Piezoelectric oxide single crystal composite substrate and method for manufacturing the same

Abstract

This invention provides an oxide single-crystal composite substrate and a method for manufacturing the same that can achieve both the recovery of polarization and crystallinity in LT and LN thin films and the non-low resistance reduction of the polysilicon layer in trap-rich silicon. [Solution] The method for manufacturing a piezoelectric oxide single crystal composite substrate according to an embodiment of the present invention is characterized by transferring and bonding a thin film of piezoelectric oxide single crystal onto a crystalline silicon support substrate having a polysilicon layer as the uppermost layer using an ion implantation exfoliation method, then performing heat treatment in a forming gas with a hydrogen concentration of 1 to 5% at a temperature of 300 to 400°C, and then performing heat treatment at a temperature of 400°C or higher in a non-reducing atmosphere.

Description

Technical Field

[0001] The present invention relates to a piezoelectric oxide single crystal composite substrate used as a material for electronic devices and the like, and a method for manufacturing the same.

Background Art

[0002] In recent years, in the market of mobile communication represented by smartphones, the communication volume has been increasing rapidly. In order to increase the number of bands required to address this problem, miniaturization and high performance of various components have inevitably become essential. Lithium tantalate (sometimes abbreviated as LT), which is a common piezoelectric material, and lithium niobate (sometimes abbreviated as LN) are widely used as materials for surface acoustic wave (SAW) devices. These materials have a large electromechanical coupling coefficient and have the advantage of being able to achieve wide bandwidth.

[0003] It is known that thinning these LTs and LNs (to 10 μm or less) improves their characteristics and expands the scope of applications. Specifically, high-performance filter devices and optical modulators can be mentioned.

[0004] When thinning, methods such as grinding and polishing LT and LN are usually used, but it is difficult to obtain film thickness uniformity at the nanometer level (nm) over the entire surface of the substrate. Generally, it is an extremely difficult task to thin a substrate of 200 - 300 μm to several hundred nm by grinding and polishing methods and obtain nanometer-level uniformity. Also, as a support substrate, it is common to use a silicon substrate with polysilicon deposited on the upper layer. This is to capture carriers such as electrons and holes induced in silicon during device operation at the grain boundaries of polysilicon and suppress deterioration of dielectric loss as much as possible. This silicon substrate with polysilicon is sometimes referred to as trap-rich silicon. It is known that the completed device exhibits a high Q value.

[0005] One method for obtaining thin films of LT or LN with good uniformity is the ion implantation exfoliation method (see, for example, Patent Document 1). This method involves implanting light elements such as hydrogen and helium into a target substrate and exfoliating at the point where the concentration is maximum (ion implantation interface). In this method, it is necessary to bond the LT or LN to a support substrate, perform some heat treatment, and then exfoliate using a method such as the SiGen method (mechanical exfoliation method). In the ion implantation exfoliation method, polishing is required after exfoliation to remove the ion implantation damage layer (approximately 150 nm) and to make the surface mirror-like. The problem here is that LT and LN have polarization, and this polarization is an essential element for embodying the properties of a piezoelectric element. Thin films of LT or LN that have undergone ion implantation exfoliation are damaged by ion implantation, and in order to restore the disordered polarization and crystallinity caused by ion implantation, some heat treatment is necessary to return them to properties close to those of bulk LT or LN.

[0006] If an oxidizing or inert atmosphere (nitrogen or argon) is used during this heat treatment, the LT and LN thin films will become insulated. This can lead to the localization of charges within the thin film, disrupting the polarization and preventing the maintenance of a single polarization. To prevent this, a reducing atmosphere, more specifically a hydrogen atmosphere, can be used to maintain the reducing properties of the thin film and restore its polarization and crystallinity. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] International Publication No. 2016 / 088466 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] However, there is one problem with this method of restoring the polarization and crystallinity of LT and LN thin films using a hydrogen atmosphere. Specifically, heat treatment using a hydrogen atmosphere causes the underlying trap-rich silicon polysilicon layer to become less resistive, leading to a degradation of the device's characteristics, more specifically, the Q factor (an indicator of signal sharpness). This is likely because hydrogen diffuses into the polysilicon layer, terminating the dangling bonds at the grain boundaries and hindering the polysilicon's expected function of capturing carriers. However, hydrogen treatment is essential for restoring the polarization and crystallinity of LT and LN thin films, making the use of hydrogen unavoidable.

[0009] This invention has been made in view of the above circumstances, and aims to provide an oxide single-crystal composite substrate and a method for manufacturing the same that can achieve both the recovery of polarization and crystallinity in thin films of LT and LN and the non-low resistance reduction of the polysilicon layer of trap-rich silicon. [Means for solving the problem]

[0010] To solve this problem, the inventors devised and implemented a method of using a reducing atmosphere with the hydrogen concentration kept to a minimum, specifically a mixed gas of hydrogen and an inert gas (forming gas), for heat treatment to restore the polarization and crystallinity of LT and LN after delamination.

[0011] In other words, in order to solve the above problems, the method for manufacturing a piezoelectric oxide single crystal composite substrate according to an embodiment of the present invention is characterized by transferring and bonding a thin film of piezoelectric oxide single crystal onto a crystalline silicon support substrate having a polysilicon layer as the uppermost layer using an ion implantation exfoliation method, then performing heat treatment in a forming gas with a hydrogen concentration of 1 to 5% at a temperature of 300 to 400°C, and then performing heat treatment at a temperature of 400°C or higher in a non-reducing atmosphere.

[0012] In this invention, the support substrate and the piezoelectric oxide single crystal are preferably joined via an intervening layer.

[0013] In the present invention, lithium tantalate or lithium niobate may be used as the piezoelectric oxide single crystal.

[0014] In the present invention, the atmosphere in the heat treatment performed at a temperature of 400 °C or higher is preferably a non-reducing atmosphere composed of nitrogen, argon, helium, oxygen, or a mixed gas thereof.

[0015] In the present invention, the intervening layer may be any one of SiO2, SiON, SiN, and SiO x (x = 1.5 to 2.5), or may contain any of these.

[0016] Further, the piezoelectric oxide single crystal composite substrate according to an embodiment of the present invention is characterized in that a thin film of a piezoelectric oxide single crystal is bonded onto a support substrate of crystalline silicon having a polysilicon layer on the uppermost layer.

[0017] In the present invention, it is preferable to have an intervening layer between the support substrate and the thin film of the piezoelectric oxide single crystal.

[0018] In the present invention, the piezoelectric oxide single crystal is preferably lithium tantalate or lithium niobate.

[0019] In the present invention, the intervening layer is preferably any one of SiO2, SiON, SiN, and SiO x (x = 1.5 to 2.5), or may contain any of the above.

Brief Description of Drawings

[0020] [Figure 1] It is a schematic diagram showing the structure of the piezoelectric oxide single crystal composite substrate 1. [Figure 2] It is a diagram showing the results of Reference Experiment 2. [[ID=4'0]] [Figure 3] It is a diagram showing the results of Reference Experiment 3. [Figure 4] It is a flowchart showing a method for manufacturing the piezoelectric oxide single crystal composite substrate 1.

Mode for Carrying Out the Invention

[0021] Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[0022] FIG. 1 is a schematic diagram showing the structure of a piezoelectric oxide single crystal composite substrate 1 according to the present embodiment. As shown in FIG. 1, the piezoelectric oxide single crystal composite substrate 1 has a structure in which a thin film of a piezoelectric oxide single crystal 3 is laminated on a support substrate 2 via an intervening layer 4 as required.

[0023] For the support substrate 2, crystalline silicon (so-called trap-rich silicon) having a polysilicon layer on the outermost layer is used. [[ID=1�]]

[0024] The piezoelectric oxide single crystal 3 is preferably lithium tantalate (LT) or lithium niobate (LN), and black LT or black LN which has been subjected to a blackening treatment to increase the conductivity is particularly preferable. Here, black LT is LT which has been subjected to a treatment to increase the conductivity by slightly reducing the amount of oxygen from the stoichiometric ratio. Since the color of LT changes from its original color to black with this treatment, this treatment is called a blackening treatment. Also, LT subjected to the blackening treatment is called black LT (Black-LT). Similarly, LN subjected to the blackening treatment is called black LN.

[0025] The piezoelectric oxide single crystal 3 is transferred as a thin film onto the support substrate 2 by a so-called ion implantation and peeling method. Since the thin film of the piezoelectric oxide single crystal 3 is in a state where polarization and crystallinity are disturbed due to damage caused by ion implantation, a treatment for recovering these is required.

[0026] The intervening layer 4 may be any of SiO2, SiON, SiN, and SiO x (x = 1.5 to 2.5).

[0027] Next, we will describe reference experiments conducted to clarify the conditions under which both the recovery of polarization and crystallinity in thin films of LT and LN, which are used as piezoelectric oxide single crystals 3, and the non-low resistance reduction of the polysilicon layer in trap-rich silicon can be achieved.

[0028] [Reference Experiment 1] The conductivity is 4.73 × 10⁻⁶. -11 A black LT with a diameter of 100 mm and a thickness of 0.35 mm and a density of ( / Ωcm) was prepared. Using the prepared black LT, an LT on SiO2 on Si substrate was fabricated by stacking a 500 nm thick LT layer and a 500 nm intermediate layer of SiO2 on a Si substrate using ion implantation exfoliation. This substrate was then subjected to sequential heat treatment for 6 hours at temperatures of 250°C, 300°C, 350°C, 400°C, 450°C, and 500°C. During this process, as shown in Table 1, the atmosphere was changed from a reducing atmosphere (hydrogen atmosphere) to an atmospheric atmosphere (20% oxygen + 80% nitrogen) at seven different temperatures. Devices were fabricated on these heat-treated substrates, and the specific bandwidth was measured. The results are shown in Table 1. These results show that samples heat-treated in a reducing atmosphere at temperatures above 300°C (conditions 3 to 7) exhibit good specific bandwidth (i.e., the polarization of the LT is restored), while samples treated in a reducing atmosphere at temperatures below 250°C (i.e., conditions 1 and 2) show degraded specific bandwidth. Therefore, it is necessary to perform heat treatment in a reducing atmosphere at temperatures above 300°C. It is thought that polarization will not be restored at lower heat treatment temperatures. Furthermore, it has been empirically proven that specific bandwidth measurement is a useful indicator for showing the characteristics of the LT thin film alone.

[0029] [Table 1]

[0030] [Reference Experiment 2] The conductivity is 1.97 × 10⁻⁶. -11A black LT substrate with a diameter of 100 mm and a thickness of 0.35 mm, with a density of ( / Ωcm), was prepared. This substrate was heat-treated at 400°C in a 100% hydrogen atmosphere or in a foaming gas atmosphere (hydrogen concentration of 0.5-4%) obtained by diluting hydrogen with nitrogen. The treatment time was 6 hours. The conductivity after treatment is shown in Figure 2. Substrates with a hydrogen concentration of 1% or higher showed higher conductivity than the original, indicating reducing properties, while those with a hydrogen concentration below 1% showed lower conductivity than the original, indicating non-reducing properties. From these results, it can be seen that a hydrogen concentration of 1% or higher is sufficient to exhibit reducing properties.

[0031] [Reference Experiment 3] A silicon substrate with approximately 1.7 μm of polysilicon deposited was subjected to heat treatment at 400°C for 6 hours in a nitrogen atmosphere, air atmosphere, hydrogen atmosphere, and various foaming gases. The resistivity of this polysilicon layer was measured using the spreading resistance (SR) method. The resistivity under each condition is shown in Figure 3. The measurement point was set near the midpoint of the polysilicon layer (approximately 0.85 μm from the surface). From Figure 3, it can be seen that the hydrogen concentration should be kept below 5% to avoid drastically reducing the original resistivity (3850 Ωcm). It was found that if the hydrogen concentration is higher than this (e.g., 10%), the polysilicon layer becomes less resistive, as shown in Figure 3.

[0032] Based on findings from reference experiments 1-3, it was determined that the temperature at which the forming gas is used should preferably be between 300°C and 400°C, and that the hydrogen concentration in the forming gas should preferably be between 1% and 5%.

[0033] Next, the manufacturing method of the piezoelectric oxide single crystal composite substrate 1, based on the above reference experiments, will be explained with reference to the flowchart shown in Figure 4.

[0034] First, a support substrate 2 and a piezoelectric oxide single crystal 3 to be bonded are prepared (step S01). The support substrate 2 is made of crystalline silicon having a polysilicon layer as the uppermost layer (so-called trap-rich silicon). The piezoelectric oxide single crystal 3 is preferably lithium tantalate (LT) or lithium niobate (LN), and black LT or black LN that has been blackened to increase conductivity is particularly preferred.

[0035] Ion implantation is performed on the piezoelectric oxide single crystal 3 from the bonding surface side so that the desired depth becomes the ion implantation interface (step S02). The ions to be implanted should preferably be light elements such as hydrogen and helium.

[0036] Furthermore, an intervening layer 4 is formed on one or both of the prepared support substrate 2 and piezoelectric oxide single crystal 3 as needed (step S03). Then, the bonding surface between the support substrate 2 and the piezoelectric oxide single crystal 3 is flattened, and surface activation treatments such as ozonated water treatment, UV ozone treatment, ion beam treatment, and plasma treatment are applied as needed (step S04). Then, the support substrate 2 and the piezoelectric oxide single crystal 3 are bonded together (through the intervening layer 4 if one is provided) (step S05). Then, the piezoelectric oxide single crystal 3 is separated (exfoliated) at the exfoliation position (ion implantation interface) of the piezoelectric oxide single crystal 3 on the bonded substrate (step S06). In this way, a thin film of piezoelectric oxide single crystal 3 is transferred onto the support substrate 2 (so-called ion implantation exfoliation method). After that, the surface of the thin film is polished to remove the ion implantation damage layer (approximately 150 nm) (step S07).

[0037] Next, a heat treatment is performed in a forming gas with a hydrogen concentration of 1-5% at a temperature of 300-400°C (Step S08). The forming gas may be hydrogen diluted with nitrogen or argon. After that, a heat treatment is performed at a temperature of 400°C or higher in a non-reducing atmosphere (Step S09). The non-reducing atmosphere at this time may be an atmosphere composed of nitrogen, argon, helium, oxygen, or a mixture of these gases.

[0038] By the above manufacturing method, a piezoelectric oxide single crystal composite substrate 1 can be obtained in which the polarization and crystallinity of the piezoelectric oxide single crystal 3 in the thin film are restored while suppressing the non-low resistance of the support substrate 2.

[0039] In the following, a method for manufacturing a piezoelectric oxide single crystal composite substrate, including heat treatment in a forming gas, will be described with reference to examples, illustrating the conditions under which a piezoelectric oxide single crystal composite substrate with desirable properties can be obtained.

[0040] [Example 1] Using the same LT substrate as in Reference Experiment 1, an LT on SiO2 on Si substrate was fabricated by laminating a 500 nm thick LT layer and a 500 nm interlayer of SiO2 on a Si substrate using ion implantation exfoliation. The silicon substrate used had a 1.7 μm polysilicon layer deposited on top. This substrate was subjected to heat treatment at 350°C for 6 hours and then at 500°C for 6 hours. The atmosphere at 350°C ranged from 0.5% to 100% hydrogen, while the 500°C heat treatment was performed in an air atmosphere. Devices were fabricated on these substrates, and the specific bandwidth and Q-value were measured. The results are shown in Table 2. Substrates with hydrogen atmospheres of 1% or more and 5% or less showed good specific bandwidth (indicating the quality of the LT thin film) and Q-value (indicating the properties of the underlying silicon). The substrate with 0.5% hydrogen did not recover its polarization and therefore had a poor specific bandwidth, making it impossible to measure the Q-value.

[0041] [Table 2]

[0042] [Example 2] Using the same LT substrate as in Example 1, an LT on SiO2 on Si substrate was fabricated by laminating a 500 nm thick LT layer and a 500 nm layer of SiO2 interlayer on a Si substrate using ion implantation exfoliation. This substrate was heat-treated at a temperature of 200-450°C for 6 hours in a 4% hydrogen atmosphere, followed by heat treatment at 500°C for 6 hours in an air atmosphere. The results are shown in Table 3. The samples heat-treated at a temperature of 300-400°C in a 4% hydrogen atmosphere showed good specific bandwidth and Q-value. For samples heat-treated at 200°C and 250°C in a 4% hydrogen atmosphere, the Q-value could not be measured, possibly because polarization recovery did not occur.

[0043] [Table 3]

[0044] [Example 3] The same experiments as in Example 1 and Example 2 were performed using an LN substrate instead of an LT substrate, and the results were almost the same.

[0045] [Example 4] In the experiments of Examples 1 to 3, the heat treatment at 500°C was performed using nitrogen, argon, helium, or oxygen instead of air, but the results were exactly the same.

[0046] [Example 5] Experiments 1-3 were conducted by changing the foaming gas from nitrogen to argon, but the results were exactly the same. It was found that any inert gas can be used as the atmosphere for diluting hydrogen, not just nitrogen.

[0047] [Example 6] In the experiments of Examples 1 to 3, structures without an intervening layer were used, but the results were exactly the same. It was found that the present invention is not affected by the presence or absence of an intervening layer.

[0048] [Example 7] In the experiments of Examples 1-3, the intervening layer was replaced with SiON, SiN, and SiO2. xWhen applying the range (x=1.5~2.5), the results were exactly the same.

[0049] The present invention is not limited to the embodiments described above. The embodiments described above are illustrative, and any configuration that is substantially identical to the technical idea described in the claims of the present invention and produces similar effects is included within the technical scope of the present invention. In other words, modifications can be made as appropriate within the scope of the technical idea expressed in the present invention, and such modified or improved forms are also included within the technical scope of the present invention. [Explanation of Symbols]

[0050] 1. Piezoelectric oxide single crystal composite substrate 2. Support substrate 3. Piezoelectric oxide single crystal 4 Intervening layer

Claims

1. A method for manufacturing a piezoelectric oxide single crystal composite substrate, characterized by transferring and bonding a thin film of piezoelectric oxide single crystal onto a crystalline silicon support substrate having a polysilicon layer as the uppermost layer using an ion implantation exfoliation method, then performing heat treatment in a forming gas with a hydrogen concentration of 1 to 5% at a temperature of 300 to 400°C, and subsequently performing heat treatment at a temperature of 400°C or higher in a non-reducing atmosphere.

2. A method for manufacturing a piezoelectric oxide single crystal composite substrate according to claim 1, characterized in that the support substrate and the piezoelectric oxide single crystal are joined via an intervening layer.

3. A method for manufacturing a piezoelectric oxide single crystal composite substrate according to claim 1 or 2, characterized in that lithium tantalate or lithium niobate is used as the piezoelectric oxide single crystal.

4. A method for manufacturing a piezoelectric oxide single crystal composite substrate according to claim 1 or 2, characterized in that the atmosphere used in the heat treatment performed at a temperature of 400°C or higher is a non-reducing atmosphere composed of nitrogen, argon, helium, oxygen, or a mixture thereof.

5. The intervening layer is SiO ２ , SiO, SiN, and SiO ｘ The method for manufacturing a piezoelectric oxide single crystal composite substrate according to claim 2, characterized in that x is any of (x = 1.5 to 2.5) or includes any of these.

6. A piezoelectric oxide single crystal composite substrate characterized by having a thin film of piezoelectric oxide single crystal bonded to a crystalline silicon support substrate having a polysilicon layer as the uppermost layer.

7. The piezoelectric oxide single crystal composite substrate according to claim 6, characterized in that it has an intervening layer between the support substrate and the thin film of the piezoelectric oxide single crystal.

8. The piezoelectric oxide single crystal composite substrate according to claim 6 or 7, characterized in that the piezoelectric oxide single crystal is lithium tantalate or lithium niobate.

9. The intervening layer is SiO ２ , SiO, SiN, and SiO ｘ The piezoelectric oxide single crystal composite substrate according to claim 7, characterized in that x is any of (x = 1.5 to 2.5) or includes any of these values.

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