Joint of piezoelectric material substrate and support substrate and method for manufacturing the same

Abstract

When a piezoelectric material substrate formed of lithium tantalate or the like is bonded to a support substrate provided with a silicon oxide layer, the bonding strength is improved. The bonded body comprises: a support substrate 4; a silicon oxide layer 5 provided on the support substrate 4; and a piezoelectric material substrate 1 formed of a material selected from the group consisting of lithium niobate, lithium tantalate, and lithium niobate-lithium tantalate, and provided on the silicon oxide layer 5. The surface resistivity of the piezoelectric material substrate 1 on the silicon oxide layer 5 side is 1.7×10 15 Ω / □ or more.

Description

Technical Field

[0001] The present invention relates to a joint body of a piezoelectric material substrate and a supporting substrate and a method for manufacturing the same. Background Art

[0002] In order to realize high-performance semiconductor elements, SOI substrates including high-resistance Si / SiO2 thin film / Si thin film are widely used. When realizing the SOI substrate, plasma activation is used. This is because it can be bonded at a relatively low temperature (400°C). In order to improve the characteristics of piezoelectric devices, a similar composite substrate including Si / SiO2 thin film / piezoelectric thin film has been proposed (Patent Document 1).

[0003] In Patent Document 1, a piezoelectric material substrate formed of lithium niobate or lithium tantalate and a silicon substrate provided with a silicon oxide layer are activated by ion implantation and then bonded.

[0004] Patent Document 2 describes that lithium tantalate and a ceramic containing any one of sapphire, aluminum oxide, aluminum nitride, or silicon nitride as a main component are bonded together by a plasma activation method via a silicon oxide layer.

[0005] Non-patent document 1 describes that a lithium tantalate substrate and a silicon substrate provided with a silicon oxide layer are bonded by successively irradiating O 2 RIE (13.56 MHz) plasma and N 2 microwave (2.45 GHz) plasma.

[0006] In the plasma activated bonding of Si and SiO2 / Si, sufficient bonding strength is obtained by forming Si-O-Si bonds at the bonding interface. In addition, at the same time, Si is oxidized to SiO2 to improve smoothness, promoting the above-mentioned bonding at the outermost surface (Non-Patent Document 2).

[0007] Prior art literature

[0008] Non-patent literature

[0009] Non-patent literature 1: ECS Transactions, 3(6)91-98(2006)

[0010] Non-patent document 2: J. Applied Physics 113, 094905 (2013)

[0011] Patent Literature

[0012] Patent Document 1: Japanese Patent Application Laid-Open No. 2016-225537

[0013] Patent Document 2: Japanese Patent No. 3774782 Summary of the invention

[0014] However, when a piezoelectric element is produced by thinning a lithium niobate or lithium tantalate substrate by ion implantation as in the prior art, there is a problem of low characteristics. This is believed to be because the crystallinity deteriorates due to damage during ion implantation.

[0015] On the other hand, when a piezoelectric material substrate such as lithium niobate or lithium tantalate is bonded to a silicon oxide layer on a silicon substrate and then the piezoelectric material substrate is ground to be thinned, the component characteristics are not degraded because the processing-degraded layer can be removed by CMP. However, during the grinding process, a large shear stress is applied to the piezoelectric material substrate. Therefore, when the piezoelectric material substrate is to be processed to be thinner by grinding, there is a problem that the piezoelectric material substrate is peeled off from the silicon substrate during grinding. In addition, if the thickness of the piezoelectric material substrate is reduced by grinding, there is also a problem that the characteristics of the obtained joint body are degraded. In particular, when the joint body is used as an elastic wave element, the characteristics as an elastic wave element (for example, the electromechanical coupling coefficient k2) are sometimes reduced.

[0016] The present invention aims to maintain the bonding strength and suppress the degradation of the properties of the bonded body when bonding a piezoelectric material substrate formed of a material selected from the group consisting of lithium niobate, lithium tantalate, and lithium niobate-lithium tantalate and a supporting substrate provided with a silicon oxide layer.

[0017] The present invention is a conjugate comprising:

[0018] A supporting substrate;

[0019] a silicon oxide layer disposed on the supporting substrate; and

[0020] a piezoelectric material substrate, the piezoelectric material substrate being formed of a material selected from the group consisting of lithium niobate, lithium tantalate, and lithium niobate-lithium tantalate, and being disposed on the silicon oxide layer;

[0021] The conjugate is characterized in that

[0022] The surface resistivity of the piezoelectric material substrate on the silicon oxide layer side is 1.7×10 15 Ω / □ or more.

[0023] In addition, the present invention is a method for bonding a piezoelectric material substrate formed of a material selected from the group consisting of lithium niobate, lithium tantalate, and lithium niobate-lithium tantalate and a supporting substrate provided with a silicon oxide layer.

[0024] The invention is characterized in that the bonding surface of the piezoelectric material substrate is irradiated with oxygen plasma at 150° C. or less so that the surface resistivity of the bonding surface is 1.7×10 15After the piezoelectric material has a bonding resistance of Ω / □ or more, the bonding surface of the piezoelectric material substrate is bonded to the bonding surface of the silicon oxide layer.

[0025] Effects of the Invention

[0026] The inventors of the present invention have conducted detailed studies on the reasons why it is difficult to maintain a certain degree of bonding strength and suppress degradation of the properties of the bonded body (for example, the electromechanical coupling coefficient k2) when directly bonding a piezoelectric material substrate formed of lithium niobate or the like and a supporting substrate provided with a silicon oxide layer, and obtained the following insights.

[0027] That is, when Si and SiO2 / Si are bonded by plasma activation, for example, Si-O-Si bonds are formed along the bonding interface, thereby obtaining a sufficiently high bonding strength. In addition, at the same time, Si is oxidized to SiO2, thereby improving the smoothness and promoting the above-mentioned bonding on the outermost surface (Non-patent Document 2: J. Applied Physics 113, 094905 (2013)).

[0028] In contrast, when lithium niobate or lithium tantalate is directly bonded to a support substrate provided with a silicon oxide layer, Ta(Nb)-O-Si bonds are formed along the bonding interface, thereby bonding. However, the density of Si atoms in the outermost surface of SiO2 / Si is 6.8 atoms / m. The density of Ta (Nb) atoms in the outermost surface of the piezoelectric material substrate depends on the cutting angle because it is anisotropic crystal, for example, 4.3 atoms / m. The atomic density in the outermost surface is low. In addition, unlike silicon, lithium niobate or lithium tantalate does not have a mechanism for smoothing by oxidation, and it is considered that for this reason, a sufficiently high bonding strength cannot be obtained.

[0029] As a result, when a piezoelectric material substrate formed of lithium niobate or lithium tantalate is directly bonded to SiO2 / Si and then processed, if the piezoelectric material substrate becomes thinner, shear force is applied to the piezoelectric material substrate, which is believed to cause the crystallinity of the piezoelectric material substrate to deteriorate.

[0030] Based on the above hypothesis, the inventors of the present invention tried to activate the bonding surface of the piezoelectric material substrate and the bonding surface of the silicon oxide layer by irradiating the bonding surface of the piezoelectric material substrate with oxygen plasma for more than 30 minutes at a temperature below 150°C, and then directly bond the activated surface of the piezoelectric material substrate and the activated surface of the silicon oxide layer. As a result, the surface resistivity of the piezoelectric material substrate on the silicon oxide layer side was 1.7×10 15 As a result, it was found that the bonding strength between the piezoelectric material substrate and the support substrate was actually maintained, and the degradation of the characteristics of the bonded body (electromechanical coupling coefficient k2) could be suppressed, thereby achieving the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 In the drawings, (a) shows a piezoelectric material substrate 1, and (b) shows a state where a bonding surface 1a of the piezoelectric material substrate 1 is activated to generate an activated surface 1c.

[0032] Figure 2 In the drawings, (a) shows a state where the silicon oxide layer 5 is formed on the surface of the support substrate 4, and (b) shows a state where the surface of the silicon oxide layer 5 is activated.

[0033] Figure 3 In the figure, (a) shows a joint body 7 obtained by directly bonding a piezoelectric material substrate 1 and a silicon oxide layer 5 on a supporting substrate 4, (b) shows a state where the piezoelectric material substrate 1A of the joint body 7 is polished to be thinned, and (c) shows an elastic wave element 10. DETAILED DESCRIPTION

[0034] Hereinafter, the present invention will be described in detail with reference to the drawings as appropriate.

[0035] First, if Figure 1 As shown in (a), a piezoelectric material substrate 1 having a pair of main surfaces 1a and 1b is prepared. In this example, 1a is used as a bonding surface. Figure 1 As shown in (b), as indicated by arrow A, the bonding surface 1a of the piezoelectric material substrate 1 is irradiated with plasma to obtain a bonding surface 1c after surface activation.

[0036] On the other hand, Figure 2 As shown in (a), a silicon oxide layer 5 is formed on the surface 4a of the support substrate 4. Figure 2 As shown in (b), as indicated by arrow A, the surface 5a of the silicon oxide layer 5 is irradiated with plasma to activate the surface, thereby forming an activated bonding surface 6.

[0037] Next, if Figure 3 As shown in (a), the activated bonding surface 1c on the piezoelectric material substrate 1 and the activated bonding surface 6 of the silicon oxide layer 5 on the supporting substrate 4 are brought into contact and directly bonded to obtain a bonded body 7. In this state, electrodes may be provided on the piezoelectric material substrate 1. However, it is preferred that Figure 3 As shown in (b), the main surface 1b of the piezoelectric material substrate 1 is processed to make the substrate 1 thinner, thereby obtaining a thinned piezoelectric material substrate 1A. 1d is the processed surface. Figure 3 As shown in (c), a predetermined electrode 8 is formed on the processed surface 1d of the piezoelectric material substrate 1A of the bonded body 7A, and an elastic wave device 10 can be obtained.

[0038] Hereinafter, each constituent element of the present invention will be described in sequence.

[0039] The material of support substrate 4 is not particularly limited, but is preferably made of a material selected from the group consisting of silicon, crystal, sialon, mullite, sapphire, and translucent alumina. This can further improve the frequency characteristics of acoustic wave device 10 .

[0040] A silicon oxide layer 5 is formed on the support substrate 4. The film forming method of the silicon oxide layer 5 is not limited, and examples thereof include sputtering, chemical vapor deposition (CVD), and evaporation. Preferably, the support substrate 4 is a silicon substrate. In this case, the silicon oxide layer 5 can be formed by sputtering or ion implanting oxygen on the surface of the silicon substrate and heating it in an oxidizing atmosphere.

[0041] From the viewpoint of the present invention, the thickness of silicon oxide layer 5 is preferably 0.05 μm or more, more preferably 0.1 μm or more, and particularly preferably 0.2 μm or more. In addition, the thickness of silicon oxide layer 5 is preferably 3 μm or less, more preferably 2.5 μm or less, and further preferably 2.0 μm or less.

[0042] The piezoelectric material substrate 1 used in the present invention is lithium tantalate (LT) single crystal, lithium niobate (LN) single crystal, lithium niobate-lithium tantalate solid solution. These materials have a fast elastic wave propagation speed and a large electromechanical coupling coefficient, so they are suitable as high-frequency and broadband elastic surface wave devices.

[0043] In addition, the normal direction of the main surfaces 1a and 1b of the piezoelectric material substrate 1 is not particularly limited. For example, when the piezoelectric material substrate 1 is formed by LT, a piezoelectric material substrate with the propagation direction of elastic surface waves, that is, the X-axis as the center and rotated 32 to 55 degrees from the Y-axis to the Z-axis is used. The Euler angle is expressed as (180°, 58 to 35°, 180°). This has a small propagation loss and is therefore preferred. When the piezoelectric material substrate 1 is formed of LN, (a) a piezoelectric material substrate with the propagation direction of the surface elastic wave, i.e., the X-axis, as the center, and the direction rotated from the Z-axis to the -Y-axis by 37.8° is preferably used, and the electromechanical coupling coefficient is large, or (i) a piezoelectric material substrate with the propagation direction of the surface elastic wave, i.e., the X-axis, as the center, and the direction rotated from the Y-axis to the Z-axis by 40 to 65° is preferably used, and the direction is preferably expressed as (180°, 50 to 25°, 180°) as the Euler angle, and a high sound velocity is obtained. In addition, the size of the piezoelectric material substrate is not particularly limited, and for example, the diameter is 100 to 200 mm and the thickness is 0.15 to 1 μm.

[0044] Next, the bonding surface 1 a of the piezoelectric material substrate 1 is irradiated with oxygen plasma at 150° C. or less to activate the bonding surface 1 a .

[0045] The pressure during surface activation is preferably 100 Pa or less, more preferably 80 Pa or less. The atmosphere may be oxygen alone, but may contain nitrogen in addition to oxygen.

[0046] The temperature during oxygen plasma irradiation is set to 150° C. or less. This provides a bonded body 7 having high bonding strength and without deterioration of the piezoelectric material. From this viewpoint, the temperature during oxygen plasma irradiation is set to 150° C. or less, more preferably 100° C. or less.

[0047] The energy during oxygen plasma irradiation is preferably 100 to 150 W. The product of the energy during oxygen plasma irradiation and the irradiation time is preferably 20 to 50 Wh. The irradiation time of oxygen plasma is preferably 30 minutes or more.

[0048] In a preferred embodiment, before plasma treatment, the bonding surface 1a of the piezoelectric material substrate 1 and the bonding surface 5a of the silicon oxide layer 5 are flattened. The methods for flattening the bonding surfaces 1a and 5a include lapping (lap), chemical mechanical polishing (CMP), etc. In addition, for the flat surface, Ra ≤ 1nm is preferred, and 0.3nm or less is more preferred.

[0049] In a preferred embodiment, the surface resistivity of the piezoelectric material substrate 1 after the oxygen plasma treatment is set to 1.7×10 15 Ω / □ or more, more preferably 2.8×10 15 Ω / □ or more. The surface resistivity measured at the bonding surface 1a increases due to the ingress of oxygen into the piezoelectric material substrate 1. Therefore, the surface resistivity is an indicator of the ingress of oxygen. It should be noted that the surface resistivity of the piezoelectric material substrate 1 after the oxygen plasma treatment is 2.0×10 16 Ω / □ or less, the bonding strength of the bonded body can be maintained. From this point of view, the surface resistivity of the piezoelectric material substrate 1 is more preferably 1.6×10 16 Ω / □ or less. This makes it easy to polish the piezoelectric material substrate 1.

[0050] The bonding surface 5a of the silicon oxide layer 5 is also subjected to plasma treatment. At this time, plasma of oxygen, nitrogen or the like can be used. The temperature during the plasma treatment is preferably 150°C or less, and more preferably 100°C or less.

[0051] The pressure when plasma is irradiated onto the bonding surface 5a of the silicon oxide layer 5 is preferably 100 Pa or less, more preferably 80 Pa or less. The energy at this time is preferably 30 to 120 W. The product of the energy and the irradiation time during plasma irradiation is preferably 1 Wh or less.

[0052] Next, the bonding surface 1c of the piezoelectric material substrate 1 and the bonding surface 6 of the silicon oxide layer 5 are brought into contact and bonded. Then, annealing treatment is preferably performed to improve the bonding strength. The temperature during the annealing treatment is preferably 100°C or more and 300°C or less.

[0053] The bonded bodies 7 and 7A of the present invention can be preferably used in the elastic wave device 10 .

[0054] As the elastic wave element 10, a surface elastic wave device, a Lamb wave element, a thin film resonator (FBAR), etc. are known. For example, the surface elastic wave device is a device in which an IDT (Interdigital Transducer) electrode (also called a comb electrode or a curtain electrode) on the input side for exciting the surface elastic wave and an IDT electrode on the output side for receiving the surface elastic wave are provided on the surface of a piezoelectric material substrate. If a high-frequency signal is applied to the IDT electrode on the input side, an electric field is generated between the electrodes, and the surface elastic wave is excited to propagate on the piezoelectric material substrate. Then, the propagated surface elastic wave can be taken out in the form of an electrical signal from the IDT electrode on the output side provided in the propagation direction.

[0055] The material constituting the electrode (electrode pattern) 8 on the piezoelectric material substrate 1A is preferably aluminum, aluminum alloy, copper, or gold, and more preferably aluminum or aluminum alloy. The aluminum alloy is preferably an aluminum alloy in which 0.3 to 5 wt% of Cu is mixed in Al. In this case, Ti, Mg, Ni, Mo, or Ta can be used instead of Cu.

[0056] Example

[0057] (Experiment A)

[0058] According to the reference Figure 1 to Figure 3 Method of explanation, making Figure 3 (b) shows a joined body 7A.

[0059] Specifically, a 0.2 mm thick 42Y-cut LiTaO3 substrate (piezoelectric material substrate) 1 with both surfaces polished to mirror finish and a 0.5 mm thick high resistance Si substrate (support substrate) 4 were prepared. A 0.5 μm thick silicon oxide layer 5 was formed on the support substrate 4 by sputtering.

[0060] Next, the bonding surface 1a of the piezoelectric material substrate 1 and the bonding surface 5a of the silicon oxide layer 5 on the supporting substrate 4 are cleaned and surface activated, respectively. Specifically, ultrasonic cleaning using pure water is implemented, and the substrate surface is dried by spin drying. Next, the cleaned supporting substrate 4 is introduced into a plasma activation chamber, and the bonding surface 5a is activated at 30°C using oxygen plasma. In addition, the piezoelectric material substrate 1 is similarly introduced into a plasma activation chamber, and the bonding surface 1a is surface activated at 30°C using oxygen plasma. For the purpose of removing particles attached during surface activation, the same ultrasonic cleaning and spin drying as described above are implemented again.

[0061] The activation treatment time was changed as shown in Table 1. The plasma irradiation energy during the activation treatment was 150 W. In addition, the surface resistivity of the piezoelectric material substrate 1 after activation was measured, and the measurement results are shown in Table 1. It should be noted that the surface resistivity of the piezoelectric material substrate 1 on the silicon oxide layer 5 side was measured as follows, that is, using Hiresta UX MCP-HT800 manufactured by Mitsubishi Chemical Analytical Technology Co., Ltd., using a probe with a double ring electrode, and applying a voltage of 1000 V for measurement.

[0062] Next, align each substrate 1 and 4, and make the activated bonding surface 1c of the piezoelectric material substrate 1 and the activated bonding surface 6 of the silicon oxide layer 5 contact at room temperature. The contact is made in such a way that the piezoelectric material substrate 1 side is on top. As a result, the close contact and expansion of the substrates 1 and 4 with each other (the so-called bonding wave) are observed, and it can be confirmed that the pre-bonding is well performed. Next, in order to increase the bonding strength, the bonded body is placed in an oven with a nitrogen atmosphere and kept at 120°C for 10 hours. Furthermore, before being placed in a high-temperature oven, in order to prevent damage to the substrate caused by thermal stress, the substrate is divided into 8 equal parts in the form of radial lines starting from the center. The annealing temperature in the oven is 250°C. The 8 bonded bodies taken out of the heating oven are evaluated for bonding strength using a blade test (Semiconductor Wafer Bonding, Q.-Y. Tong & U. Gosele, p25).

[0063] Then, the surface 1b of the piezoelectric material substrate 1 of the heated joint 7 is subjected to grinding, lapping and CMP processing to reduce the thickness of the piezoelectric material substrate 1A. Electrodes 8 formed of aluminum metal are provided on the surface 1d of the ground piezoelectric material substrate 1A by photolithography. The period λ of the electrode is 5μm so that the oscillation frequency is about 850MHz. The thickness of the electrode 8 is 200nm, and 80 pairs of reflectors are provided on both sides of 200 pairs of electrodes 8 to produce a single-port elastic wave element 10 (SAW resonator). The impedance characteristics of the produced elastic wave element 10 (SAW resonator) are measured using a network analyzer E5071C manufactured by Agilent. As a result, a resonance peak is observed near 820MHz (fs), and an anti-resonance peak is observed near 850 to 860MHz (fr). The difference Δf between the resonance frequency fs and the anti-resonance frequency fr is measured. It is known that the difference Δf is generally proportional to the electromechanical coupling coefficient k2 of the piezoelectric element. The results are shown in Table 1.

[0064] [Table 1]

[0065]

[0066] As shown in Table 1, when the activation treatment time is short (0.1 to 15 minutes), the surface resistivity of the piezoelectric material substrate 1A on the silicon oxide layer 5 side is 1.2×10 13 Ω / □~8.8×10 14 Ω / □, Δf(MHz) is only 31.8 to 34.3. As a result, Δf(%) = Δf(MHz) / 850(MHz) is 3.7% to 4.0%.

[0067] On the other hand, when the activation treatment time is long (30 minutes to 150 minutes), the surface resistivity of the piezoelectric material substrate 1A on the silicon oxide layer 5 side is 1.73×10 15 Ω / □~1.51×10 16 Ω / □ is about 38.8 to 39.6. As a result, Δf(%)=Δf(MHz) / 850(MHz) is about 4.6% to 4.7%.

[0068] If the surface resistivity of the piezoelectric material substrate 1A on the silicon oxide layer 5 side exceeds 2×10 16 Ω / □, the bonding strength is significantly reduced. Therefore, when the thickness of the piezoelectric material substrate 1A is reduced, peeling occurs.

[0069] As a result, according to the present invention, the bonding surface 1a of the piezoelectric material substrate 1 was activated by oxygen plasma to a surface resistivity of 1.7×10 15 Ω / □More than 1.6×1016 When Ω / □ is less than or equal to Ω / □, since the bonding strength is maintained, the thickness of the piezoelectric material substrate 1 can be reduced, and degradation of the characteristics of the bonded body (for example, the electromechanical coupling coefficient k2) can be suppressed.

[0070] (Experiment B)

[0071] In Experiment A, the material of the piezoelectric material substrate 1 was changed to lithium niobate. As a result, the same result as that of Experiment A was obtained.

Claims

1. A conjugate comprising: A supporting substrate; a silicon oxide layer disposed on the supporting substrate; and a piezoelectric material substrate, the piezoelectric material substrate being formed of a material selected from the group consisting of lithium niobate, lithium tantalate, and lithium niobate-lithium tantalate, and being disposed on the silicon oxide layer; The conjugate is characterized in that The surface resistivity of the piezoelectric material substrate on the silicon oxide layer side is 1.7×10 15 Ω / □More than 2.0×10 16 Ω / □ or less.

2. The joint body according to claim 1, characterized in that: The support substrate is formed of a material selected from the group consisting of silicon, crystal, sialon, mullite, sapphire, and translucent alumina.

3. A method for manufacturing a piezoelectric material substrate and a support substrate joint body, which is a method for joining a piezoelectric material substrate formed of a material selected from the group consisting of lithium niobate, lithium tantalate, and lithium niobate-lithium tantalate and a support substrate provided with a silicon oxide layer, The method for manufacturing a bonded body of a piezoelectric material substrate and a supporting substrate is characterized in that: The bonding surface of the piezoelectric material substrate is irradiated with oxygen plasma at 150° C. or less, so that the surface resistivity of the bonding surface is 1.7×10 15 Ω / □More than 2.0×10 16 After the piezoelectric material has a bonding resistance of 0.054 W / m3 or less, the bonding surface of the piezoelectric material substrate is bonded to the bonding surface of the silicon oxide layer.

4. The method according to claim 3, characterized in that: After the bonding surface of the silicon oxide layer is activated by irradiating it with oxygen plasma at 150° C. or less, the bonding surface of the piezoelectric material substrate is bonded to the bonding surface of the silicon oxide layer.

5. The method according to claim 3 or 4, characterized in that: After the bonding surface of the piezoelectric material substrate is bonded to the bonding surface of the silicon oxide layer, the thickness of the piezoelectric material substrate is reduced by polishing.

6. The method according to claim 5, characterized in that The thickness of the piezoelectric material substrate is reduced to less than 2 μm by grinding.

7. The method according to claim 3 or 4, characterized in that: The support substrate is formed of a material selected from the group consisting of silicon, crystal, sialon, mullite, sapphire, and translucent alumina.

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