JP6668292B2 - Bonded body of piezoelectric material substrate, bonding method, and elastic wave element - Google Patents

Description

  The present invention relates to a method for bonding a piezoelectric material substrate, a bonded body, and an acoustic wave device.

For the purpose of realizing a high-performance semiconductor element, an SOI substrate composed of a high-resistance Si / SiO ₂ thin film / Si thin film is widely used. In realizing an SOI substrate, plasma activation is used. This is because bonding can be performed at a relatively low temperature (400 ° C.). A composite substrate composed of a similar Si / SiO ₂ thin film / piezoelectric thin film has been proposed to improve the characteristics of a piezoelectric device (Patent Document 1).

In Patent Literature 1, a piezoelectric material substrate made of lithium niobate or lithium tantalate and a silicon substrate provided with a silicon oxide layer are exposed to a reduced-pressure plasma (for example, 0.2 to 1.0 mTorr) for about 5 to 60 seconds. After the surface is activated by plasma treatment, bonding is performed. As the plasma gas, O ₂ , N ₂ , Ar, or a mixed gas thereof is used.

  Patent Literature 2 describes that lithium tantalate and sapphire or ceramics are joined via a silicon oxide layer by a plasma activation method.

Non-Patent Document 1 discloses that a lithium tantalate substrate and a silicon substrate provided with a silicon oxide layer are successively irradiated with O ₂ RIE (13.56 MHz) plasma and N ₂ microwave (2.4 G5 Hz) plasma. Joining is described.

ECS Transactions, 3 (6) 91-98 (2006) J. Applied Physics 113, 094905 (2013))

JP 2016-225537 Patent No.3774782 JP 2014-086400

  However, when a piezoelectric element is manufactured by thinning a lithium niobate or lithium tantalate substrate by ion implantation as in the prior art, there is a problem that characteristics are low. This is considered to be due to the fact that crystallinity is deteriorated due to damage during ion implantation.

  On the other hand, when a piezoelectric material substrate such as lithium niobate or lithium tantalate is bonded to a silicon oxide film on a silicon substrate and then the piezoelectric single crystal substrate is polished and thinned, the deteriorated layer is processed by CMP. Since it can be removed, the element characteristics do not deteriorate. However, a large shear stress is applied to the piezoelectric material substrate during the polishing process. Therefore, when the piezoelectric material substrate is processed to be thin by polishing, there is a problem that the piezoelectric material substrate is peeled off from the silicon substrate during polishing.

  An object of the present invention is to join a piezoelectric substrate made of a material selected from the group consisting of lithium niobate, lithium tantalate, and lithium niobate-lithium tantalate to a support substrate provided with a silicon oxide layer. In this case, it is to improve the bonding strength.

The present invention provides a method for joining a piezoelectric material substrate made of a material selected from the group consisting of lithium niobate, lithium tantalate, and lithium niobate-lithium tantalate to a support substrate provided with a silicon oxide layer. So,
Providing, on the piezoelectric material substrate, an atomic layer having a thickness of 0.1 to 0.8 nm consisting of only one or more atoms selected from the group consisting of tantalum and niobium, and a bonding surface of the piezoelectric material substrate Is bonded to the silicon oxide layer.

Also, the present invention
Support substrate,
A silicon oxide layer provided on a supporting substrate,
A piezoelectric material substrate made of a material selected from the group consisting of lithium niobate, lithium tantalate, and lithium niobate-lithium tantalate; and a piezoelectric material substrate provided on the piezoelectric material substrate, selected from the group consisting of tantalum and niobium. An atomic layer having a thickness of 0.1 to 0.8 nm composed of only one or more types of atoms, wherein a bonding surface of the piezoelectric material substrate is bonded to the silicon oxide layer. The present invention relates to a joined body of piezoelectric material substrates.

  The present inventors have examined in detail the reason why it is difficult to improve the bonding strength to a certain extent when directly bonding a piezoelectric material substrate made of lithium niobate or the like and a supporting substrate provided with a silicon oxide layer. And led to the following findings.

That is, when, for example, plasma-activated bonding of Si and SiO ₂ / Si is performed, a sufficiently high bonding strength can be obtained by forming a Si—O—Si bond along the bonding interface. Simultaneously, Si is oxidized to SiO ₂ , so that the smoothness is improved and the above-described bonding is promoted at the outermost surface (Non-Patent Document 2: J. Applied Physics 113, 094905 (2013)).

In contrast, when lithium niobate or lithium tantalate is directly bonded to a supporting substrate provided with a silicon oxide layer, a Ta (Nb) -O-Si bond is formed along the bonding interface. Thus, joining is performed. However, the density of Si atoms on the outermost surface of SiO ₂ / Si is 6.8 / Å ² . In contrast, the density of Ta (Nb) atoms in the outermost surface of the piezoelectric material substrate depends on the cut angle for an anisotropic crystal, for example, at 4.3 pieces / Å ² or less, most Low atomic density at the surface. Furthermore, unlike lithium silicon, lithium niobate and lithium tantalate do not have a mechanism for smoothing by oxidation, so it is considered that a sufficiently high bonding strength could not be obtained.

As a result, even when the piezoelectric material substrate made of lithium niobate or lithium tantalate is directly bonded to SiO ₂ / Si and then the piezoelectric material substrate is polished and thinned, the piezoelectric material substrate is also subjected to the shearing force. Is considered to have peeled off.

Based on such a hypothesis, the present inventor provided an extremely thin atomic layer of at least one atom selected from the group consisting of tantalum and niobium and having a thickness of 0.1 to 0.8 nm on a piezoelectric material substrate. Then, an attempt was made to directly bond the piezoelectric material substrate to the silicon oxide film. This is because a small amount of niobium atoms and tantalum atoms are arranged on the outermost surface (bonding surface) of the piezoelectric material substrate, so that Ta (Nb) —O— is formed along the bonding interface between the piezoelectric material substrate and the silicon oxide film. This is for promoting the generation of a Si bond or a Si—O—Si bond. As a result, the present inventors have found that the bonding strength between the piezoelectric material substrate and the support substrate is significantly improved, and have reached the present invention.

(A) shows the piezoelectric material substrate 1, (b) schematically shows a state in which the atomic layer 2 is provided on the bonding surface 1a of the piezoelectric material substrate 1, and (c) shows the piezoelectric material substrate. 2 shows a state in which the bonding surface is activated. (A) shows a state where the silicon oxide film 5 is formed on the surface of the support substrate 4, and (b) shows a state where the silicon oxide film 5 is activated. (A) shows a joined body 7 obtained by directly joining the piezoelectric material substrate 1 and the silicon oxide film 5 on the support substrate 4, and (b) shows the piezoelectric material substrate 1 </ b> A of the joined body 7. This shows a state of being polished and thinned, and (c) shows an elastic wave element.

Hereinafter, the present invention will be described in detail with reference to the drawings as appropriate.
First, as shown in FIG. 1A, a piezoelectric material substrate 1 having a pair of main surfaces 1a and 1b is prepared. In this example, 1a is the bonding surface. Next, as shown in FIG. 1B, an atomic layer 2 is formed on the bonding surface 1a of the piezoelectric material substrate 1 as described later. Next, the bonding surface of the piezoelectric material substrate is irradiated with plasma as indicated by arrow A to activate the surface.

  On the other hand, as shown in FIG. 2A, a silicon oxide film 5 is formed on the surface 4a of the support substrate 4. Next, as shown in FIG. 2B, the surface of the silicon oxide film 5 is activated by irradiating the surface thereof with plasma as shown by an arrow A to form an activated surface 6.

  Next, as shown in FIG. 3A, the activation surface 3 on the piezoelectric material substrate 1 and the bonding surface 6 of the silicon oxide film 5 on the support substrate 4 are brought into contact with each other, and are directly bonded to each other. Get. In this state, electrodes may be provided on the piezoelectric material substrate 2. However, preferably, as shown in FIG. 3B, the main surface 1b of the piezoelectric material substrate 1 is processed to make the substrate 1 thinner, and a thinned piezoelectric material substrate 1A is obtained. 1c is a processing surface. Next, as shown in FIG. 3C, a predetermined electrode 8 is formed on the processed surface 1c of the piezoelectric material substrate 1A of the bonded body 7A, and the acoustic wave device 10 can be obtained.

Hereinafter, each component of the present invention will be sequentially described.
The material of the support substrate is not particularly limited, but is preferably made of a material selected from the group consisting of silicon, quartz, sialon, mullite, sapphire, and translucent alumina. Thereby, the temperature characteristics of the frequency of the elastic wave element can be further improved.

  A silicon oxide film is formed over the supporting substrate. The method for forming the silicon oxide film is not limited, and examples thereof include sputtering, chemical vapor deposition (CVD), and vapor deposition. Preferably, the supporting substrate is a silicon substrate. In this case, a silicon oxide film can be formed by sputtering, ion implantation, or heating in an oxidizing atmosphere of oxygen on the surface of the silicon substrate.

  From the viewpoint of the present invention, the thickness of the silicon oxide film is preferably 0.05 μm or more, more preferably 0.1 μm or more, and particularly preferably 0.3 μm or more. Further, the thickness of the silicon oxide film is preferably 3 μm or less, more preferably 2.5 μm or less, and further preferably 2.0 μm or less.

  The piezoelectric material substrate used in the present invention is a lithium tantalate (LT) single crystal, a lithium niobate (LN) single crystal, or a lithium niobate-lithium tantalate solid solution. These are suitable as high-frequency and wide-band surface acoustic wave devices because they have a high propagation speed of elastic waves and a large electromechanical coupling coefficient.

  The direction of the normal to the main surface of the piezoelectric material substrate is not particularly limited. For example, when the piezoelectric material substrate is formed of LT, the Z-axis extends from the Y-axis around the X-axis, which is the propagation direction of the surface acoustic wave. It is preferable to use the direction rotated by 32 to 50 degrees about the axis, that is, (180, 58 to 40, 180 degrees) in the Euler angle display because the propagation loss is small. When the piezoelectric material substrate is made of LN, a direction rotated by 37.8 ° from the Z axis to the −Y axis around the X axis, which is the propagation direction of the surface acoustic wave, is expressed by Euler angles (0 °, 37.8). (0, 0 °) is preferable because the electromechanical coupling coefficient is large. Further, the size of the piezoelectric material substrate is not particularly limited, but is, for example, 100 to 200 mm in diameter and 0.15 to 1 μm in thickness.

In the present invention, an atomic layer having a thickness of 0.1 to 0.8 nm made of at least one atom selected from the group consisting of tantalum and niobium is provided on the bonding surface of the piezoelectric material substrate. For example, the atomic radius of tantalum is about 0.15 nm, and the thickness of a tantalum monolayer is about 0.30 nm. Therefore, the atomic layer formed according to the present invention needs to be extremely thin, from the state in which atoms are attached to the bonding surface so that the average does not reach a monoatomic layer in the plane, to about 3 atomic layers. . Such an extremely thin atomic layer promotes generation of a Ta (Nb) -O-Si bond or a Si-O-Si bond along a bonding interface between the piezoelectric material substrate and the silicon oxide film, and It has been found that the joining strength between the material substrate and the supporting substrate is significantly improved.

  From the viewpoint of the present invention, the thickness of the atomic layer formed on the bonding surface of the piezoelectric material substrate is more preferably 0.2 nm or more, and further preferably 0.6 nm or less.

  Further, the atomic layer formed on the bonding surface of the piezoelectric material substrate may be composed only of tantalum atoms or may be composed only of niobium atoms. Alternatively, it may be composed of two kinds of atoms selected from these.

  The atomic layer is preferably formed by a sputtering method or an ion-assisted evaporation method. In this method, atoms are attached to a bonding surface using a target composed of target atoms. At this time, in the present invention, a case where atoms are attached to the bonding surface at a density that does not reach the formation of a monoatomic layer is also defined as an atomic layer. Note that the thickness of the atomic layer is determined by the method described in Examples.

  In the case where the bonding surface of the piezoelectric material substrate and the bonding surface of the silicon oxide layer on the supporting substrate are directly bonded, it is preferable to planarize these and then activate them. Methods for flattening each bonding surface include lap polishing and chemical mechanical polishing (CMP). The flat surface preferably has Ra ≦ 1 nm, and more preferably 0.3 nm or less.

  Next, as a method of activating each bonding surface, the surface is preferably activated by a plasma irradiation method. In a preferred embodiment, the bonding surface is irradiated with plasma (N2, O2, Ar, etc.) in a low vacuum ((10 Pa) to activate the surface. After the irradiation, the exposed surfaces are brought into the air, and the joining surfaces are brought into contact with each other to be joined. After joining, heating is performed at 200 to 300 ° C. to improve joining strength.

The joined body of the present invention can be suitably used for an acoustic wave device.
As the acoustic wave device, a surface acoustic wave device, a Lamb wave device, a thin film resonator (FBAR), and the like are known. For example, a surface acoustic wave device has an input side IDT (Interdigital Transducer) electrode (also referred to as a comb-shaped electrode or an interdigital electrode) for exciting a surface acoustic wave and an output side for receiving a surface acoustic wave on a surface of a piezoelectric material substrate. IDT electrodes are provided. When a high-frequency signal is applied to the IDT electrode on the input side, an electric field is generated between the electrodes, and a surface acoustic wave is excited and propagates on the piezoelectric material substrate. Then, the propagated surface acoustic wave can be extracted as an electric signal from the output-side IDT electrode provided in the propagation direction.

  The material constituting the electrode pattern on the piezoelectric material substrate is preferably aluminum, an aluminum alloy, copper, or gold, and more preferably aluminum or an aluminum alloy. It is preferable to use an aluminum alloy obtained by mixing 0.3 to 5% by weight of Cu with Al. In this case, Ti, Mg, Ni, Mo, or Ta may be used instead of Cu.

(Experiment A)
According to the method described with reference to FIGS. 1 to 3, the joined body 7A shown in FIG. 3B was manufactured.

Specifically, a 42Y-cut LiTaO ₃ substrate (piezoelectric material substrate) 1 having a thickness of 0.2 mm and both surfaces of which are mirror-polished, and a high-resistance Si substrate (support substrate) 4 having a thickness of 0.5 mm are provided. Prepared. On the supporting substrate 4, a silicon oxide layer 5 was formed to a thickness of 0.5 μm by a sputtering method.

  The piezoelectric material substrate 1 was placed in a sputtering apparatus with the -Z plane serving as the bonding surface 1a facing upward. The sputtering device was a parallel plate type, and the distance between the Ta target and the surface of the piezoelectric material substrate was relatively large, 80 mm. This aims to finely control the amount of film formation by intentionally lowering the film formation rate. After the chamber was evacuated, Ar gas was introduced into the chamber to generate plasma. The discharge power at this time was 100 W. Sputter deposition was performed for 25 seconds so that Ta was uniformly deposited on the piezoelectric material substrate.

  Here, the thickness of the Ta atomic layer was measured as follows. That is, the film formation rate of Ta was investigated in advance as follows. First, a film of Ta was formed on a Si substrate under the same sputtering conditions as described above for 15 minutes (900 seconds). The substrate on which the film was formed was cut into small angles, and the thickness of Ta was measured by a transmission electron microscope (TEM) to find that it was 8.0 nm. Therefore, the Ta deposition rate under the above-described sputtering conditions is 8.0 / 900 (nm / sec). In this example, since the Ta sputtering time is 25 seconds, the thickness of the Ta atomic layer is 0.2 nm.

  Next, the bonding surface of the piezoelectric material substrate 1 and the silicon oxide layer on the support substrate 4 were cleaned and surface activated. Specifically, ultrasonic cleaning using pure water was performed, and the substrate surface was dried by spin drying. Next, each of the cleaned substrates was introduced into a plasma activation chamber, and each bonding surface was activated with nitrogen gas plasma for 40 seconds. Next, the same ultrasonic cleaning and spin drying as described above were performed again in order to remove particles attached during the surface activation.

  Next, each substrate was aligned, and the activated surfaces of both substrates were brought into contact at room temperature. Since the piezoelectric material substrate 1 was contacted with its side facing upward, a state in which the substrates were closely adhered to each other (a so-called bonding wave) was observed, and it was confirmed that the preliminary bonding was successfully performed. At this point, the bonding strength was low, and therefore, it was put into an oven in a nitrogen atmosphere and kept at 120 ° C. for 10 hours for the purpose of increasing the bonding strength. Before the substrate was further placed in a high-temperature oven, the substrate was divided into eight equal parts along radiation from the center to prevent damage to the substrate due to thermal stress.

A blade test (Semiconductor) was performed on the eight joints taken out of the oven.
Wafer Bonding, Q.-Y. Tong & U. Goesele, p25) was used to evaluate the bonding strength. When the strength was calculated from the respective physical property values of lithium tantalate and silicon and the insertion distance of the blade, it was found that a bonding strength of 2.7 J / m ² was obtained in an average of eight times.

  Next, another joined body was further prepared under the same joining conditions as described above. Then, the surface of the piezoelectric material substrate made of lithium tantalate of the joined body was ground and polished to reduce the thickness. As a result, polishing was possible without peeling off from the supporting substrate until the thickness of the piezoelectric material substrate became 2 μm. From this, it was found that the present invention can realize a bonding strength that can withstand polishing.

  Next, in the same manner as described above, the joined bodies of the respective examples shown in Table 1 were further produced. However, the thickness of each Ta atomic layer was changed to 0.04, 0.1, 0.2, 0.4, 0.8, 1.0, and 1.6 nm by variously changing the Ta film formation time. Was changed to. Other than that, the joined body of each example was produced in the same manner as in the above-described example, the joining strength was measured, and polishing was performed. However, the polishing was continued until the thickness of the piezoelectric material substrate reached 2 μm. Table 1 shows the results.

  From the results in Table 1, it can be seen that by providing an atomic layer made of tantalum atoms with a thickness of 0.1 to 0.8 nm on the piezoelectric material substrate, the bonding strength to the silicon oxide layer on the supporting substrate is significantly improved, and polishing is performed. It was found that no peeling occurred during processing.

Further, as a comparative example, a bonded substrate was prepared under exactly the same conditions as in the above-described example except that a piezoelectric material substrate on which no Ta film was formed was used. As a result, the joint strength of the joined body was 1.3 J / m ² , which was lower than that of the example. When the piezoelectric material substrate of the joined body was polished, when the thickness of the piezoelectric material substrate reached about 12 μm, the end of the joined body was peeled off. As the thickness was further reduced, the peeled area increased at a stretch. The results are also shown in Table 1.

(Experiment B)
In the experiment A, the material of the piezoelectric material substrate was changed to lithium niobate, and Nb was adhered to the joining surface of the piezoelectric material substrate by sputtering using Nb target instead of Ta. Then, as in Experiment A, the thickness of the Nb atomic layer was changed as shown in Table 2 by changing the Nb deposition time.

  For each of the obtained joined bodies, the joining strength and the presence or absence of peeling during polishing were measured in the same manner as in Experiment A. Table 2 shows the results. Although the bonding strength was slightly lower than in the case of lithium tantalate, it was found that the preferred range of the thickness of the Nb atomic layer showed the same tendency as in Example 1 in which the Ta atomic phase was provided.

Claims (11)

A method for bonding a piezoelectric material substrate made 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,
Providing, on the piezoelectric material substrate, an atomic layer having a thickness of 0.1 to 0.8 nm consisting of only one or more atoms selected from the group consisting of tantalum and niobium, and a bonding surface of the piezoelectric material substrate Bonding the piezoelectric material to the silicon oxide layer.   2. The piezoelectric material substrate according to claim 1, wherein after joining the bonding surface of the piezoelectric material substrate to the silicon oxide layer, the thickness of the piezoelectric material substrate is reduced by polishing. 3. Joining method.   3. The method according to claim 1, wherein the atomic layer is provided on the bonding surface of the piezoelectric material substrate by a sputtering method or a vapor deposition method. 4.   After irradiating the bonding surface of the piezoelectric material substrate with plasma to activate the bonding surface, the bonding surface of the piezoelectric material substrate is bonded to the silicon oxide layer. The method for bonding a piezoelectric material substrate according to claim 3.   5. The method according to claim 1, wherein the bonding surface of the piezoelectric material substrate is bonded to the silicon oxide layer after the silicon oxide layer is activated by irradiating the silicon oxide layer with plasma. A method for bonding a piezoelectric material substrate according to any one of the preceding claims.   The method according to any one of claims 1 to 5, wherein the supporting substrate is made of a material selected from the group consisting of silicon, quartz, sialon, mullite, sapphire, and translucent alumina. A method for joining a piezoelectric material substrate. Support substrate,
A silicon oxide layer provided on a supporting substrate,
A piezoelectric material substrate made of a material selected from the group consisting of lithium niobate, lithium tantalate, and lithium niobate-lithium tantalate; and a piezoelectric material substrate provided on the piezoelectric material substrate, selected from the group consisting of tantalum and niobium. An atomic layer having a thickness of 0.1 to 0.8 nm composed of only one or more types of atoms, wherein a bonding surface of the piezoelectric material substrate is bonded to the silicon oxide layer. A bonded body of a piezoelectric material substrate.   The joined body according to claim 7, wherein the joining surface of the piezoelectric material substrate is activated by plasma.   9. The joined body according to claim 7, wherein the silicon oxide layer is activated by plasma.   The method according to any one of claims 7 to 9, wherein the support substrate is made of a material selected from the group consisting of silicon, quartz, sialon, mullite, sapphire, and translucent alumina. Joint. An acoustic wave device comprising: the joined body according to any one of claims 7 to 10; and an electrode provided on the piezoelectric material substrate.

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