JP7561202B2 - Bonded Substrate - Google Patents

Description

The present invention relates to a bonded substrate.

Surface acoustic wave elements (SAW elements) are fabricated using a bonded substrate in which a piezoelectric substrate such as lithium tantalate (LT) or lithium niobate (LN) is bonded to a support substrate such as silicon. For example, such bonded substrates have an intermediate film on the bonding surface, as described in Patent Documents 1 and 2.

If the bonding strength of the bonded substrate is low, it may peel off during processing such as machining and heat treatment. For example, if the bonded substrate is exposed to a high-temperature environment, the difference in thermal expansion between the piezoelectric substrate and the support substrate increases the likelihood of the piezoelectric substrate and the support substrate peeling off. In order to prevent peeling between the piezoelectric substrate and the support substrate, the bonded substrate is required to have high bonding strength.

Patent No. 3774782 Patent No. 6621561

The bonded substrate according to the present disclosure includes a support substrate made of single crystal silicon, a piezoelectric substrate made of single crystal lithium tantalate or single crystal lithium niobate, and a bonding layer for bonding the support substrate and the piezoelectric substrate. The bonding layer is an amorphous bonding layer that contains silicon and the constituent elements of the piezoelectric substrate as main components and also contains carbon.

FIG. 1 is a cross-sectional view illustrating a bonded substrate according to an embodiment of the present disclosure. FIG. 1 is a schematic diagram showing a fast atom beam irradiation device. 1 is a graph showing the results of a qualitative analysis, by X-ray photoelectron spectroscopy (XPS), of a bonding interface between a support substrate and a bonding layer in a bonded substrate according to an embodiment of the present disclosure. 1 is a graph showing the results of a state analysis by XPS of a bonding interface between a support substrate and a bonding layer in a bonded substrate according to an embodiment of the present disclosure.

As mentioned above, conventional bonded substrates with low bonding strength are likely to cause the piezoelectric substrate and the support substrate to peel off during processing or heat treatment. Therefore, there is a demand for bonded substrates in which the piezoelectric substrate and the support substrate are firmly bonded.

In the bonded substrate according to the present disclosure, the bonding layer for bonding the support substrate and the piezoelectric substrate is an amorphous bonding layer that contains silicon and the constituent elements of the piezoelectric substrate as main components and also contains carbon. Therefore, according to the present disclosure, it is possible to provide a bonded substrate in which the piezoelectric substrate and the support substrate are firmly bonded.

The bonded substrate according to the present disclosure will now be described with reference to the drawings. Figure 1 is a cross-sectional view showing a schematic diagram of a bonded substrate according to one embodiment of the present disclosure.

The bonded substrate 1 according to one embodiment shown in Figure 1 has a structure in which a support substrate 2 formed of single crystal silicon and a piezoelectric substrate 3 formed of single crystal lithium tantalate (LT) or single crystal lithium niobate (LN) are bonded together via a bonding layer 4 formed between the support substrate 2 and the piezoelectric substrate 3.

The support substrate 2 is made of single crystal silicon and is used to support the piezoelectric substrate 3 in the bonded substrate 1. The thermal expansion coefficient of the support substrate 2 is smaller than that of the piezoelectric substrate 3. The thickness of the support substrate 2 is not limited and is, for example, 0.1 mm or more and 1.0 mm or less.

The piezoelectric substrate 3 is provided on the surface of the support substrate 2 via a bonding layer 4 described later. The piezoelectric substrate 3 is a piezoelectric material film supported by the support substrate 2. The piezoelectric substrate 3 is formed of a single crystal of lithium tantalate or a single crystal of lithium niobate. The piezoelectric substrate 3 has a thin film shape that is processed, for example, by grinding and polishing to a thickness of about 2 μm to 50 μm. It is preferable that the piezoelectric substrate 3 has a single polarization.

The bonding layer 4 bonds the support substrate 2 and the piezoelectric substrate 3. The bonding layer 4 is an amorphous bonding layer containing silicon, which is a constituent element of the support substrate 2, and the constituent elements of the piezoelectric substrate 3 (lithium, tantalum, niobium, and oxygen), as main components, and also containing carbon. "Main component" means that the target element is contained in the bonding layer 4 at a ratio of 50 atm% or more (hereinafter, the element ratio of the bonding layer 4 indicates the atomic ratio (atm%) unless otherwise specified). The bonding layer 4 is located on the support substrate 2 side and is composed of a first bonding layer mainly composed of silicon, and a second bonding layer located on the piezoelectric substrate 3 side and is mainly composed of the constituent elements of the piezoelectric substrate 3. The existence of the bonding layer 4 and the first and second bonding layers can be confirmed by a cross-sectional TEM (transmission electron microscope) image. The composition of the main component elements can be confirmed by energy dispersive X-ray analysis (EDS). In addition to silicon, the constituent elements of the piezoelectric substrate 3, and carbon, the bonding layer 4 may also contain elements (e.g., Ar) irradiated in the activation process for bonding. The first bonding layer preferably contains silicon at a ratio of, for example, about 50% to 99%. The thickness of the bonding layer 4 is not limited, and is preferably, for example, about 1 nm to 50 nm.

The bonding layer 4, i.e., the first bonding layer and the second bonding layer, may contain, for example, 1% to 10% carbon. By containing carbon in such a ratio in the bonding layer 4, the support substrate 2 and the piezoelectric substrate 3 are more firmly bonded. The presence or absence of carbon can be analyzed, for example, by X-ray photoelectron spectroscopy (XPS).

Si-C bonds may be present at the bonding interface between the support substrate 2 and the bonding layer 4. The atomic distance of the Si-C bond (1.88 Å) is closer to the atomic distance of the Ta-O bond in lithium tantalate (1.89 Å or 2.08 Å) and the atomic distance of the Nb-O bond in lithium niobate (1.92 Å or 2.06 Å) than the atomic distance of the Si-Si bond (2.35 Å). Therefore, crystal defects are less likely to occur at the bonding interface between the piezoelectric substrate 3 and the bonding layer 4, and the bonding strength between the support substrate 2 and the piezoelectric substrate 3 is further improved.

Whether or not Si-C bonds are present at the bonding interface between the support substrate 2 and the bonding layer 4 can be analyzed, for example, by the above-mentioned XPS. When the state of the bonding interface between the support substrate 2 and the bonding layer 4 is analyzed by XPS and a peak derived from the Si-C bond is detected, it can be recognized that the Si-C bond is present.

The method for manufacturing the bonded substrate 1 according to one embodiment of the present disclosure is not limited. The bonded substrate 1 according to one embodiment can be obtained, for example, by the following method.

First, the support substrate 2 and the piezoelectric substrate 3 are prepared. The support substrate 2 is made of single crystal silicon. The piezoelectric substrate 3 is made of single crystal lithium tantalate or single crystal lithium niobate. Below, the piezoelectric substrate 3 will be described in the case where a piezoelectric substrate made of single crystal lithium tantalate is used.

The surfaces of the substrate made of single crystal silicon and the substrate made of single crystal lithium tantalate may be subjected to a planarization process. For example, it is preferable that the surface roughness of the bonding surface of each substrate is set to 1.0 nm or less in terms of arithmetic mean roughness Ra.

Next, the surface of the support substrate 2, which will be the bonding surface, and the surface of the piezoelectric substrate 3 are activated with neutral particles (atoms or molecules) or charged particles (ions, plasma, or electrons). There are no limitations on the activation method, and an example is a method of irradiating a fast atom beam using a fast atom beam (hereinafter sometimes referred to as "Fab") gun. If a Fab gun is used, for example, a highly concentrated, electrically neutralized Ar atom beam can be obtained.

For example, a Fab gun is configured as follows: A plate-shaped anode electrode and two cathode electrodes are arranged in parallel. An inert gas (Ar in this embodiment) is injected under high vacuum, and a high voltage is applied to the central anode electrode to cause glow discharge. The electric field distribution in the plasma is configured so that the direction of ion acceleration is perpendicular to the cathode electrode on the exit side. The accelerated ions combine with electrons near the cathode electrode and are emitted as a high-speed atomic beam.

Therefore, the Fab emitted from the Fab emission hole at the exit becomes a beam with excellent linearity. Therefore, by using a Fab gun, Fab irradiation is performed under high reduced pressure. As a result, inactive films such as adsorbed molecules and oxide films on the substrate surface are removed by the inert gas beam (i.e., Ar neutral atom beam), exposing the active surface.

Figure 2 shows a schematic diagram of a fast atom beam (Fab) irradiation device. The Fab irradiation device (bonding device) used in this embodiment is equipped with two Fab guns 5, one for the support substrate 2 and one for the piezoelectric substrate 3. A plate (aperture plate) 6 having an opening is disposed at each of the irradiation ports on the front of the Fab guns 5. The support substrate 2 and the piezoelectric substrate 3 are disposed opposite each other, and each is activated by Fab irradiation. Due to the configuration of the device, Fab is irradiated obliquely onto the support substrate 2 and the piezoelectric substrate 3.

Here, a carbonaceous target can be placed in the bonding device and the target can also be irradiated with Fab, thereby allowing carbon to be contained in the bonding layer 4. The method of containing carbon in the bonding layer 4 is not limited to this method.

When Fab irradiation is performed on both the support substrate 2 and the piezoelectric substrate 3, the bonding strength is further improved. Furthermore, the temperature of both substrates during bonding can be adjusted by adjusting the amount of irradiation (irradiation energy and irradiation time) or the time from the end of irradiation to bonding, thereby adjusting the amount of thermal expansion. For example, the amount of irradiation on the piezoelectric substrate 3 (lithium tantalate), which has a larger thermal expansion coefficient than the support substrate 2 (single crystal silicon), can be reduced to suppress temperature rise.

The amount of irradiation by the Fab gun may be adjusted by adjusting the irradiation angle of the Fab relative to the irradiation surface. For example, the closer the irradiation angle of the Fab gun is to perpendicular to the irradiation surface, the greater the amount of irradiation. Furthermore, the amount of irradiation may be adjusted by the irradiation energy of the Fab gun. The irradiation energy can be controlled by adjusting the current value, particularly the current value, which are the Fab irradiation conditions, the current value, the acceleration voltage value, and the irradiation time. The irradiation energy required for the amount of irradiation is not limited, and is preferably about 20 kJ or more and 80 kJ or less.

Next, after activation by Fab irradiation, the support substrate 2 and the piezoelectric substrate 3 are bonded together. Specifically, the surface of the support substrate 2 activated by Ar and the surface of the piezoelectric substrate 3 are brought together and pressure is applied to bond them. Because the surfaces are activated, bonding without heating (room temperature bonding) is possible. This bonding forms a bonding layer 4 between the support substrate 2 and the piezoelectric substrate 3. The pressure used during bonding should be approximately 0.5 kN or more and 20 kN or less.

Next, the piezoelectric substrate 3 is subjected to grinding and polishing processes to thin it to the desired thickness (for example, about 2 μm to 50 μm). After that, it is subjected to heat treatment as necessary to obtain the bonded substrate 1.

Specifically, a bonded substrate 1 according to one embodiment was obtained by the following procedure using a substrate (diameter 100 mm) formed of a silicon single crystal and a substrate (diameter 100 mm) formed of a lithium tantalate single crystal.

First, a planarization process was performed so that the surfaces of the substrate (support substrate 2) formed of single crystal silicon and the substrate (piezoelectric substrate 3) formed of single crystal lithium tantalate had an arithmetic mean roughness Ra of 1.0 nm or less. Next, as shown in Figure 2, two Fab guns 5 and a carbon-containing aperture plate 6 were used to perform an activation process on the surfaces of the substrate formed of single crystal silicon and the substrate formed of single crystal lithium tantalate. The irradiation energy of the Fab guns 5 was set to about 30 kJ in both cases.

Next, the surfaces of the substrates that had been subjected to the activation treatment were brought together and pressed with a force of about 5 kN to form a bonding layer 4 (about 5 nm thick) between the substrate made of silicon single crystal and the substrate made of lithium tantalate single crystal. Then, grinding and polishing were performed and a heat treatment was performed so that the thickness of the substrate made of lithium tantalate single crystal was about 15 μm. In this way, a bonded substrate 1 having a thickness of 245 μm according to one embodiment was obtained.

The first bonding layer of the obtained bonded substrate 1 according to one embodiment was analyzed by XPS. The qualitative analysis results are shown in FIG. 3, and the state analysis results are shown in FIG. 4.

The qualitative analysis results shown in Figure 3 show the presence of a peak (C1s) derived from carbon. This carbon is thought to originate from the aperture plate 6. In this qualitative analysis, the detection limit for carbon is approximately 1%. Therefore, the presence of a carbon peak indicates that the bonding interface between the support substrate 2 and the bonding layer 4 contains carbon at a rate of 1% or more. "Fab non-irradiation" in the graph of Figure 3 and the graph of Figure 4 described below indicates the qualitative analysis results of the surface of a silicon single crystal substrate that has not been subjected to activation treatment by Fab irradiation.

Furthermore, when the bonding interface between the support substrate 2 and the bonding layer 4 was analyzed by Rutherford backscattering spectroscopy (RBS) and nuclear reaction analysis (NRA), which have a carbon detection limit of about 10%, no carbon was detected. Therefore, it can be seen that the proportion of carbon contained in the bonding layer 4 in the bonded substrate 1 according to one embodiment is about 1% or more and 10% or less.

Next, the state analysis results shown in Figure 4 show that there is a peak due to the Si-C bond. Therefore, it is clear that the Si-C bond is present at the bonding interface between the support substrate 2 and the bonding layer 4.

Although not specifically shown, similar behavior was observed when a bonded substrate using lithium niobate as the piezoelectric substrate 3 was used instead of a substrate formed of lithium tantalate single crystal.

REFERENCE SIGNS LIST 1 Bonding substrate 2 Support substrate 3 Piezoelectric substrate 4 Bonding layer 5 Fab gun 6 Plate having an opening (aperture plate)

Claims (4)

A support substrate made of single crystal silicon;
A piezoelectric substrate formed of a single crystal of lithium tantalate or a single crystal of lithium niobate; and a bonding layer for bonding the support substrate and the piezoelectric substrate.
Including,
A method for manufacturing a bonded substrate, wherein the bonding layer is an amorphous layer containing silicon and a constituent element of the piezoelectric substrate as main components and containing carbon,
a surface of the support substrate and a surface of the piezoelectric substrate are directly opposed to each other without a shielding plate therebetween, and an activation process is performed by Fab irradiation together with a target containing carbon, and then the support substrate and the piezoelectric substrate are bonded to each other to form the bonding layer.
A method for manufacturing a bonded substrate. The method for manufacturing a bonded substrate according to claim 1 , wherein the bonding layer includes a first bonding layer located on the support substrate side and containing silicon as a main component and carbon. 3. The method for producing a bonded substrate according to claim 1, wherein a Si--C bond is present at a bonding interface between the support substrate and the bonding layer. 4. The method for producing a bonded substrate according to claim 1, wherein the carbon is contained in the bonding layer at a ratio of 1 atm % to 10 atm %.

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