Method for manufacturing bonded body and bonding method

Abstract

Provided is a method for producing a bonded body, the method comprising: an activation step for activating the surface of each of a first substrate and a second substrate, each of which has a surface having SiO2 as the main component, by means of plasma; a bonding step in which the activated surfaces of the first substrate and the second substrate are bonded to each other at a vacuum degree of from 1 mbar to 400 mbar (inclusive); and a heating step in which the bonded first substrate and second substrate are heated. As a result, provided are a method for manufacturing a bonded body and a bonding method that can simultaneously reduce the occurrence of voids and achieve bonding strength.

Description

Technical Field

[0001] The present invention relates to a method for producing a joined body and a joining method. Background Art

[0002] To achieve high-performance semiconductor devices, SOI substrates, for example, consisting of high-resistance Si / SiO2 thin film / Si thin film, are widely used. Plasma activation is used to create SOI substrates. This method enables the substrates to be bonded at relatively low temperatures (400°C). Furthermore, composite substrates, similar to SOI substrates, consisting of Si / SiO2 thin film / piezoelectric thin film have been proposed to improve the characteristics of piezoelectric devices.

[0003] Patent Document 1 discloses a method for manufacturing a composite wafer. This method includes at least the following steps: implanting hydrogen atomic ions or hydrogen molecular ions from the surface to form an ion implantation layer within an oxide single crystal wafer; performing a surface activation treatment on at least one of the ion implanted surface of the oxide single crystal wafer and the surface of a support wafer; bonding the ion implanted surface of the oxide single crystal wafer and the surface of the support wafer to obtain a bonded structure; heat treating the bonded structure at a temperature of 90°C or higher to prevent cracking; and irradiating the heat-treated bonded structure with visible light to peel off the ion implanted layer along the ion implantation layer to obtain an oxide single crystal thin film transferred to the support wafer.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2016-225537 Summary of the Invention

[0007] In order to produce a substrate having a structure of Si substrate / SiO 2 thin film / piezoelectric thin film, for example, SiO 2 formed on the Si substrate and SiO 2 formed on the piezoelectric material are plasma activated and bonded together.

[0008] Annealing then forms covalent bonds via OH groups generated by plasma activity, improving the bond strength. However, if there is excessive moisture at the bond interface, this moisture may become voids after heating, leaving room for improvement. On the other hand, insufficient moisture at the bond interface results in insufficient bond strength.

[0009] An object of the present invention is to provide a method for producing a bonded body and a bonding method that can achieve both reduction in the generation of voids and increased bonding strength.

[0010] In order to solve the above-mentioned problems, the present invention provides a method for manufacturing a bonded body, which includes, in the following order: an activation process, in which plasma is used to activate the surfaces of a first substrate and a second substrate having a surface mainly composed of SiO2; a bonding process, in which the surfaces of the first substrate and the second substrate are bonded to each other at a vacuum degree of not less than 1 mbar and not more than 400 mbar; and a heating process, in which the bonded first substrate and second substrate are heated.

[0011] In addition, the present invention provides a bonding method, which includes, in the following order: an activation process, in which the surfaces of the first SiO2 layer and the second SiO2 layer are activated by plasma; a bonding process, in which the surfaces of the first SiO2 layer and the second SiO2 layer are bonded to each other at a vacuum degree of more than 1 mbar and less than 400 mbar; and a heating process, in which the bonded first SiO2 layer and the second SiO2 layer are heated to remove water generated on the bonding surface.

[0012] Effects of the Invention

[0013] According to the present invention, a method for producing a bonded body and a bonding method can be provided, which can achieve both reduction in the generation of voids and strong bonding. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a figure which shows the bonded body of this embodiment.

[0015] Figure 2 It is a flowchart explaining the method of manufacturing the bonded body 1 .

[0016] Figure 3 In the figure, (A) to (E) are shown. Figure 2 The diagram shows the status of each process.

[0017] Figure 4 This is a graph showing the results of Example 1.

[0018] Figure 5 In the figures, (A) to (C) are graphs showing the results of Comparative Examples 1 to 3.

[0019] Figure 6 is a graph showing the relationship between the degree of vacuum and the number of voids. DETAILED DESCRIPTION

[0020] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0021] <Description of the structure of the joint>

[0022] Figure 1 This is a diagram showing a joined body 1 according to this embodiment.

[0023] The bonded body 1 shown in the figure has a structure in which a piezoelectric layer 11 a , a dielectric layer 12 , and a supporting substrate 13 are stacked in this order from the top in the figure.

[0024] The piezoelectric layer 11a is formed of a piezoelectric material. The piezoelectric material is selected based on the intended use of the bonded structure 1. Examples of piezoelectric materials include, but are not limited to, LiNbO3 (LN) and LiTaO3 (LT). Suitable piezoelectric materials include silicon (Si), gallium arsenide (GaAs), silicon carbide (SiC), gallium nitride (GaN), zinc oxide (ZnO), and solid solution ceramics (PZT).

[0025] The dielectric layer 12 is a layer disposed below the piezoelectric layer 11a. In this embodiment, the dielectric layer 12 contains SiO2 as a main component. That is, the dielectric layer 12 can also be called a SiO2 film or a SiO2 layer.

[0026] The support substrate 13 supports the entire assembly 1. Furthermore, the support substrate 13 is bonded to the piezoelectric layer 11a via the dielectric layer 12. Any suitable substrate can be used as the support substrate 13. The support substrate 13 can be made of either a single crystal or a polycrystalline material. Furthermore, it can be made of metal.

[0027] The material constituting the support substrate 13 is preferably selected from the group consisting of silicon, sialon, sapphire, cordierite, mullite, glass, quartz, crystal, alumina, SUS, iron-nickel alloy (42 alloy), and brass. The thickness of the support substrate 13 is, for example, 0.2 to 1 mm, but any other appropriate thickness may be employed.

[0028] The silicon may be single crystal silicon, polycrystalline silicon, or high-resistance silicon. In addition, the support substrate 13 may be SOI (Silicon on Insulator).

[0029] Typically, the sialon ceramics are ceramics obtained by sintering a mixture of silicon nitride and aluminum oxide, for example, 6－w Al w O w N 8－w Specifically, the sialon ceramic has a composition in which aluminum oxide is mixed with silicon nitride, and w in the formula represents the mixing ratio of aluminum oxide. w is preferably greater than or equal to 0.5 and less than or equal to 4.0.

[0030] Typically, the sapphire is a single crystal having a composition of Al 2 O 3 , and the aluminum oxide is a polycrystalline having a composition of Al 2 O 3 . The aluminum oxide is preferably translucent aluminum oxide.

[0031] Typically, the cordierite is a ceramic having a composition of 2MgO·2Al 2 O 3 ·5SiO 2 , and the mullite is a ceramic having a composition ranging from 3Al 2 O 3 ·2SiO 2 to 2Al 2 O 3 ·SiO 2 .

[0032] <Device>

[0033] The structure of the illustrated bonded body 1 can be used as the structure of various devices, such as high-frequency devices, power semiconductors, semiconductor lasers, surface acoustic wave filters (SAW filters), and thin-film piezoelectric MEMS (Micro Electro Mechanical Systems).

[0034] <Description of the Manufacturing Method of the Joint 1>

[0035] Next, a method for producing the bonded body 1 will be described.

[0036] Figure 2 Flowchart for explaining the method of manufacturing the joint body 1. Figure 3 (A) to (E) show Figure 2 The diagram shows the status of each process.

[0037] First, a piezoelectric material substrate 11 is prepared, and a dielectric layer 12a is formed on the surface of the piezoelectric material substrate 11 (step 101). Separately, a support substrate 13 is prepared, and a dielectric layer 12b is formed on the surface of the support substrate 13 (step 102). Through steps 101 and 102, the dielectric layers 12a and 12b are formed on the surfaces of the piezoelectric material substrate 11 and the support substrate 13 (dielectric layer formation process: Figure 3 (A) in FIG. 1 ). It should be noted that the order of steps 101 and 102 can be reversed. In addition, the “surface” here refers to the main surface of the piezoelectric material substrate 11 and the support substrate 13, not the side surface.

[0038] In this embodiment, the piezoelectric material substrate 11 on which the dielectric layer 12a is formed is an example of a first substrate having a surface (surface layer) primarily composed of SiO2. Furthermore, the supporting substrate 13 on which the dielectric layer 12b is formed is an example of a second substrate having a surface (surface layer) primarily composed of SiO2. In this case, it can also be said that the first substrate is formed by depositing a SiO2 film on the piezoelectric material substrate 11, and the second substrate is formed by depositing a SiO2 film on the supporting substrate 13.

[0039] The dielectric layers 12a and 12b are mainly composed of SiO2. The dielectric layers 12a and 12b are integrated by bonding in a subsequent process to form a dielectric layer 12 mainly composed of SiO2. The dielectric layers 12a and 12b can be formed by reactive sputtering using a reactive sputtering device. Specifically, a piezoelectric material substrate 11 and a support substrate 13 are arranged in the reactive sputtering device. In addition, a target composed of silicon (Si) is arranged in the reactive sputtering device. In addition, argon (Ar) and oxygen free radicals are introduced into the reactive sputtering device. Then, a sputtering power source is used to sputter the silicon constituting the target, and a silicon film is formed on the piezoelectric material substrate 11 and the support substrate 13, which is oxidized by oxygen free radicals to form a silicon oxide (SiO2) film. Accordingly, dielectric layers 12a and 12b mainly composed of SiO2 can be formed on the surface of the piezoelectric material substrate 11 and the support substrate 13.

[0040] The dielectric layers 12a and 12b may be polished and flattened, thereby improving the bonding strength during bonding in a subsequent step.

[0041] Next, the surfaces of the dielectric layers 12a and 12b are activated by plasma (step 103: activation process). Figure 3 (B) in FIG). As the plasma, N2 plasma can be used. Thus, as Figure 3 As shown in (C), SiO2 constituting the dielectric layers 12a and 12b is activated to generate hydroxyl groups (OH groups) as hydrophilic functional groups. Therefore, this step can also be regarded as a hydrophilization step of hydrophilizing the surfaces of the dielectric layers 12a and 12b using plasma.

[0042] Furthermore, during the activation process, the plasma discharge output power directed toward the surfaces of the piezoelectric material substrate 11 and the support substrate 13 is preferably 30 to 100 W. A plasma discharge output power of 30 W or greater provides greater plasma stability, allowing for sufficient generation of hydroxyl groups, further improving bonding strength in subsequent steps. On the other hand, even if the plasma discharge output power exceeds 100 W, the reflected wave from the plasma increases, resulting in a constant degree of activation and no expectation of further improvement in bonding strength. Therefore, from an efficiency perspective, a plasma discharge output power of 100 W or less is preferably employed.

[0043] Furthermore, the surfaces of the dielectric layers 12a and 12b after the activation process are bonded to each other (step 104: bonding process) ( Figure 3 (D) in FIG. 1 ). Bonding is performed by, for example, bringing the surfaces of the dielectric layers 12 a and 12 b into contact with each other and pressing them with a predetermined pressure. Thus, the piezoelectric material substrate 11 and the support substrate 13 are bonded via the dielectric layers 12 a and 12 b.

[0044] In addition, at this time, the bonding is performed at a vacuum level of 1 mbar or more and 400 mbar or less. It should be noted that this can also be referred to as: bonding in an atmosphere of 1 mbar or more and 400 mbar or less. In this way, a bonded body 1 can be manufactured that can simultaneously achieve a reduction in the generation of voids and a high bonding strength. If the vacuum level is less than 1 mbar, the bonding strength is likely to be insufficient. On the other hand, if the vacuum level exceeds 400 mbar, voids are likely to be excessively generated.

[0045] The vacuum time during the bonding process is preferably 30 to 120 seconds. A vacuum time of 30 seconds or longer facilitates controlling the amount of OH groups using the aforementioned vacuum level, thereby ensuring optimal and sufficient bonding strength. Furthermore, a vacuum time of 120 seconds or less is preferred because it also suppresses particles from adhering to the wafer surface.

[0046] Then, the bonded piezoelectric material substrate 11 and the supporting substrate 13 are heated (step 105: heating step) ( Figure 3 (E) in the figure). For example, the bonded piezoelectric material substrate 11 and the supporting substrate 13 are placed in a heating device such as an oven and heated at a predetermined temperature and time. By heating, the hydroxyl groups generated on the surfaces of the dielectric layers 12a and 12b are covalently bonded. Then, the dielectric layers 12a and 12b are integrated to form the dielectric layer 12. As a result, the piezoelectric material substrate 11 and the supporting substrate 13 are firmly bonded with the aid of the dielectric layer 12. In addition, at this time, the reaction [Si-OH]+[OH+Si]→[Si-O-Si]+H2O occurs to generate water (H2O). Although the water is released to the outside of the dielectric layer 12, if excessive water is generated, it will remain in the dielectric layer 12 in the form of voids.

[0047] It should be noted that the heating step can also be regarded as a step of annealing the bonded piezoelectric material substrate 11 and the supporting substrate 13 (annealing step).

[0048] In addition, a step (grinding step) of grinding the heated piezoelectric material substrate 11 and the support substrate 13 may be provided. In this embodiment, the piezoelectric material substrate 11 is ground to be thinned, thereby forming Figure 1 The piezoelectric layer 11a is shown. It should be noted that the edges of the piezoelectric material substrate 11 and the support substrate 13 may be ground. The bonded body 1 can be manufactured through the above steps.

[0049] Example

[0050] (Example 1)

[0051] A 0.25 mm thick 42Y-cut black LiTaO3 (LT) substrate with both surfaces polished to mirror finishes was prepared as the piezoelectric material substrate 11. A 0.23 mm thick high-resistance (≥2 kΩ·cm) Si substrate was prepared as the support substrate 13.

[0052] Next, 0.5 μm SiO2 films were formed on the LT substrate and the Si substrate as dielectric layers 12a and 12b, respectively (dielectric layer formation step). The surfaces were polished to about 0.1 μm by CMP (Chemical Mechanical Polishing) for planarization.

[0053] The SiO2 film surfaces on the LT substrate and Si substrate were activated with N2 plasma at a discharge power of 100W (LT substrate side) and 65W (Si substrate side), respectively (activation step). The bonding process was then performed at a predetermined vacuum level (bonding step). The vacuum level in the bonding chamber was 30.2 mbar. The vacuum duration during the bonding step was 120 seconds.

[0054] To improve the bonding strength, the bonded substrates were placed in an oven at 130° C. and heated for 4 hours (heating step). The LT surface of the bonded substrates removed from the oven was thinned to 1 μm by grinding and polishing.

[0055] In this way, the bonded body 1 is manufactured, and the entire surface of the wafer is inspected using a high-resolution appearance inspection device.

[0056] The results are shown in Figure 4 .

[0057] like Figure 4 As shown, no voids were generated and the bonding strength was sufficient.

[0058] (Comparative Example 1)

[0059] The bonded body 1 was produced in the same manner as in Example 1 except that the vacuum degree during bonding of the LT substrate and the SiO2 film surface of the Si substrate was set to 1013 mbar (1 atmosphere).

[0060] The results are shown in Figure 5 (A) in the.

[0061] like Figure 5 As shown in (A), it is clear that excess water forms voids V near the periphery after heating. The bonding strength is sufficient. This is believed to be because excessive vacuum levels result in excess water remaining at the bonding interface.

[0062] (Comparative Example 2)

[0063] The bonded body 1 was produced in the same manner as in Example 1 except that the vacuum degree during bonding of the LT substrate and the SiO 2 film surface of the Si substrate was set to 0.16 mbar.

[0064] The results are shown in Figure 5 (B) in.

[0065] like Figure 5 As shown in (B), insufficient moisture at the bonding interface resulted in delamination H due to insufficient bonding strength. This is believed to be due to insufficient vacuum, resulting in insufficient residual moisture at the bonding interface and insufficient OH groups for covalent bonding. Furthermore, Comparative Example 2 also produced voids throughout the wafer.

[0066] (Comparative Example 3)

[0067] The bonded body 1 was produced in the same manner as in Example 1 except that the vacuum degree during bonding of the LT substrate and the SiO 2 film surface of the Si substrate was set to 0.0001 mbar.

[0068] The results are shown in Figure 5 (C) in.

[0069] like Figure 5 As shown in (C), the moisture content at the bonding interface is very low, resulting in significantly lower bonding strength and a significant amount of delamination H. This is believed to be because the vacuum level is further reduced compared to Comparative Example 2, resulting in a lower amount of moisture remaining at the bonding interface and an even lower number of OH groups for covalent bonding. Furthermore, Comparative Example 3 also produces voids throughout the wafer.

[0070] As described above, it is understood that the number of voids and the bonding strength depend on the degree of vacuum in the bonding process.

[0071] Furthermore, the conditions for activating the SiO 2 film surfaces of the LT substrate and the Si substrate with N 2 plasma, as well as the vacuum time and vacuum degree during bonding were further changed to produce a bonded body 1 .

[0072] Table 1 below shows the modified conditions. These include changes to the plasma discharge output power on the LT substrate side during activation (Plasma Power (upper)), the plasma discharge output power on the Si substrate side (Plasma Power (lower)), the vacuum time during bonding, and the vacuum level. Furthermore, the number of voids was calculated for each condition. Note that the number of voids is the number of voids per wafer with a diameter of 150 mm.

[0073] If there are too many voids, the device is considered unqualified. Here, a device with 500 or fewer voids is considered acceptable, and a device with more than 500 voids is considered unqualified. The number of voids is shown in Table 1 below as the void count.

[0074] [Table 1]

[0075]

[0076] In Table 1, No. 1 to 2, 4 to 9, 11 to 12, 14 to 15 passed, and the others failed. The above results show that the number of voids depends mainly on the degree of vacuum.

[0077] Figure 6 is a graph showing the relationship between the degree of vacuum and the number of voids.

[0078] Figure 6 In FIG. 1 , the horizontal axis represents the vacuum degree, and the vertical axis represents the number of voids. Furthermore, it is shown that for a given vacuum degree, the number of voids mainly falls within the region enclosed by the dotted line.

[0079] As shown in the figure, when the vacuum level during bonding is between 1 mbar and 400 mbar, the number of voids falls below 500. On the other hand, when the vacuum level is less than 1 mbar or exceeds 400 mbar, the number of voids easily exceeds 500. In other words, bonding under this vacuum level can optimize the moisture content at the bonding interface. By setting this vacuum level during bonding, the amount of moisture adsorbed at the bonding interface is reduced, suppressing voids after heating. On the other hand, the vacuum level is set to maintain the moisture required to ensure bonding strength.

[0080] Based on the above results, the degree of vacuum in the bonding step is preferably determined based on the bonding strength between the LT substrate and the Si substrate after the heating step and the extent of voids generated at the bonding surface.

[0081] In the activation step, it is preferable to determine the discharge output of the plasma on the surfaces of the first substrate and the second substrate based on the bonding strength and the degree of the gap after the heating step.

[0082] It should be noted that the above process is described in the form of a method for manufacturing the bonded body 1. However, it can also be regarded as a method for bonding two SiO2 layers. That is, it can also be regarded as a bonding method, which includes, in the following order: an activation process, in which the surfaces of the first SiO2 layer (in the above example, the dielectric layer 12a) and the second SiO2 layer (in the above example, the dielectric layer 12b) are activated by plasma; a bonding process, in which the surfaces of the first SiO2 layer and the second SiO2 layer are bonded to each other at a vacuum degree of more than 1 mbar and less than 400 mbar; and a heating process, in which the bonded first SiO2 layer and the second SiO2 layer are heated to remove water generated on the bonding surface.

[0083] Although the present embodiment has been described above, the technical scope of the present invention is not limited to the scope described in the above embodiment. As can be seen from the description of the claims, various changes or improvements to the above embodiment are also included in the technical scope of the present invention.

[0084] Explanation of symbols

[0085] 1...bonded body, 11...piezoelectric material substrate, 11a...piezoelectric layer, 12, 12a, 12b...dielectric layer, 13...support substrate, V...gap, H...peeling.

Claims

1. A method for manufacturing a bonded body, comprising: an activation step in which the surfaces of the first substrate and the second substrate are activated by plasma, the first substrate and the second substrate having surfaces mainly composed of SiO2; a bonding step of bonding the surfaces of the first substrate and the second substrate to each other at a vacuum level of not less than 1 mbar and not more than 400 mbar; as well as A heating step in which the bonded first substrate and second substrate are heated.

2. The method for producing a bonded body according to claim 1, wherein: The vacuum time of the bonding process is 30 to 120 seconds.

3. The method for producing a bonded body according to claim 2, wherein: In the activation step, discharge output power of the plasma on the surfaces of each of the first substrate and the second substrate is 30 to 100 W.

4. The method for producing a joined body according to any one of claims 1 to 3, wherein: The method further includes a grinding step in which the heated first substrate is ground.

5. The method for producing a joined body according to any one of claims 1 to 3, wherein: The first substrate is formed by depositing a SiO 2 film on a piezoelectric material substrate, and the second substrate is formed by depositing a SiO 2 film on a supporting substrate.

6. A joining method comprising: an activation step in which the surfaces of the first SiO2 layer and the second SiO2 layer are activated using plasma; a bonding step of bonding the surfaces of the first SiO 2 layer and the second SiO 2 layer to each other at a vacuum level of not less than 1 mbar and not more than 400 mbar; as well as A heating step in which the first SiO 2 layer and the second SiO 2 layer that have been bonded are heated to remove water generated on the bonding surface.

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