JP2020043591A - Composite wafer manufacturing method - Google Patents

Abstract

To provide a composite wafer manufacturing method arranged so that in a composite wafer, a lithium tantalate film with a high thermal expansion coefficient is laminated on a support substrate with a low thermal expansion coefficient, which can reduce the spurious produced by the reflection of incident signals at a junction interface of the lithium tantalate film and the support substrate.SOLUTION: A method for manufacturing a composite wafer comprises the steps of: bonding a lithium tantalate wafer of a high thermal expansion coefficient to a support wafer of a small thermal expansion coefficient; and injecting ions from a bonding face of each or one of the lithium tantalate wafer and the support wafer, thereby disturbing the crystallinity in the vicinities of the bonding faces of the respective wafers before the bonding.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for manufacturing a composite wafer used as a material for a surface acoustic wave device.

  2. Description of the Related Art In recent years, in a mobile communication market represented by a smartphone, a communication amount is rapidly increasing. As the number of bands required to cope with this problem increases, it is inevitably necessary to reduce the size and performance of various components. Common piezoelectric materials such as lithium tantalate (LT) and lithium niobate (LN) are widely used as materials for surface acoustic wave (SAW) devices. However, these materials have a large electromechanical coupling coefficient and can broaden the band, but have a problem that the temperature stability is low and the corresponding frequency shifts due to a temperature change. This is because lithium tantalate and lithium niobate have very high thermal expansion coefficients.

In order to reduce this problem, a material having a low thermal expansion coefficient is attached to lithium tantalate (LiTaO ₃ : LT) or lithium niobate (LiNbO ₃ : LN) which is a piezoelectric material, and a low thermal expansion coefficient material of the piezoelectric material is used. A method has been proposed in which the surface to be not bonded is thinned to several μm to several tens μm by grinding or the like (Non-Patent Document 1). In this method, the thermal expansion of LT or LN is suppressed by bonding a low thermal expansion coefficient material (sapphire, silicon, or the like), and the temperature characteristics are improved. FIG. 12 shows a graph of the thermal expansion coefficients of various materials.

  However, according to this method, another problem that noise called spurious or ripple is generated in the anti-resonance frequency band by laminating a thin LT film or LN film on the support substrate. This noise is generated by reflection from the interface between the LT film or LN film and the support substrate. FIG. 13 shows the return loss (S11) of the resonator formed on the LT film laminated on the silicon substrate. FIG. 13 shows that the spurious waveform repeats peaks and valleys in accordance with the change in frequency. The difference between the peaks and valleys of the spurious waveform is called spurious intensity (amplitude).

Several methods are proposed in reference 2 to solve this problem. For example, _a method has been proposed in which the bonding surface of the LT is roughened with _a No. 1000 grinding stone to obtain a roughness of 300 nm in _Ra value, and then bonded to a support substrate via an adhesive. However, since it is difficult to use an adhesive from the viewpoint of reliability in an actual device, a method of depositing and polishing an inorganic material, for example, SiO ₂ instead of the adhesive has been further proposed. However, it is difficult to process unevenness to an atomic level smoothness ( _Ra value of 1 nm or less) that can withstand bonding, and there is a problem in cost.

Taiyo Yuden Co., Ltd., "Temperature Compensation Technology of SAW-Duplexer Used for Smartphone RF Front End", Denpa Shimbun High Technology, November 2012 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

  An object of the present invention is to provide a composite wafer in which an LT film or an LN film having a high coefficient of thermal expansion is laminated on a supporting substrate having a low coefficient of thermal expansion, and an incident signal is reflected at a bonding interface between the LT film or the like and the supporting substrate. It is an object of the present invention to provide a method of manufacturing a composite wafer capable of reducing spurious generated by the above.

  (1) The method of manufacturing a composite wafer according to the present invention comprises bonding a lithium tantalate wafer or a lithium niobate wafer (hereinafter, referred to as a “laminated wafer”) to a supporting wafer having a smaller coefficient of thermal expansion than the composite wafer. Prior to bonding, implanting ions from the bonded surfaces of the laminated wafer and / or the supporting wafer, and disturbing the crystallinity in the vicinity of each bonded surface. It is characterized by executing. Thus, at the interface where the laminated wafer and the supporting wafer are bonded, the signal incident from the laminated wafer, which is a piezoelectric material, is absorbed and scattered in the vicinity of the interface and the reflection is suppressed, so that spurious can be reduced.

(2) The ions to be implanted into each wafer are hydrogen ions (H ⁺ ), hydrogen molecule ions (H ₂ ⁺ ), or helium ions (He ⁺ ), and the dose in each case is 1.0 × 10 ¹⁶ atoms / cm ² or more and 1.0 × 10 ¹⁷ atoms / cm ² or less, 5.0 × 10 ¹⁵ atoms / cm ² or more and 5.0 × 10 ¹⁶ atoms / cm ² or less, 1.0 × 10 ¹⁶ atoms / cm ² or more and 1.0 × 10 ¹⁷ atoms / cm ² or less It may be. Since these light element ions can be deeply implanted into the wafer with a small acceleration voltage, they are not easily restricted by the implantation apparatus. In addition, by controlling the dose in this manner, the reflection suppressing effect can be enhanced, and the substrate can be prevented from being damaged when heat treatment is performed after bonding.

  (3) After the execution of the ion implantation step, prior to the bonding, the surface on which the surface to be bonded of the laminated wafer and / or the supporting wafer is subjected to the surface activation treatment by ozone water treatment, UV ozone treatment, ion beam treatment, or plasma treatment. An activation step may be performed. As a result, the atoms on the bonding surface of each wafer are brought into an active state in which a chemical bond is easily formed, and a stronger bond can be obtained.

  (4) Silicon or sapphire may be applied as the material of the supporting wafer. Since these materials have a small coefficient of thermal expansion, the thermal expansion of a laminated wafer having a large coefficient of thermal expansion can be effectively suppressed, and the temperature characteristics of the device can be improved.

(5) Prior to the ion implantation step, an insulating film forming step of forming an insulating film of SiO ₂ , SiON, or SiN on the bonding surface of the laminated wafer and / or the supporting wafer may be performed. By forming an insulating film and implanting ions therethrough, channeling of implanted ions can be suppressed.

  (6) As the laminated wafer, a wafer having a higher lithium concentration as approaching the bonding surface in the thickness direction may be used. By using a laminated wafer having such a concentration distribution, for example, when a resonator is formed on the wafer, Dip appearing in the input impedance waveform can be reduced.

It is a figure showing an outline of a manufacturing method of a compound wafer of the present invention. It is a figure showing an example of a manufacturing flow of a compound wafer of the present invention. FIG. 3 is a diagram illustrating an example of a concentration distribution of hydrogen atoms in a silicon wafer in a depth direction from a bonding surface. FIG. 4 is a diagram illustrating a comparison of spurious intensity between a case where ions are not implanted into a silicon wafer and a case where ions are implanted. FIG. 3 is a diagram illustrating a relationship between an ion dose amount to a silicon wafer and spurious intensity. FIG. 6 is a diagram showing a comparison of spurious intensity between a case where ions are not implanted into a sapphire wafer and a case where ions are implanted. It is a figure which shows the comparison of the spurious intensity of the case where ions are not implanted into the silicon wafer and the LT wafer and the case where ions are implanted into both. It is a figure which shows an example of the thickness direction profile of the Li density | concentration of an LT wafer. It is a figure which shows the comparison of the input impedance waveform in the resonator produced in each of the composite wafer produced using the LT wafer which does not have the density of the Li density | concentration in the thickness direction, and the composite wafer produced using the LT wafer which has the density. FIG. 11 is another diagram showing a comparison of input impedance waveforms in resonators created in each of a composite wafer created using an LT wafer having no density shading in the thickness direction and a composite wafer created using an LT wafer having shading. is there. It is a figure which shows the comparison of the Q value in the resonator produced in each of the composite wafer produced using the LT wafer which does not have the density of Li density in the thickness direction, and the composite wafer produced using the LT wafer which has the density. It is a figure showing the coefficient of thermal expansion of various materials. FIG. 9 is a diagram illustrating an example of the return loss of a resonator formed on an LT film stacked on a silicon substrate.

  In the present invention, when a composite wafer is manufactured by bonding a lithium tantalate wafer or a lithium niobate wafer (laminated wafer) to a supporting wafer having a smaller coefficient of thermal expansion, the bonding of the laminated wafer and / or the supporting wafer is performed. Ions are implanted from the mating surfaces to disturb the crystallinity near the respective bonding surfaces. That is, prior to laminating the laminated wafer 10 and the supporting wafer 20 as shown in FIG. 1A on the laminating surfaces 11 and 21 as shown in FIG. Ions are implanted from the mating surface (the bonding surface 21 in FIG. 1B) to form an ion-implanted region 22.

  As a result, at the interface 31 where the laminated wafer 10 and the supporting wafer 20 are bonded together, a signal incident from the laminated wafer 10 that is a piezoelectric material is absorbed and scattered in the vicinity of the interface 31 to suppress reflection, thereby reducing spurious. can do.

  The material used for the support wafer 20 is a material having a small thermal expansion coefficient, which effectively suppresses the thermal expansion of the laminated wafer 10 having a large thermal expansion coefficient and contributes to the improvement of the temperature characteristics of the SAW device formed on the laminated wafer 10. , Silicon and sapphire are preferred. Further, as the laminated wafer 10, it is preferable to use a laminated wafer having a lithium concentration that increases as approaching the bonding surface in the thickness direction. By using a laminated wafer having such a concentration distribution, for example, when a resonator is formed on the wafer, Dip appearing in the input impedance waveform can be reduced.

  FIG. 2 shows an example of a specific manufacturing flow of the method for manufacturing a composite wafer of the present invention.

First, an insulating film is formed on a bonding surface of a wafer into which ions are implanted (S1). By forming an insulating film and implanting ions therethrough, channeling of implanted ions can be suppressed. As a material of the insulating film, for example, SiO ₂ , SiON, or SiN is preferable.

Subsequently, ions are implanted from the bonding surface of the wafer on which the insulating film has been formed (S2). The ions to be implanted are not particularly limited as long as they disturb the crystallinity, but light element ions that can be implanted deeply with a low acceleration voltage and are not easily restricted by the implantation apparatus, for example, hydrogen ions (H ⁺ ), hydrogen molecule ions ( H ₂ ⁺ ) and helium ions (He ⁺ ) are preferred. The antireflection effect is remarkable when the dose of ions is a certain amount or more, but, on the other hand, when the amount is too large, the excessively present element destabilizes the bonding at the bonding interface, and the heat treatment after bonding is performed. Problems such as breakage of the substrate occur at the stage. From this viewpoint, the dose amount is 1.0 × 10 ¹⁶ atoms / cm ² or more and 1.0 × 10 ¹⁷ atoms / cm ² or less for hydrogen ions, and 5.0 × 10 ¹⁵ atoms / cm ² or more and 5.0 × 10 ^{16 for} hydrogen molecule ions. atoms / cm ² or less, and in the case of helium ions, it is desirable to be 1.0 × 10 ¹⁶ atoms / cm ² or more and 1.0 × 10 ¹⁷ atoms / cm ² or less.

  Subsequently, the insulating film is removed (S3), and a surface activation treatment is performed on the bonding surface of the wafer into which the ions have been implanted (S4). By performing the surface activation treatment, atoms on the bonding surface are brought into an active state in which a chemical bond is easily formed, so that a stronger bond can be obtained. The surface activation treatment may be performed by, for example, ozone water treatment, UV ozone treatment, ion beam treatment, or plasma treatment.

  Subsequently, the respective wafers are bonded on a bonding surface (S5), and a heat treatment is performed to prevent the introduction of crystal defects due to the displacement of the bonding interface (S6). Then, after laminating the laminated wafer to a necessary degree by grinding and polishing (S7), a SAW device such as a resonator is formed (S8).

  Note that the present invention is not limited to the above embodiment. The above embodiment is an exemplification, and the present invention has substantially the same configuration as the technical idea described in the claims of the present invention, and has the same effect. Within the technical scope of

<Example 1>
A silicon wafer having a diameter of 100 mm and a thickness of 0.55 mm was prepared, and a thermal oxide film was grown at a temperature of 1000 ° C. to about 480 nm. It was confirmed that the surface roughness of both the lithium tantalate wafer (LT wafer) as the laminated wafer and the silicon wafer as the support wafer was 1.0 nm or less by RMS. Hydrogen molecular ions were implanted into the bonding surface of the silicon wafer at an energy of 92 keV so that the dose became 2.0 × 10 ¹⁶ atms / cm ² . After the ion implantation, the thermal oxide film was removed with a 10% hydrofluoric acid solution. FIG. 3 shows the concentration distribution of hydrogen atoms in the silicon wafer in the depth direction from the bonding surface at this time. FIG. 3 shows that the hydrogen concentration near the bonding surface is high.

  These wafers were subjected to a plasma activation treatment to activate the surface, and then bonded together. After heat treatment at 120 ° C. for 6 hours after bonding, the LT wafer was thinned to 20 μm by grinding and polishing. On this wafer, a one-stage ladder filter was formed by combining one parallel resonator and one series resonator. The wavelength of the one-stage ladder filter was 5 μm.

  For comparison, a one-stage ladder filter prepared in the same manner as described above except that ions were not implanted into the silicon wafer was prepared.

  FIG. 4 shows the comparison results. The vertical axis represents the spurious intensity (dB) in the S11 characteristic of the one-stage ladder filter, and the horizontal axis represents the normalized LT film thickness (LT film thickness / wavelength) generally used for evaluating the characteristics of the one-stage ladder filter. From FIG. 4, it can be seen that the spurious intensity is significantly reduced when the ion-implanted silicon wafer is used as the supporting wafer as compared with the case where no ion is implanted.

<Example 2>
The same test was performed by changing the surface activation treatment method in Example 1 to vacuum ion beam activation, activation by ozone water treatment, and activation by UV ozone treatment. The difference between the results of Example 1 and Example 1 was within the range of the error, and it was confirmed that the same effect was obtained by any of the processing methods.

<Example 3>
In Example 1, a test was performed by performing ion implantation with hydrogen ions as the ion species to be implanted, implantation energy of 46 KeV (half of Example 1), and dose of 4 × 10 ¹⁶ atoms / cm ² . The difference between the result of Example 1 and that of Example 1 was within the range of error, and it was confirmed that the same effect can be obtained even if hydrogen ions were implanted.

<Example 4>
In the first embodiment, the spurious intensity when the ion species to be implanted is hydrogen atoms and the dose of ions is changed in the range of 0.8 × 10 ¹⁶ atoms / cm ² to 1.0 × 10 ¹⁷ atoms / cm ² A confirmation test was performed. FIG. 5 shows the results. Reference is a result when no ion implantation is performed. From FIG. 5, it has been confirmed that the effect of the ion implantation becomes remarkable when the dose is 1.0 × 10 ¹⁶ atoms / cm ² . In addition, although the case where the dose amount was more than 1.0 × 10 ¹⁷ atoms / cm ² was carried out, the bonded substrate was broken at the stage of heat treatment after bonding. This is presumed to be because excess hydrogen made the bonding unstable at the bonding interface.

<Example 5>
A test was performed in Example 1 in which the oxide film on silicon was not removed after ion implantation. The difference from the result of Example 1 was within the range of the error, and it was confirmed that the same effect was obtained regardless of the presence or absence of the oxide film.

<Example 6>
In Example 1, a test was performed in which a SiN film formed by an LPCVD method or a SiON film formed by a PECVD method was formed in place of the thermal oxide film, and bonding was performed with the ion implantation left as it was. The difference from the result of Example 1 was within the range of the error, and it was confirmed that the same effect was obtained regardless of the presence or absence of the oxide film.

<Example 7>
In Example 1, the same test was performed using a sapphire wafer having no oxide film or the like instead of the silicon wafer. FIG. 6 shows the results. From FIG. 6, it is found that the spurious reduction effect is obtained in the case of the sapphire wafer as compared with the case of the silicon wafer, but the effect is obtained.

<Example 8>
In Example 1, the bonding was performed by performing ion implantation on the LT wafer instead of the silicon wafer (other conditions were the same). The difference between the result of Example 1 and that of Example 1 lies in the range of the error, and it was confirmed that the same effect can be obtained whether the ion implantation is performed on the silicon wafer or the LT wafer.

<Example 9>
In Example 1, a test was performed by implanting ions into both the silicon wafer and the LT wafer (other conditions were the same). FIG. 7 shows the results. From FIG. 7, it can be seen that the spurious reduction effect is slightly larger than when one is ion-implanted. That is, it was confirmed that performing the ion implantation on both the silicon wafer and the LT wafer can obtain an effect equal to or greater than performing the ion implantation on one of the silicon wafer and the LT wafer.

<Example 10>
In Example 1, a test was performed by implanting helium ions instead of hydrogen ions. The dose was 4 × 10 ¹⁶ atoms / cm ² and the acceleration voltage was 140 KeV. The difference from the result of Example 1 was within the range of the error, and it was confirmed that the same effect could be obtained even if helium ions were implanted.

<Example 11>
A silicon wafer having a diameter of 100 mm and a thickness of 0.55 mm was prepared, and a thermal oxide film was grown at a temperature of 1000 ° C. to about 480 nm. It was confirmed that the surface roughness of both the lithium tantalate wafer (LT wafer) as the laminated wafer and the silicon wafer as the support wafer was 1.0 nm or less by RMS. Hydrogen molecular ions were implanted into the bonding surface of the silicon wafer at an energy of 92 KeV so that the dose became 2.0 × 10 ¹⁶ atms / cm ² . After the ion implantation, the oxide film was removed with a 10% hydrofluoric acid solution. FIG. 3 shows the concentration distribution of hydrogen atoms in silicon in the depth direction from the bonding surface at this time. FIG. 3 shows that the hydrogen concentration is high near the bonding surface.

  These wafers were subjected to a plasma activation treatment to activate the surface, and then bonded together. After heat treatment at 120 ° C. for 6 hours after bonding, the LT wafer was thinned to 45 μm by grinding and polishing.

The LT wafer used was prepared as follows. First, a 4-inch diameter LiTaO ₃ single crystal ingot having an approximate congruent composition of Li: Ta = 48.3: 51.7 is sliced, and a 46.3 ° rotation Y-cut LiTaO ₃ substrate is cut into a thickness of 370 μm. Was. Thereafter, if necessary, the surface roughness of each slice wafer was adjusted to an arithmetic average roughness Ra value of 0.15 μm by a lapping process, and the finished thickness was 350 μm.

Next, the substrate whose front and back surfaces were finished to a quasi-mirror surface with a Ra value of 0.01 μm by plane polishing was embedded in a powder composed of Li, Ta, and O mainly containing Li ₃ TaO ₄ . At this time, as a powder mainly composed of Li ₃ TaO ₄ , a powder of Li ₃ TaO ₄ : Ta ₂ O ₅ mixed at a molar ratio of 7: 3 and fired at 1300 ° C. for 12 hours was used. . Then, such a powder mainly composed of Li ₃ TaO ₄ was spread in a small container, and a plurality of slice wafers were embedded in the Li ₃ TaO ₄ powder.

Then, the small container was set in an electric furnace, and the furnace was heated to 900 ° C. for 24 hours in an N ₂ atmosphere to diffuse Li from the surface of the sliced wafer to the center. Thereafter, in the temperature lowering process of this process, annealing is performed at 800 ° C. for 12 hours in the atmosphere, and an electric field of 4,000 V / m approximately in the + Z-axis direction during 770 ° C. to 500 ° C. in the process of further lowering the temperature of the wafer. After applying, a process of lowering the temperature to room temperature was performed.

After this treatment, the rough side is finished by sandblasting to an Ra value of about 0.15 μm, and the rough mirror side is polished to 3 μm to obtain a plurality of LiTaO ₃ single crystal substrates. It was created.

  FIG. 8 shows the profile in the thickness direction of the Li concentration of the LT wafer prepared as described above. The thickness indicates the depth from the bonding surface of 0 μm. As can be seen from FIG. 8, the Li concentration of the LT wafer is the highest on the bonding surface, and decreases with increasing depth.

For comparison, a composite wafer was similarly prepared for a LiTaO ₃ single crystal substrate having no Li concentration shading in the thickness direction without performing the treatment in the Li ₃ TaO ₄ powder described above.

  For each of the composite wafer prepared using the LT wafer having the above-described Li concentration in the thickness direction and the LT wafer having no Li concentration in the thickness direction, the wavelength on the wafer is different. A 5 μm resonator was made. FIG. 9 shows an input impedance waveform (main resonance expanded waveform) of each resonator. It can be seen that Dip on the main resonance waveform is smaller and preferable when there is such a concentration of Li concentration in the thickness direction as compared to when there is no distribution of Li concentration in the thickness direction.

  FIG. 10 shows an expanded frequency range of the frequency-input impedance characteristics shown in FIG. From FIG. 10, the spurious response at 900 to 1200 MHz higher than the main resonance frequency shows a large amplitude when the density of the Li concentration is in the thickness direction of the LT wafer and when the density of the Li concentration is not in the thickness direction. It turns out that there is no difference.

  FIG. 11 shows the Q value of the resonator. From FIG. 11, it can be seen that the Q value is larger in the case where the density of the Li concentration is as described above in the thickness direction of the LT wafer than in the case where there is no distribution of the Li concentration in the thickness direction. Therefore, when the density of the Li concentration varies in the thickness direction of the LT wafer, the spurious of the main resonance decreases and the Q value increases. On the other hand, the spurious response at a frequency higher than the main resonance showed almost the same results with and without the Li concentration in the thickness direction of the LT wafer.

DESCRIPTION OF SYMBOLS 10 Laminated wafer 11, 21 Bonding surface 20 Support wafer 22 Ion implantation area 31 Interface

Claims (8)

In a composite wafer manufacturing method for manufacturing a composite wafer by bonding a lithium tantalate wafer or a lithium niobate wafer (hereinafter, referred to as a “laminated wafer”) to a supporting wafer having a smaller coefficient of thermal expansion,
Prior to the bonding, an ion implantation step of implanting ions from a bonding surface of the laminated wafer and / or the supporting wafer and disturbing crystallinity near each bonding surface is performed. Production method. The composite wafer according to claim 1, wherein the ions are hydrogen ions (H ⁺ ), and the dose is not less than 1.0 × 10 ¹⁶ atoms / cm ^{2 and not} more than 1.0 × 10 ¹⁷ atoms / cm ² . Production method. _2. The composite according to claim 1, wherein the ions are hydrogen molecular ions (H ₂ ⁺ ), and the dose is not less than 5.0 × 10 ¹⁵ atoms / cm ^{2 and not} more than 5.0 × 10 ¹⁶ atoms / cm ^2. 3. Wafer manufacturing method. The composite wafer according to claim 1, wherein the ions are helium ions (He ⁺ ), and the dose is not less than 1.0 × 10 ¹⁶ atoms / cm ^{2 and not} more than 1.0 × 10 ¹⁷ atoms / cm ² . Production method.   After the execution of the ion implantation step, a surface activation treatment by an ozone water treatment, a UV ozone treatment, an ion beam treatment, or a plasma treatment is performed on the bonding surface of the laminated wafer and / or the support wafer before the bonding. The method according to claim 1, wherein a surface activation step is performed.   The method of manufacturing a composite wafer according to any one of claims 1 to 5, wherein a material of the support wafer is silicon or sapphire. _2. An insulating film forming step of forming an insulating film of SiO ₂ , SiON, or SiN on a bonding surface of the laminated wafer and / or the supporting wafer prior to the ion implantation step. 7. The method for manufacturing a composite wafer according to any one of 6. The method of manufacturing a composite wafer according to any one of claims 1 to 7, wherein the laminated wafer has a higher lithium concentration as approaching a bonding surface in a thickness direction.