic cleaning with pure water was performed, followed by spin drying to dry the substrate surface. Next, the cleaned support substrate was introduced into a plasma activation chamber, where the bonding surfaces were activated using nitrogen plasma at a temperature of 30°C. Similarly, the piezoelectric material substrate was introduced into the plasma activation chamber, where the bonding surfaces were surface activated using nitrogen plasma at a temperature of 30°C. The surface activation time was set to 40 seconds, and the energy was set to 100W. To remove particles adhering during surface activation, the same ultrasonic cleaning and spin drying process was performed again.

[0113] Next, the substrates were aligned, and the activated bonding surfaces of the two substrates were brought into contact at room temperature. The piezoelectric material substrate side was facing upwards during contact. As a result, a close-bonding expansion pattern (the so-called bonding wave) was observed between the substrates, confirming that good pre-bonding had been achieved. Next, to increase the bonding strength, the bond was placed in a nitrogen atmosphere oven and maintained at 150°C for 10 hours. The bonding strength of the bond removed from the oven was measured using the crack-opening method, and the result showed that 2.3 J / m² was achieved. 2 Sufficient bonding strength.

[0114] The surface of the piezoelectric material substrate of the heated bonding body is ground, precision ground, and CMP processed to achieve a thickness of 20μm.

[0115] Next, to confirm the effectiveness of the present invention, a resonator for a surface elastic wave element was fabricated by forming a comb-tooth electrode made of aluminum on a piezoelectric material substrate of the bonding body. Its specifications are shown below.

[0116]

[0117] Its reflection characteristics were measured using a network analyzer, and the results are as follows: Figure 6 As shown, the maximum parasitic signal magnitude in the region above the anti-resonant frequency is 2.2 dB.

[0118] The following is a summary of the physical properties of the support substrate, intermediate layer, and piezoelectric material substrate in this embodiment. Additionally, Figure 4 The speed of sound for each part is shown in the figure.

[0119]

[0120] The speed of sound for each part is defined as follows.

[0121] That is, given that the elastic modulus of the material is E and the density is ρ, the speed of sound V can be calculated using the following formula.

[0122]

Mathematical Formula 1

[0123]

[0124] In the case of piezoelectric crystallization, various reports have been made regarding these parameters (e.g., the Japan Society for the Promotion of Science's Committee on Elastic Wave Devices, Committee 150, which provides more details), however, it is necessary to measure the thin films individually. Films for each material were fabricated on a Si substrate using sputtering. The thickness was set to approximately 1 μm. For these films, firstly, the density was measured using X-ray reflectance analysis. Then, the elastic modulus was measured using nanoindentation testing, and the sound velocity of each film was calculated based on the above formula.

[0125] In addition, regarding the arithmetic mean roughness of each surface, an atomic force microscope (AFM) manufactured by Hitachi High-Tech was used to observe a range of 10x10μm, and the arithmetic mean roughness was calculated based on the surface roughness data.

[0126] (Example 2)

[0127] Prepared in the same manner as in Example 1 Figure 2 (c) The joint 7B shown therein is then implemented. Figure 3 The process shown yields SAW elements.

[0128] However, intermediate layers X, 3, 2, and bonding layer M are disposed on the support substrate. Specifically, after cleaning the surface Sa of the silicon-based support substrate S with an arithmetic mean roughness Ra of 3.2 nm, an intermediate layer X made of aluminum oxide is formed using a sputtering apparatus. The thickness of the intermediate layer X at this point is 600 nm. The wafer after film deposition is temporarily removed, and the arithmetic mean roughness Ra of the surface Xa of the intermediate layer X is measured. x Measurements were performed, and the roughness was reduced to 2.7 nm. A 1000 nm thick intermediate layer 3 made of silicon oxide was then continuously formed on the same wafer. The arithmetic mean roughness at this point was 1.6 nm. Next, after forming a 300 nm thick intermediate layer 2 made of silicon, a 350 nm thick bonding layer M made of silicon oxide was continuously formed, ultimately obtaining a support substrate with a four-layer film structure. The arithmetic mean roughness of the intermediate layer 2 and the bonding layer M at this point were 1.3 nm and 1.2 nm, respectively, thus obtaining a significantly smoother surface compared to the initial 3.2 nm. The surface Ma of the outermost bonding layer M was subjected to CMP processing to remove approximately 30 nm. As a result, the arithmetic mean roughness of the surface Ma of the bonding layer M reached 0.55 nm.

[0129] In addition, it was made in the same manner as in Example 1. Figure 3 (b) Using the same SAW element and performing the same measurements, the maximum parasitic signal size was 1.3 dB.

[0130] The surface roughness and sound velocity of each layer are as follows. Additionally, Figure 5 The speed of sound for each part is shown in the figure.

[0131]

[0132] (Comparative Example 1)

[0133] Prepared in the same manner as in Example 1 Figure 1 The assembly shown is then implemented. Figure 3 The process shown yields SAW elements.

[0134] However, in this example, the materials of each intermediate layer were changed compared to Example 1. Specifically, an intermediate layer X (600 nm) made of aluminum oxide, an intermediate layer 2 (1200 nm) made of silicon oxide, and a bonding layer M (400 nm) made of silicon on the outermost surface were continuously formed on the surface Sa of a silicon-based support substrate S with an arithmetic mean roughness Ra = 2.8 nm, thereby obtaining a three-layer structure. The arithmetic mean roughnesses of each intermediate layer 2 and bonding layer M are 1.2 nm, 1.0 nm, and 0.9 nm, respectively. The outermost bonding layer was CMP polished at approximately 20 nm to form a mirror finish. Next, after irradiating the surfaces of the bonding layer on the piezoelectric material substrate and the surface of the intermediate layer on the outermost surface of the support substrate with neutral Ar atoms, direct bonding was performed.

[0135] Similar to Example 1, the piezoelectric material substrate was processed to a thickness of 20 μm, and then the frequency characteristics were measured. The results were obtained. Figure 7 S shown 11 The graph shows the frequency variation. Additionally, the maximum parasitic signal magnitude is 14.4 dB.

[0136] The surface roughness and sound velocity of each layer are as follows.

[0137]

[0138] (Comparative Example 2)

[0139] Prepared in the same manner as in Example 1 Figure 1 The assembly shown is then implemented. Figure 3 The process shown yields SAW elements.

[0140] However, in this example, the materials of each intermediate layer were changed compared to Example 1. Specifically, an intermediate layer X (600 nm) made of silicon, an intermediate layer 2 (1200 nm) made of aluminum oxide, and a bonding layer M (400 nm) made of silicon on the outermost surface of a silicon-based support substrate S with an arithmetic mean roughness Ra = 2.9 nm were continuously formed on the surface Sa of the silicon-based support substrate S. The arithmetic mean roughness of each intermediate layer 2 and bonding layer M at the time of formation were 2.2 nm, 1.7 nm, and 1.6 nm, respectively. The outermost bonding layer was CMP polished at approximately 80 nm to form a mirror finish. Next, the surface of the piezoelectric material substrate and the surface of the bonding layer on the outermost surface of the support substrate were irradiated with neutral Ar atoms, and then direct bonding was performed.

[0141] Similar to Example 1, the piezoelectric material substrate was processed to a thickness of 20 μm, and then the frequency characteristics were measured. The result showed that the maximum parasitic signal was 17.8 dB.

[0142] The surface roughness and sound velocity of each layer are as follows.

[0143]

Claims

1. A composite substrate for an elastic wave device, characterized in that, The composite substrate for the elastic wave device comprises: a piezoelectric material layer; a support substrate; and an intermediate layer of x layers located between the piezoelectric material layer and the support substrate, wherein x is an integer greater than or equal to 3. The piezoelectric material layer, the supporting substrate, and the intermediate layer satisfy the following formula (1), and when x is an even number, they satisfy the following formula (2), and when x is an odd number, they satisfy the following formula (3). R n <R n+1 ··· (1) In equation (1), n represents all integers from 1 to x. R n The roughness is the arithmetic mean of the surface roughness of the piezoelectric material layer side of the intermediate layer of the nth layer, as viewed from the piezoelectric material layer. R n With R n+1 The difference is more than 0.2nm. R x+1 The arithmetic mean roughness of the surface of the piezoelectric material layer side of the supporting substrate is 1.5–5 nm. V n-1 <V n ··· (2) In equation (2), n represents all even numbers greater than 2 and less than x. V n The sound velocity of the intermediate layer in the nth layer as observed from the piezoelectric material layer. V n-1 >V n ··· (3) In equation (3), n represents all odd numbers greater than 1 and less than x. V n The sound velocity of the intermediate layer in the nth layer as observed from the piezoelectric material layer. V0 is the sound velocity of the piezoelectric material layer. In equations (2) and (3), V n-1 With V n The difference is between 200 m / sec and 3000 m / sec.

2. The composite substrate for elastic wave devices according to claim 1, characterized in that, The intermediate layer is formed of a material selected from the group consisting of silicon, silicon oxide, aluminum oxide, tantalum pentoxide, niobium pentoxide, hafnium oxide and titanium oxide.

3. The composite substrate for elastic wave devices according to claim 1 or 2, characterized in that, The surface of the piezoelectric material layer side of the supporting substrate is roughened by grinding or sandblasting.

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