Composite wafer and method for producing the same

Abstract

To provide a composite wafer in which an LT film or an LN film having a high thermal expansion coefficient is laminated on a support substrate having a low thermal expansion coefficient, and which can reduce spurious caused by reflection of an incident signal on a bonding interface between the LT film or the like and the support substrate, and to provide a method of manufacturing the same.SOLUTION: A method for producing a composite wafer according to an embodiment of the present invention includes a step of performing ion implantation into a single crystal wafer serving as a support substrate, a step of performing surface activation treatment on one or both of a piezoelectric wafer of lithium tantalate or lithium niobate and the single crystal support substrate, and a step of thinning the piezoelectric wafer after bonding.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a composite substrate in which an oxide single crystal thin film is bonded to a support wafer, and a method for producing the same. [Background technology]

[0002] In recent years, the mobile communications market, typified by smartphones, has seen a rapid increase in communication traffic. To address this issue, the number of bands required has increased, inevitably necessitating the miniaturization and performance improvement of various components. Common piezoelectric materials, lithium tantalate (also abbreviated as LT) and lithium niobate (also abbreviated as LN), are widely used as materials for surface acoustic wave (SAW) devices. However, while these materials have a large electromechanical coupling coefficient and can achieve wide bandwidths, they suffer from poor temperature stability, resulting in a shift in the frequency range they can support due to temperature changes. This is due to the extremely high thermal expansion coefficients of lithium tantalate and lithium niobate.

[0003] To alleviate this problem, a method has been proposed in which a material with a low thermal expansion coefficient is bonded to the piezoelectric material, lithium tantalate (LiTaO3:LT) or lithium niobate (LiNbO3:LN), and the surface of the piezoelectric material that is not bonded to the low thermal expansion coefficient material is thinned to a few micrometers to a few tens of micrometers by grinding or other methods (Non-Patent Document 1). In this method, bonding a low thermal expansion coefficient material (sapphire, silicon, etc.) suppresses the thermal expansion of LT or LN, improving its temperature characteristics. Figure 1 shows a graph of the thermal expansion coefficients of various materials.

[0004] However, this method presents another problem: when a thin LT or LN film is laminated onto a support substrate, noise called spurious or ripple occurs in the anti-resonance frequency band. This noise is generated by reflection from the interface between the LT or LN film and the support substrate. Figure 2 shows the return loss (S11) of a resonator created on an LT film laminated onto a silicon substrate. Figure 2 shows that the spurious waveform alternates between peaks and valleys as the frequency changes. The difference between the peaks and valleys of the spurious waveform is called the spurious amplitude. Empirically, it can be determined that if the magnitude of this spurious is 1 dB or less, there is almost no adverse effect.

[0005] Several methods have been proposed to solve this problem of spurious signals (see, for example, Non-Patent Document 2). Non-Patent Document 2 proposes a method in which the bonding surface of the LT is roughened with a 1000-grit grinding stone to achieve a roughness of 300 nm Ra, and then the LT is bonded to the support substrate via an adhesive. However, in actual devices, it is difficult to use an adhesive from the standpoint of reliability. Furthermore, with the method of depositing and polishing an inorganic material such as SiO2 instead of an adhesive, it is extremely difficult and expensive to achieve atomic-level smoothness (Ra of 1 nm or less) that can withstand bonding. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] Taiyo Yuden Co., Ltd., "Temperature Compensation Technology for SAW-Duplexers Used in the RF Front-End of Smartphones," Dempa Shimbun High Technology, November 2012 [Non-patent document 2] H.Kobayashi et al., "A study on Temperature-Compensated Hybrid Substrates for Surface Acoustic Wave Filters", IEEE International Ultrasonics Symposium, 2010, Vol.1, p.637-640 Summary of the Invention [Problem to be solved by the invention]

[0007] The object of the present invention is to provide a composite wafer in which an LT film or an LN film with a high thermal expansion coefficient is laminated on a support substrate with a low thermal expansion coefficient, and to provide a method for manufacturing the same that can reduce spurious signals that occur when an incident signal is reflected at the bonding interface between the LT film or the like and the support substrate. [Means for solving the problem]

[0008] In order to solve the above problems, the present inventors have devised the following method: Namely, ions are implanted into a single crystal silicon wafer serving as a support substrate to leave ion species near the surface, and then a lithium tantalate or lithium niobate wafer and the single crystal silicon wafer are bonded together.

[0009] This allows the signal incident from the piezoelectric body to be absorbed and scattered near the interface, reducing spurious emissions in the anti-resonance frequency band. There are no particular restrictions on the type of ions to be implanted, but light elements such as hydrogen or helium are preferable because they can be implanted deeply. These atoms can be implanted with a low acceleration voltage, so they are less subject to equipment restrictions.

[0010] When implanting ions into the support substrate, it is important not to significantly disrupt the crystallinity of the support substrate. Specifically, it is preferable to perform ion implantation under conditions that maintain the single crystallinity of the support substrate (i.e., unchanged from before ion implantation). While it is possible to further reduce spurious signals by increasing the ion implantation dose, this increases high-frequency dielectric loss, resulting in a degradation of the Q value (sharpness of the signal peak) of devices (resonators) fabricated using the composite wafer. It is important to reduce spurious signals without degrading the Q value. Furthermore, it is preferable to set the ion implantation interface (the depth at which the distribution of implanted ions peaks) within 0.15 μm (150 nm) of the surface of the support substrate. This allows the implanted ions to remain very close to the surface, which is expected to result in a greater spurious reduction effect.

[0011] Note that "having single crystallinity" is defined above as a diffraction spot pattern that clearly shows the crystallinity characteristic of a single crystal in the electron beam diffraction image of the ion-implanted area. For reference, Figure 3 shows the diffraction patterns of single crystal Si (Figure 3(a)) and polycrystalline Si (Figure 3(b)) excerpted from the website of the Materials Science and Technology Foundation, as well as the diffraction pattern of amorphous Si (Figure 3(c)) obtained by the inventors.

[0012] In order to solve the above problems, a method for manufacturing a composite wafer according to an embodiment of the present invention includes the steps of: implanting ions into a single crystal wafer that serves as a support substrate; performing a surface activation process on one or both of a lithium tantalate or lithium niobate piezoelectric wafer and the single crystal wafer; and thinning the piezoelectric wafer after bonding.

[0013] The present invention may further include a step of performing a heat treatment after the bonding.

[0014] In the present invention, it is preferable to have an intervening layer between the piezoelectric wafer and the single crystal wafer. The intervening layer may be made of SiO, SiON, or SiN, or a combination of these. In the step of implanting ions into the single crystal wafer, an insulating film may be formed on the bonding surface of the single crystal wafer, and ions may be implanted through the insulating film, which may then be used as the intervening layer without being removed.

[0015] In the present invention, it is preferable that the single crystallinity of the single crystal wafer is not destroyed in the step of implanting ions into the single crystal wafer.

[0016] In the present invention, when the ion species to be implanted in the step of implanting ions into the single crystal wafer are hydrogen molecular ions, the dose is 1.0 to 2.5 × 10 16 / cm 2 In addition, when the ion species to be implanted in the step of implanting ions into the single crystal wafer are hydrogen ions, and the dose amount is 2.0 to 5.0 × 10 16 atoms / cm 2 In addition, when the ion species to be implanted in the step of implanting ions into the single crystal wafer are helium ions, and the dose amount is 1.0 to 2.5 × 10 16 atoms / cm 2 It is recommended to set the range as follows.

[0017] In the present invention, it is preferable that the ion implantation depth peak is in the range of 50 to 150 nm from the surface of the single crystal wafer.

[0018] In the present invention, the surface activation treatment carried out in the step of carrying out the surface activation treatment may be any one of ozone water treatment, UV ozone treatment, ion beam treatment, and plasma treatment.

[0019] In the present invention, the single crystal wafer may be silicon, thermally oxidized silicon, or sapphire.

[0020] In the present invention, in the step of thinning the piezoelectric wafer, the piezoelectric wafer may be thinned by grinding and polishing. Alternatively, the piezoelectric wafer may be thinned by an ion implantation delamination method.

[0021] A composite wafer according to an embodiment of the present invention includes a single crystal wafer serving as a support substrate, and a piezoelectric wafer of lithium tantalate or lithium niobate bonded to the single crystal wafer, wherein the single crystal wafer has an ion-implanted layer.

[0022] In the present invention, it is preferable to have an intervening layer between the piezoelectric wafer and the single crystal wafer, and the intervening layer may be made of SiO2, SiON, or SiN, or a combination thereof.

[0023] In the present invention, the ion-implanted layer preferably has an ion implantation depth peak in the range of 50 to 150 nm from the surface of the single crystal wafer.

[0024] In the present invention, it is preferable that the single crystal wafer maintains its single crystallinity.

[0025] In the present invention, the single crystal wafer may be silicon, thermally oxidized silicon, or sapphire. [Effects of the Invention]

[0026] According to the present invention, it is possible to reduce spurious signals that occur when an incident signal is reflected at the bonding interface between the LT film or the like and the support substrate in a composite wafer. [Brief explanation of the drawings]

[0027] [Figure 1] FIG. 1 is a diagram showing the thermal expansion coefficients of various materials. [Figure 2] FIG. 10 is a diagram showing an example of the return loss of a resonator fabricated in an LT film laminated on a silicon substrate. [Figure 3]Examples of diffraction spot patterns in electron beam diffraction images are shown in Figure 3(a) for single crystal Si, Figure 3(b) for polycrystalline Si, and Figure 3(c) for amorphous Si. [Figure 4] FIG. 2 is a schematic diagram showing the configuration of a composite wafer. [Figure 5] FIG. 2 is a diagram showing an example of a specific manufacturing flow of a method for manufacturing a composite wafer. [Figure 6] 10 is a graph showing spurious intensity at each dose. [Figure 7] 10 is a graph showing the Q value at each dose. [Figure 8] The cross-sectional TEM image is shown when the dose is 2.0×1016 / cm2. [Figure 9] Electron beam diffraction patterns when the dose is 2.0×10 16 / cm 2 and 4.0×10 16 / cm 2 are shown. DETAILED DESCRIPTION OF THE INVENTION

[0028] In the present invention, a composite wafer 1 is manufactured by bonding a piezoelectric wafer 2 made of lithium tantalate or lithium niobate to a single crystal wafer 3 that serves as a support substrate. The single crystal wafer 3 is preferably made of a material with a smaller thermal expansion coefficient than the piezoelectric wafer, specifically silicon, thermally oxidized silicon, sapphire, or the like. Using these materials as the support substrate effectively suppresses thermal expansion, improving the temperature characteristics of the SAW device formed on the piezoelectric wafer 2.

[0029] Prior to bonding, ions are implanted into the bonding surface of the single crystal wafer 3 to form an ion-implanted layer 31 as shown in Fig. 4. As a result, at the interface where the piezoelectric wafer 2 and the single crystal wafer 3 are bonded, the signal incident from the piezoelectric wafer is absorbed and scattered near the interface, suppressing reflection and reducing spurious signals.

[0030] The ion species to be implanted are hydrogen ions (H +), hydrogen molecular ion (H2 + ), and helium ions (He + When the ion species to be implanted are hydrogen molecular ions, the dose is preferably 1.0 to 2.5 × 10 16 / cm 2 When the ion species to be implanted is hydrogen ions, the dose is set to 2.0 to 5.0 × 10 16 atoms / cm 2 When the ion species to be implanted is helium ions, the dose is set to 1.0 to 2.5 × 10 16 atoms / cm 2 It is recommended to set the range as follows.

[0031] Ion implantation is preferably performed by forming an insulating film 4 on the bonding surface of the single crystal wafer and then implanting ions through the insulating film. This makes it possible to suppress channeling of the implanted ions. Suitable materials for the insulating film 4 include, for example, SiO2, SiON, and SiN. Ion implantation is preferably performed under conditions that do not destroy the crystallinity of the single crystal wafer 3 (i.e., conditions under which an electron beam diffraction image of the ion-implanted area can be clearly observed even after implantation). The ion implantation depth peak (the depth at which the distribution of implanted ions peaks) is preferably set in the range of 50 to 150 nm from the surface of the single crystal wafer 3 (the interface between the single crystal wafer 3 and the insulating film 4).

[0032] Furthermore, an intermediate layer may be provided on the bonding surfaces of the piezoelectric wafer 2 and / or the single crystal wafer 3 prior to bonding. The intermediate layer is preferably made of SiO2, SiON, or SiN, or a combination thereof. As shown in Figure 4, the insulating film 4 formed on the bonding surface of the single crystal wafer during ion implantation may be left as it is and used as the intermediate layer.

[0033] FIG. 5 shows an example of a specific manufacturing flow of the method for manufacturing a composite wafer of the present invention.

[0034] First, an insulating film 4 is formed on the bonding surface of the single crystal wafer 3 into which ions are to be implanted (S1). Next, ions are implanted into the bonding surface of the single crystal wafer 3 on which the insulating film 4 is formed, forming an ion-implanted layer 31 (S2). Next, the insulating film is removed as necessary (S3), and the bonding surface of the ion-implanted wafer is subjected to a surface activation process (S4). By performing the surface activation process, atoms on the bonding surface are activated to facilitate chemical bond formation, thereby achieving stronger bonding. The surface activation process can be performed by, for example, plasma activation, vacuum ion beam activation, activation by ozone water treatment, or activation by UV ozone treatment. Step S3 may be omitted, and the insulating film 4 may be left as an intermediate layer.

[0035] Next, the wafers are bonded together at their bonding surfaces (S5) and heat treated (S6).The piezoelectric wafers are then thinned to the required thickness by grinding and polishing (S7), and SAW devices such as resonators are formed (S8).

[0036] In this way, the composite wafer 1 according to the embodiment of the present invention can be produced. In the examples described below, an experiment conducted to clarify the conditions under which the composite wafer 1 can achieve the spurious reduction effect that is the object of the present invention will be described. [Example]

[0037] [Example 1] A single crystal silicon wafer with a diameter of 100 mm and a thickness of 0.55 mm was prepared, and a thermal oxide film (SiO2) of approximately 480 nm was grown at a temperature of 1000°C. The surface roughness of both the lithium tantalate wafer (LT) and the silicon wafer serving as the support substrate was confirmed to be 1.0 nm or less in RMS. The silicon wafer was previously doped with hydrogen molecular ions (H2 + ) at an energy of 92 KeV and a dose of 0.1 to 5 × 10 16 / cm 2 The implantation depth was set to about 100 nm from the SiO2 / Si interface.

[0038] These wafers were subjected to plasma activation treatment to activate the surfaces before being bonded together. After bonding, they were heat treated at 120°C for 6 hours, and then the LT wafers were ground and polished to thin them down to 11 μm to form a composite wafer. A resonator was fabricated on this composite wafer, and the levels of spurious intensity and Q value were compared. The results are shown in Figures 6 and 7. Figure 6 is a graph showing the spurious intensity at each dose. Figure 7 is a graph showing the Q value at each dose. As the ion implantation dose increased, the spurious intensity decreased, but at the same time, a deterioration in the Q value was also observed. Both spurious reduction and maintenance of the Q value were achieved when the dose was 1.0 to 2.5 × 10 16 / cm 2 In addition, cross-sectional TEM (transmission electron microscope) observations were performed at each dose level, and electron diffraction images were obtained at the ion-implanted interface. The results are shown in Table 1.

[0039] [Table 1]

[0040] For example, if the dose is 2.0 × 10 16 / cm 2 The cross-sectional TEM image at 4.0 × 10 is shown in Fig. 8. The electron diffraction pattern at the same dose (Fig. 9(a)) and the 16 / cm 2 The electron diffraction pattern (Fig. 9(b)) is shown. 16 / cm 2 When the diffraction pattern is 4.0 × 10 16 / cm 2 It can be seen that when the diffraction pattern is

[0041] [Example 2] The ion species to be implanted is hydrogen atomic ions (H + ) was used to carry out an experiment similar to that in Example 1. The results were the same as in Example 1, except that the acceleration voltage was half and the dose was doubled compared to that in Example 1.

[0042] [Example 3] Similar tests were conducted by changing the surface activation method in Example 1 to vacuum ion beam activation, activation by ozone water treatment, and activation by UV ozone treatment. The results were the same as in Example 1 within the margin of error, and it was found that the results were not affected by the surface activation method.

[0043] [Example 4] Dose amount 2.0×10 16 / cm 2 An experiment similar to that in Example 1 was carried out by fixing the ion implantation depth at 50 nm to 150 nm, and varying only the accelerating voltage. Other conditions were the same as in Example 1. The results are shown in Table 2. It can be seen that a reduction in spurious intensity and a high Q value can both be achieved when the ion implantation depth (depth from the interface between SiO2 and Si) is between 50 nm and 150 nm.

[0044] [Table 2]

[0045] [Example 5] After ion implantation, the oxide film on the silicon was removed and an experiment similar to that in Example 1 was performed. The results were the same as in Example 1 within the margin of error, and it was found that the results were not affected by the presence or absence of the oxide film.

[0046] [Example 6] Instead of a thermal oxide film, an SiO2 film, a SiN film, and an SiON film formed by CVD were interposed, and after ion implantation, the two films were left as they were and bonded together, and an experiment similar to that in Example 1 was conducted. No significant difference was observed in the results, so it is thought that the intervening film has almost no effect.

[0047] [Example 7] An experiment similar to that in Example 1 was carried out using a sapphire wafer without an oxide film, etc., instead of a silicon wafer. Although the spurious reduction effect was less than that in the case of silicon, the effect was still confirmed.

[0048] [Example 8] Ion implantation was performed using helium ions instead of hydrogen ions, and an experiment similar to that in Example 1 was carried out. The results were the same as those in Example 1 within the margin of error. Therefore, it was found that the results are independent of the ion species as long as ion implantation is possible.

[0049] [Example 9] The LT wafer is pre-doped with hydrogen ions (H + ) at an acceleration voltage of 160 keV and a dose of 9.0 atoms / cm 2 The composite wafer was then bonded to a silicon substrate with an oxide film that had been subjected to the same ion implantation as in Example 1. After heat treatment at 110°C for 24 hours, it was peeled off, polished, and then heat treated at 500°C. The thickness of the LT film remaining on the composite wafer after peeling and polishing was 750 nm. As a result, the spurious response (however, the spurious response intensity was lower compared to Example 1) and the Q value tendencies were the same as in Example 1. It is believed that this method is independent of the thinning method of the piezoelectric wafer.

[0050] The experiments conducted in each of the above examples clarified the conditions under which the composite wafer 1 can achieve the spurious reduction effect aimed at by the present invention (more preferably, the conditions under which both the reduction of spurious intensity and the maintenance of a high Q value can be achieved).

[0051] The present invention is not limited to the above-described embodiments. The above-described embodiments are merely examples, and anything that has substantially the same configuration as the technical idea described in the claims of the present invention and exhibits similar effects is included within the technical scope of the present invention. In other words, appropriate modifications are possible within the scope of the technical idea expressed in the present invention, and forms incorporating such modifications and improvements are also included within the technical scope of the present invention. [Explanation of symbols]

[0052] 1. Composite wafer 2. Piezoelectric wafer 3. Single crystal wafers 4. Insulating film

Claims

1. A step of implanting ions into a single crystal wafer that serves as a support substrate; a step of performing a surface activation treatment on one or both of the lithium tantalate or lithium niobate piezoelectric wafer and the single crystal wafer; a step of thinning the piezoelectric wafer after bonding; A method for manufacturing a composite wafer comprising:

2. 2. The method for producing a composite wafer according to claim 1, further comprising the step of performing a heat treatment after the bonding.

3. 2. The method for producing a composite wafer according to claim 1, further comprising the step of: providing an intervening layer between the piezoelectric wafer and the single crystal wafer.

4. The intermediate layer is SiO ２ 4. The method for producing a composite wafer according to claim 3, wherein the material is any one of Si, SiON, and SiN, or a combination thereof.

5. 5. The method for producing a composite wafer according to claim 4, wherein in the step of implanting ions into the single crystal wafer, an insulating film is formed on the bonding surface of the single crystal wafer, and then ions are implanted through the insulating film, and the insulating film is used as the intermediate layer without being removed.

6. 2. The method for producing a composite wafer according to claim 1, wherein the single crystallinity of the single crystal wafer is not destroyed in the step of implanting ions into the single crystal wafer.

7. In the step of implanting ions into the single crystal wafer, the ion species implanted are hydrogen molecular ions, and the dose is 1.0 to 2.5×10 １６ / cm ２ 2. The method for producing a composite wafer according to claim 1, wherein the range is:

8. In the step of implanting ions into the single crystal wafer, the ion species implanted are hydrogen ions, and the dose amount is 2.0 to 5.0×10 １６ atoms / cm ２ 2. The method for producing a composite wafer according to claim 1, wherein the range is:

9. In the step of implanting ions into the single crystal wafer, the ion species implanted are helium ions, and the dose amount is 1.0 to 2.5×10 １６ atoms / cm ２ 2. The method for producing a composite wafer according to claim 1, wherein the range is:

10. 2. The method for producing a composite wafer according to claim 1, wherein the ion implantation depth peak is in the range of 50 to 150 nm from the surface of the single crystal wafer.

11. 2. The method for manufacturing a composite wafer according to claim 1, wherein the surface activation treatment carried out in the step of carrying out the surface activation treatment is any one of ozone water treatment, UV ozone treatment, ion beam treatment, and plasma treatment.

12. 2. The method for producing a composite wafer according to claim 1, wherein the single crystal wafer is silicon, thermally oxidized silicon, or sapphire.

13. 2. The method for producing a composite wafer according to claim 1, wherein in the step of thinning the piezoelectric wafer, the piezoelectric wafer is thinned by grinding and polishing.

14. 2. The method for producing a composite wafer according to claim 1, wherein in the step of thinning the piezoelectric wafer, the piezoelectric wafer is thinned by an ion implantation delamination method.

15. a single crystal wafer serving as a support substrate; a piezoelectric wafer of lithium tantalate or lithium niobate bonded to the single crystal wafer; A composite wafer comprising: The composite wafer is characterized in that the single crystal wafer has an ion-implanted layer.

16. 16. The composite wafer according to claim 15, further comprising an intervening layer between the piezoelectric wafer and the single crystal wafer.

17. The intermediate layer is SiO ２ 17. The composite wafer of claim 16, wherein the composite wafer is one of or a combination of SiN, SiON, and SiN.

18. 16. The composite wafer according to claim 15, wherein the ion-implanted layer has an ion implantation depth peak in the range of 50 to 150 nm from the surface of the single crystal wafer.

19. 16. The composite wafer of claim 15, wherein the single crystal wafer maintains its single crystallinity.

20. 16. The composite wafer of claim 15, wherein the single crystal wafer is silicon, thermally oxidized silicon, or sapphire.

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