Manufacturing method of composite substrate

Abstract

The invention provides a composite substrate having exceptional durability. This method for producing a composite substrate according to an embodiment of the present invention comprises: forming a first layer on a lower surface side of a piezoelectric substrate having an upper surface and the lower surface on opposite sides from each other, and having an electrode provided on the lower surface; planarizing the first layer so that the waviness of the surface is greater than 2 nm but no greater than 70 nm; and joining a support substrate to the first layer side of the piezoelectric substrate having the first layer formed thereon.

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing a composite substrate. [Background technology]

[0002] Surface acoustic wave (SAW) devices are known as acoustic wave devices that utilize acoustic waves. SAW devices are used, for example, as filters in communication devices such as mobile phones. In recent years, aiming to improve the characteristics of devices, devices have been proposed that have a structure in which a piezoelectric layer is sandwiched between electrodes and a hollow portion is formed between the piezoelectric layer and a support substrate, as disclosed in Patent Document 1. Such a structure can be obtained, for example, by processing a composite substrate in which a piezoelectric substrate and a support substrate are bonded via an intermediate layer. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 5650553 Summary of the Invention [Problem to be solved by the invention]

[0004] The composite substrate is required to have durability in processing, and it is a primary object of the present invention to provide a composite substrate having excellent durability. [Means for solving the problem]

[0005] A method for manufacturing a composite substrate according to an embodiment of the present invention includes forming a first layer on a lower surface side of a piezoelectric substrate having upper and lower surfaces opposing each other and an electrode provided on the lower surface, reducing the surface waviness of the first layer to more than 2 nm and not more than 70 nm by a planarization process, and joining a support substrate to the first layer side of the piezoelectric substrate on which the first layer is formed. In one embodiment, at the time of the bonding, an activation treatment is performed on the bonding surface of the first layer and the bonding surface on the supporting substrate side. In one embodiment, the activation treatment is carried out by plasma irradiation. In one embodiment, the manufacturing method further includes polishing the top surface of the piezoelectric substrate after the bonding. In one embodiment, the first layer comprises silicon oxide. Effect of the Invention

[0006] According to an embodiment of the present invention, a composite substrate having excellent durability can be provided. [Brief description of the drawings]

[0007] [Figure 1] 1 is a schematic cross-sectional view showing an outline of a configuration of a composite substrate according to one embodiment of the present invention. [Figure 2A] 1A to 1C are diagrams illustrating an example of a manufacturing process for a composite substrate according to one embodiment. [Figure 2B] This is a continuation of Figure 2A. [Figure 2C] This is a continuation of Figure 2B. [Figure 2D] This is a continuation of Figure 2C. [Figure 2E] This is a continuation of Figure 2D. [Figure 3A] 1A to 1C are diagrams illustrating an example of a manufacturing process for an acoustic wave device according to an embodiment. [Figure 3B] This is a continuation of Figure 3A. [Figure 4] 4 is a graph showing the waviness state of the surface of the silicon oxide layer in Example 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Hereinafter, the embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to these embodiments. In addition, in order to clarify the description, the width, thickness, shape, etc. of each part may be shown more diagrammatically than in the embodiment, but these are merely examples and do not limit the interpretation of the present invention.

[0009] A. Composite board FIG. 1 is a schematic cross-sectional view showing an outline of a composite substrate according to an embodiment of the present invention. The composite substrate 100 includes a piezoelectric layer 10, an intermediate layer 20, and a support substrate 30 in this order. Specifically, the support substrate 30 is disposed on the second principal surface 10b side of the piezoelectric layer 10 having a first principal surface 10a and a second principal surface 10b facing each other, and the intermediate layer 20 is disposed between the support substrate 30 and the piezoelectric layer 10. An electrode 41 is provided on the second principal surface 10b of the piezoelectric layer 10, and the intermediate layer 20 is disposed so as to cover the electrode 41. Specifically, the intermediate layer 20 is in contact with the electrode 41 and the electrode-free region of the piezoelectric layer 10 where the electrode 41 is not formed.

[0010] Although not shown, composite substrate 100 may further include any layer. The types, functions, number, combination, arrangement, etc. of such layers may be appropriately set depending on the purpose.

[0011] The composite substrate 100 may be manufactured in any suitable shape. In one embodiment, the composite substrate 100 may be manufactured in the form of a so-called wafer. The size of the composite substrate 100 may be appropriately set depending on the purpose. The diameter of the wafer is, for example, 100 mm to 200 mm.

[0012] A-1. Piezoelectric layer Any suitable piezoelectric material may be used as the material constituting the piezoelectric layer. As the piezoelectric material, a single crystal having a composition of LiAO3 is preferably used. Here, A is one or more elements selected from the group consisting of niobium and tantalum. Specifically, LiAO3 may be lithium niobate (LiNbO3), lithium tantalate (LiTaO3), or a lithium niobate-lithium tantalate solid solution.

[0013] When the piezoelectric material is lithium tantalate, for example, when the X-axis (crystal axis) of the piezoelectric material is the propagation direction (X1) of the surface acoustic wave, the direction rotated 32° to 55° (e.g., 42°) from the Y-axis toward the Z-axis corresponds to the direction perpendicular to the main surface of the piezoelectric layer (X3), specifically, it is preferable that the Euler angles are (180°, 58° to 35°, 180°).

[0014] When the piezoelectric material is lithium niobate, for example, when the X-axis (crystal axis) of the piezoelectric material is the propagation direction (X1) of the surface acoustic wave, the direction rotated from the Z-axis toward the -Y-axis by 0° to 40° (for example, 37.8°) of the piezoelectric layer corresponds to the direction perpendicular to the main surface of the piezoelectric layer (X3), specifically, it is preferable that the Euler angle is (0°, 0° to 40°, 0°). When the piezoelectric material is lithium niobate, for example, when the X-axis (crystal axis) of the piezoelectric material is the propagation direction (X1) of the surface acoustic wave, the direction rotated from the Y-axis toward the Z-axis by 40° to 65° of the piezoelectric layer corresponds to the direction perpendicular to the main surface of the piezoelectric layer (X3), specifically, it is preferable that the Euler angle is (180°, 50° to 25°, 180°).

[0015] The thickness of the piezoelectric layer can be set to any appropriate thickness depending on the method and application of the composite substrate, and is, for example, 0.2 μm or more and 30 μm or less.

[0016] A-2. Electrode The electrodes may be made of, for example, metals such as Au, Ag, Al, Pt, Mo, and Ru. These may be used alone or in combination of two or more. The thickness of the electrodes is, for example, 0.1 μm to 1 μm.

[0017] Typically, the electrodes are formed by patterning a metal film formed on the piezoelectric body by sputtering, vacuum deposition or the like.

[0018] A-3. Middle class Examples of materials that can be used to form the intermediate layer include silicon oxide (SiO2), tantalum oxide (Ta2O5), niobium oxide (Nb2O5), and silicon (PVD-Si). Silicon oxide is preferably used. The thickness of the intermediate layer (including the thickness in the region facing the electrode) is, for example, 1 μm or more and 6 μm or less, and preferably 2 μm or more and 3 μm or less.

[0019] The intermediate layer may be deposited by any suitable method, such as sputtering, physical vapor deposition such as ion-beam assisted deposition (IAD), chemical vapor deposition, or atomic layer deposition (ALD).

[0020] A-4.Support board Any suitable substrate may be used as the support substrate. The support substrate may be made of a single crystal, a polycrystalline material, or a combination of these. The material constituting the support substrate is preferably selected from the group consisting of silicon, sapphire, glass, quartz, crystal, and alumina.

[0021] The silicon may be single crystal silicon with a polycrystalline layer or an amorphous layer formed on the surface thereof, or may be high resistance silicon.

[0022] Typically, the sapphire is a single crystal having a composition of Al2O3, and the alumina is a polycrystalline body having a composition of Al2O3. The alumina is preferably translucent alumina.

[0023] The thermal expansion coefficient of the material constituting the support substrate is preferably smaller than that of the material constituting the piezoelectric layer. Such a support substrate can suppress changes in the shape and size of the piezoelectric layer when the temperature changes, and can suppress changes in the frequency characteristics of the resulting surface acoustic wave device, for example.

[0024] The thickness of the support substrate may be any appropriate thickness, and is, for example, 100 μm to 1000 μm.

[0025] A-5. Manufacturing method A method for manufacturing a composite substrate according to one embodiment of the present invention includes forming a first layer on a lower surface side of a piezoelectric substrate having upper and lower surfaces facing each other and an electrode provided on the lower surface, planarizing a surface of the first layer, and joining a support substrate to the first layer side of the piezoelectric substrate.

[0026] 2A to 2E are diagrams illustrating an example process for manufacturing a composite substrate according to one embodiment.

[0027] 2A shows a state in which the formation of the electrode 41 has been completed on the lower surface 12b of a piezoelectric substrate 12 having an upper surface 12a and a lower surface 12b opposed to each other, and FIG. 2B shows a state in which the first layer 21 has been formed on the lower surface side of the piezoelectric substrate 12. The first layer 21 can be formed by the above-mentioned method for forming an intermediate layer. The thickness of the first layer 21 can be set, for example, to a thickness that can sufficiently cover the electrode 41. The thickness of the first layer 21 is, for example, 2 μm or more and 6 μm or less.

[0028] FIG. 2C shows a state where the planarization process (e.g., lapping and / or chemical mechanical polishing) of the surface 21a of the first layer 21 is completed. The planarization process causes the waviness of the surface 21a of the first layer 21 to preferably exceed 2 nm, more preferably be 3 nm or more, and even more preferably be 5 nm or more. On the other hand, the waviness of the surface 21a of the first layer 21 is preferably set to 70 nm or less, more preferably be 60 nm or less, and even more preferably be 50 nm or less. The waviness (surface shape) of the surface can be measured by a step gauge. The waviness of the surface 21a of the first layer 21 can be controlled by adjusting the planarization process. For example, the value of the waviness can be controlled by adjusting the polishing amount of the lapping and the polishing amount of the chemical mechanical polishing.

[0029] 2D shows a process of bonding (direct bonding) the piezoelectric substrate 12 on which the first layer 21 has been formed to the support substrate 30. Specifically, the bonding surface 21a of the first layer 21 and the bonding surface 30a of the support substrate 30 are brought into contact with each other and bonded to each other. In this way, as shown in FIG. 2E, a composite substrate 110 is obtained in which the piezoelectric substrate 12 and the support substrate 30 are bonded to each other via the intermediate layer 20.

[0030] Although not shown, a second layer may be formed on the side of the support substrate 30 to which the piezoelectric substrate 12 is bonded, and the bonding surface 21a of the first layer 21 formed on the piezoelectric substrate 12 and the bonding surface of the second layer formed on the support substrate 30 may be brought into contact with each other to be bonded. In this case, the first layer and the second layer are bonded to each other to form an intermediate layer. In one embodiment, the material constituting the first layer 21 and the material constituting the second layer are substantially the same. For example, the first layer 21 and the second layer are formed by sputtering using the same target (e.g., a Si target) under the same conditions. Note that any appropriate material may be selected as the material constituting the first layer 1 and the material constituting the second layer, as long as the above bonding can be performed.

[0031] When the piezoelectric substrate 12 and the support substrate 30 are brought into contact with each other, it is preferable that the bonding surface on the piezoelectric substrate 12 side and the bonding surface on the support substrate 30 side have been subjected to an activation treatment in advance. In one embodiment, the activation treatment is performed by plasma irradiation. Examples of gases contained in the atmosphere during the activation treatment include oxygen, nitrogen, hydrogen, and argon. These may be used alone or in combination of two or more kinds (as a mixed gas). Nitrogen is preferably used.

[0032] The atmospheric pressure during activation treatment by plasma irradiation is preferably 10 kPa to 100 kPa, more preferably 50 kPa to 80 kPa. The energy during plasma irradiation is preferably 30 W to 150 W, more preferably 60 W to 120 W. The time of plasma irradiation is preferably 5 seconds to 30 seconds.

[0033] It is preferable to heat the bonded body after bringing the bonding surface on the piezoelectric substrate 12 side into contact with the bonding surface on the support substrate 30 side. Heating can further improve the bonding strength between the piezoelectric substrate 12 and the support substrate 30. The heating temperature is, for example, 100°C to 400°C. The heating time is, for example, 1 hour to 25 hours. The contact and heating may be performed in an inert gas atmosphere such as nitrogen or argon, or in the air.

[0034] In one embodiment, the heating includes a first heating step and a second heating (annealing) step in this order. In the first heating step, the bonded body is heated from room temperature to a temperature T1 (for example, 100°C to 150°C). In the second heating step, the bonded body is placed under a condition of a temperature T2 for a predetermined time (for example, 3 hours to 25 hours). The temperature T2 is, for example, 180°C or higher, may be 200°C or higher, may be 230°C or higher, may be 250°C or higher, or may be 270°C or higher. On the other hand, the temperature T2 is, for example, preferably 350°C or lower, more preferably 300°C or lower, from the viewpoint of preventing damage to the bonded body. After the second heating step, the bonded body is typically naturally cooled.

[0035] In the above film formation and bonding, the arithmetic mean roughness Ra of the surface of each layer is preferably 1 nm or less, more preferably 0.3 nm or less. Such Ra can be achieved, for example, by mirror polishing using chemical mechanical polishing (CMP). The arithmetic mean roughness Ra is a value measured in a field of view of 10 μm × 10 μm using an atomic force microscope (AFM).

[0036] During the above film formation and bonding, it is preferable to clean the surface of each layer, for example, to remove abrasive residues, processing-degraded layers, etc. Examples of cleaning methods include wet cleaning, dry cleaning, and scrub cleaning. Among these, scrub cleaning is preferable because it can clean easily and efficiently. A specific example of scrub cleaning is a method in which a cleaning agent (e.g., Sun Wash series manufactured by Lion Corporation) is used, followed by cleaning with a scrub cleaner using a solvent (e.g., a mixed solution of acetone and isopropyl alcohol (IPA)).

[0037] Typically, the upper surface 12a of the piezoelectric substrate 12 of the obtained composite substrate 110 is subjected to processing such as grinding and polishing so as to form a piezoelectric layer of the desired thickness. In this manner, the composite substrate 100 shown in FIG. 1 can be obtained. Since the bonding surface is adjusted to the above-mentioned predetermined waviness, the composite substrate 110 can have excellent durability. For example, the composite substrate 110 can have excellent durability during processing such as grinding and polishing. Specifically, the occurrence of peeling of the composite substrate (specifically, peeling at the bonding interface) due to processing such as grinding and polishing can be suppressed. As a result, a composite substrate of excellent quality without peeling can be obtained.

[0038] B.How to use The composite substrate according to the embodiment of the present invention is typically used in an acoustic wave device. FIGS. 3A and 3B are diagrams showing an example of a manufacturing process for an acoustic wave device according to one embodiment. FIG. 3A is a diagram showing a state in which the formation of a second electrode (surface electrode) 42 is completed on the surface of the composite substrate 100 (the first main surface 10a of the piezoelectric layer 10). Thereafter, a through hole leading to the intermediate layer 20 is formed in the piezoelectric layer 10 (not shown), and the intermediate layer 20 is partially etched by, for example, a wet etching method using an etching solution to form a hollow portion 24. According to the composite substrate according to the embodiment of the present invention, for example, the bonding strength between the piezoelectric layer 10 (intermediate layer 20) and the support substrate 30 can be excellent, so that the hollow portion 24 can be formed well. EXAMPLES

[0039] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

[0040] [Example 1] A black lithium niobate (LN) substrate having a diameter of 150 mm and a thickness of 0.5 mm, the front and back surfaces of which were mirror-polished, was prepared. In addition, a high-resistivity (>2 kΩ·cm) silicon substrate with a diameter of 150 mm and a thickness of 0.5 mm was prepared.

[0041] A 2 μm-thick Au film was formed on an LN substrate by sputtering, and the resulting Au film was then patterned by lithography (pattern width: 30 μm) to form an electrode. Next, a silicon oxide layer (first layer) having a thickness of 5 μm was formed on the pattern-formed surface of the LN substrate by sputtering (output: 4 kW) using a Si target in a carousel system. The surface of the silicon oxide layer was flattened by lapping to a depth of 2 μm and then polished by CMP to a depth of 0.5 μm, resulting in a surface waviness of 18.5 nm, as shown in Fig. 4. Here, the surface waviness is the difference between the maximum and minimum heights measured in a range of 800 μm along the orientation flat (OF) using a stylus-type profiling system (made by BRUKER, model number "DektakXT (registered trademark)") with a measuring needle having a diameter of 12.5 μm.

[0042] After cleaning the surface of the silicon substrate and the surface of the silicon oxide layer of the LN substrate, they were introduced into a plasma activation chamber to activate the surface of the silicon substrate and the surface of the silicon oxide layer of the LN substrate. Specifically, activation treatment was performed for 10 seconds at room temperature using nitrogen gas plasma (energy: 100 W). After that, these substrates were ultrasonically cleaned using pure water and spin-dried to remove particles adhering to the activated surfaces. Next, the substrates were aligned, and the activated surfaces of both substrates were overlapped at room temperature in the atmosphere to obtain a bonded body.

[0043] The resulting bonded body was then placed in an oven (130°C) in a nitrogen atmosphere and heated for 4 hours. After that, the LN substrate of the bonded body (composite substrate) was taken out of the oven and ground and lapped, and further the thickness was reduced to 1 μm by CMP processing to obtain a composite substrate.

[0044] [Example 2] A composite substrate was obtained in the same manner as in Example 1, except that the processing conditions for the planarization treatment of the silicon oxide layer on the LN substrate were changed to set the surface waviness to 5 nm.

[0045] [Example 3] A composite substrate was obtained in the same manner as in Example 1, except that the processing conditions for the planarization treatment of the silicon oxide layer on the LN substrate were changed to set the surface waviness to 50 nm.

[0046] [Comparative Example 1] A composite substrate was obtained in the same manner as in Example 1, except that the processing conditions for the planarization treatment of the silicon oxide layer on the LN substrate were changed to set the surface waviness to 2 nm.

[0047] [Comparative Example 2] A composite substrate was obtained in the same manner as in Example 1, except that the processing conditions for the planarization treatment of the silicon oxide layer on the LN substrate were changed to set the surface waviness to 80 nm.

[0048] <Evaluation> The following evaluations were carried out for the Examples and Comparative Examples. The evaluation results are summarized in Table 1. 1. Bonding Wave The silicon substrate and the LN substrate were placed on top of each other after activation, and the two substrates were partially pressed together to observe how the adhesion between the substrates spontaneously spread from the pressed area (the so-called bonding wave). 2. Check for peeling The obtained composite substrate was photographed from the LN substrate side with a digital camera, and the proportion of the area where peeling had occurred (visually determinable) was determined from the photograph obtained.

[0049] [Table 1]

[0050] In Comparative Example 1, 10% peeling was confirmed due to the processing load of thinning the LN substrate. When the cross section of the bonded body of Comparative Example 1 (before processing of thinning the LN substrate) was observed with a scanning electron microscope (SEM), it was confirmed that gaps were formed at the bonding interface (the bonding wave spread quickly and microscopically formed gaps at the bonding interface), and it is considered that this was caused by insufficient bonding strength being obtained. In Comparative Example 2, peeling was observed in the areas where the bonding wave did not spread and voids occurred due to the processing load of thinning the LN substrate.

[0051] In each example, it is believed that the predetermined waviness (unevenness) slowed down the spreading of the bonding wave and allowed sufficient microscopic adhesion, resulting in high bonding strength. [Industrial Applicability]

[0052] The composite substrate according to the embodiment of the present invention can be suitably used for an acoustic wave device. [Explanation of symbols]

[0053] 10 Piezoelectric layer 12 Piezoelectric substrate 20 Middle Class 21 First layer (middle layer) 30 Support substrate 41 electrode 100 Composite Board

Claims

1. forming a first layer on a lower surface side of a piezoelectric substrate having upper and lower surfaces opposed to each other and an electrode provided on the lower surface; A planarization process is performed to reduce the waviness of the surface of the first layer to 5 nm or more and 50 nm or less; and bonding a support substrate to a side of the piezoelectric substrate on which the first layer is formed, A method for manufacturing a composite substrate.

2. The method according to claim 1 , wherein the bonding surface of the first layer and the bonding surface on the support substrate side have been subjected to an activation treatment at the time of bonding.

3. The method according to claim 2 , wherein the activation treatment is performed by plasma irradiation.

4. The method of claim 1 , further comprising polishing the top surface of the piezoelectric substrate after the bonding.

5. The method of claim 1 , wherein the first layer comprises silicon oxide.

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