A single-domain nano-pyroelectric monocrystal thin film and a preparation method thereof

Abstract

This invention relates to a single-domain nanoscale pyroelectric single-crystal thin film and its preparation method. The single-domain nanoscale pyroelectric single-crystal thin film of this invention adds a thin pyroelectric elimination layer between the insulating pyroelectric thin film layer and the insulating layer. The material of this layer is the same as that of the pyroelectric thin film layer, but the polarity of its surface normal is reversed. This achieves the effect of improving charge accumulation in the pyroelectric crystal thin film. Simultaneously, the material of this layer is the same as that of the pyroelectric thin film layer, possessing the same physical properties, and has no effect on the piezoelectricity, pyroelectricity, or electro-optic coefficient of the thin film. The pyroelectric thin film layer in the structure of this invention exhibits high polarization intensity. Compared with the prior art method of placing electrodes on both sides of a composite thin film, this method does not introduce additional electrodes or other materials heterogeneous to the thin film, resulting in a simpler process, lower cost, less stress, and avoids problems such as thin film detachment between the pyroelectric thin film layer and the electrodes.

Description

Technical Field

[0001] This invention relates to a single-domain nanoscale pyroelectric single-crystal thin film and its preparation method, belonging to the field of semiconductor technology. Background Technology

[0002] Lithium niobate (LN), lithium tantalate (LT), quartz, and other oxide single crystals, as piezoelectric materials, are widely used in components such as surface acoustic wave (SAW) filters due to their excellent electromechanical coupling coefficients, enabling filters to achieve high bandwidth characteristics. Traditional grinding and polishing processes can obtain oxide single crystal thin films of several micrometers to tens of micrometers. However, with the trend of miniaturization in communication equipment, the size of components within these devices is also required to be further miniaturized and to achieve higher performance. Therefore, the method of preparing nanoscale thickness thin films using smart exfoliation technology has also been used to prepare oxide single crystal thin films with nanoscale thickness.

[0003] Smart lift-off technology primarily uses light ions, such as H+ and He, to bombard the wafer, causing charged ions to enter the wafer and form an ion-implanted layer. Then, through a heat treatment process, the single-crystal thin film is peeled off from the original wafer at the location of the ion-implanted layer. However, the preparation process requires relatively high energy, such as 500 keV, to bombard the oxide single-crystal wafer, which can introduce numerous defects into the single-crystal thin film. Therefore, it is essential to select an appropriate temperature for annealing the obtained nanoscale single-crystal thin film to eliminate the defects caused by ion implantation.

[0004] Generally, charge mainly accumulates near the bonding surface between the crystal and the substrate. Taking lithium niobate as an example, in the paraelectric phase, Li and Nb are located at the oxygen plane and the center of the oxygen octahedron, respectively, without spontaneous polarization; in the ferroelectric phase, both Li and Nb exhibit electric dipole moments along the Z-axis, i.e., spontaneous polarization occurs, with the spontaneous polarization vector Ps along the Z-axis direction. Therefore, it can be seen that lithium niobate crystals obtained by Z-axis cutting (hereinafter referred to as Z-tangential lithium niobate crystals) possess spontaneous polarization and can exhibit polarization reversal under the influence of an external electric field.

[0005] Because lithium niobate crystals have a very high pyroelectric coefficient, as high as 4 × 10⁻⁶. -5 C / m 2 K, and also due to its high resistivity (10 13(Above Ωcm). During the thin film production process, a large amount of static charge easily accumulates on the wafer surface with temperature changes. Although this static charge can be spontaneously released under natural conditions, the release process is very slow. Therefore, the resulting charge accumulates between the substrate and the pyroelectric thin film, thus forming an electric field. Under the influence of this electric field, the spontaneous polarization vector Ps of the Z-tangential lithium niobate wafer can undergo polarization reversal, ultimately leading to polarization defects in the Z-tangential lithium niobate wafer during the composite thin film heat treatment process. Specifically, multiple polarization spots appear on the surface of the Z-tangential lithium niobate wafer. Although these polarization spots may not affect the ferroelectric properties of the composite thin film, they will reduce its piezoelectricity. If this composite thin film is applied to the piezoelectric direction, it is difficult to meet the corresponding performance requirements. Taking Z-tangential lithium niobate crystal as an example, the piezoelectric coefficient d33 of single-domain lithium niobate is approximately 28 pC / N. Polarization reversal will lead to a decrease in the piezoelectric coefficient, which can even decrease by 90% in severe cases.

[0006] The existing method for single-domain piezoelectric crystals involves creating electrodes at both ends of the crystal, heating it to above the Curie temperature, applying a current, and then gradually cooling it down. However, this method is unsuitable for fabricating thin-film substrates because it requires creating electrodes on both sides of each wafer, resulting in high costs and complex processes for mass production, making large-scale production impossible. Furthermore, due to the significant difference in thermal expansion coefficients between silicon substrates and lithium niobate and lithium tantalate, the thin film is highly susceptible to breakage when heated above the Curie temperature.

[0007] How to make nanoscale pyroelectric single-crystal thin films obtained using intelligent exfoliation technology have polarization intensities close to those of bulk materials has become an urgent problem to be solved. Summary of the Invention

[0008] To address the shortcomings of existing technologies, this invention provides a single-domain nanoscale pyroelectric single-crystal thin film and its preparation method.

[0009] The method of the present invention avoids the polarization reversal caused by the charge generated by the pyroelectric properties of the thin film during the thermal annealing process, improves the polarization intensity performance of the nanoscale pyroelectric single crystal thin film, and makes the polarization characteristics of the obtained nanoscale single crystal thin film close to those of the bulk material.

[0010] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0011] A method for preparing a single-domain nanoscale pyroelectric single-crystal thin film includes the following steps:

[0012] 1) Provide a substrate wafer, and after cleaning, prepare a layer of silicon dioxide as an isolation layer on the substrate wafer;

[0013] 2) Provide a pyroelectric thin film wafer a. After cleaning, helium ions (He+) are implanted into the pyroelectric thin film wafer a by ion implantation.

[0014] 3) The +Z polarization surface of the pyroelectric thin film wafer a implanted by ions in step 2) is bonded to the silicon dioxide layer of the substrate wafer in step 1) for the first time to obtain the bond body a;

[0015] 4) The bond body a is kept at a constant temperature of 150℃~400℃ to achieve the first peeling of the pyroelectric thin film and form a pyroelectric elimination layer a with the +Z polarization surface facing the silicon dioxide.

[0016] 5) Provide a pyroelectric thin film wafer b. After cleaning, helium ions (He+) are implanted into the pyroelectric thin film wafer b by ion implantation.

[0017] 6) The -Z polarization surface of the pyroelectric thin film wafer b implanted by ions in step 5) is bonded a second time with the -Z polarization surface of the pyroelectric elimination layer of the bond body a obtained in step 4) to obtain the bond body b.

[0018] 7) The bond body b is kept at a constant temperature of 150℃~400℃ to achieve the second peeling of the pyroelectric thin film, forming a pyroelectric thin film layer with the +Z polarization surface facing away from the bond body b. The pyroelectric thin film layer is polished to obtain a single-domain pyroelectric crystal thin film b.

[0019] According to a preferred embodiment of the present invention, in step 1), the substrate wafer is selected from silicon, silicon carbide, sapphire, quartz, or fused silica, with a size of 3 to 12 inches and a thickness of 0.2 to 1 mm.

[0020] More preferably, in step 1), the substrate wafer is selected from silicon wafers and has a size of 4 inches.

[0021] According to a preferred embodiment of the present invention, in step 1), the smooth surface of the substrate wafer is cleaned, and after cleaning, a silicon dioxide layer is prepared on the substrate wafer by thermal oxidation, PECVD, LPCVD or CVD, and the thickness of the silicon dioxide layer is 50nm to 5μm.

[0022] More preferably, in step 1), the deposited silica layer is annealed to eliminate impurity ions in the deposited silica layer to reduce absorption loss.

[0023] More preferably, in step 1), the roughness of the silicon dioxide layer is less than or equal to 0.5 nm.

[0024] According to a preferred embodiment of the present invention, in step 2), the pyroelectric thin film wafer a is lithium niobate, lithium tantalate, or silicon carbide, with a size of 3 to 12 inches and a thickness of 0.2 to 1 mm.

[0025] In a further preferred embodiment, in step 2), the pyroelectric thin film wafer a is a lithium niobate wafer with a z-oriented orientation, a size of 4 inches, and an injection surface that is a +Z polarization surface.

[0026] According to a preferred embodiment of the present invention, in step 2), the helium ion implantation energy is 30 keV to 5 MeV, and the dose is 1 × 10⁻⁶. 15 ions / cm 2 ~1×10 17 ions / cm 2 Helium ions are implanted to form a thin-film wafer with a three-layer structure consisting of a thin film layer, a separation layer, and a residue layer.

[0027] According to a preferred embodiment of the present invention, in step 3), the bonding is performed by plasma bonding. The bonding body a structure consists of, from top to bottom, a lithium niobate thin film with the -Z polarization surface facing away from silicon dioxide, a silicon dioxide isolation layer, and a silicon substrate.

[0028] According to a preferred embodiment of the present invention, in step 4), the pyroelectric elimination layer a with the +Z polarization surface facing silicon dioxide is polished and thinned to 5nm-100nm, with a surface roughness of less than or equal to 0.5nm, to obtain a bonded body a with a pyroelectric elimination layer.

[0029] According to a preferred embodiment of the present invention, in step 5), the pyroelectric thin film wafer b is the same as the pyroelectric thin film wafer a in step 2), but the ion implantation surface is a -Z polarization surface, which is opposite to the implantation surface of the pyroelectric thin film wafer a.

[0030] According to a preferred embodiment of the present invention, in step 5), the helium ion implantation energy is 30 keV to 5 MeV, and the dose is 1 × 10⁻⁶. 15 ions / cm 2 ~1×10 17 ions / cm 2 The injection energy and dose of helium ions may be the same as or different from those in step 2).

[0031] According to a preferred embodiment of the present invention, in step 6), the bonding is performed by plasma bonding, and the implantation surface of the pyroelectric thin film wafer b obtained by ion implantation is bonded a second time to the pyroelectric elimination layer of the bond body a obtained in step 4) that is away from the silicon dioxide layer.

[0032] According to a preferred embodiment of the present invention, in step 7), polishing the pyroelectric thin film layer involves polishing the pyroelectric thin film layer to reduce its thickness to 50 nm-20 μm.

[0033] A single-domain nanoscale pyroelectric single-crystal thin film was prepared using the method described above.

[0034] The single-domain nanoscale pyroelectric single crystal thin film is provided with a substrate layer, an isolation layer, a pyroelectric elimination layer, and a pyroelectric thin film layer from bottom to top.

[0035] According to a preferred embodiment of the present invention, the pyroelectric thin film layer material is lithium niobate or lithium tantalate pyroelectric thin film layer.

[0036] According to a preferred embodiment of the present invention, the pyroelectric elimination layer material is the same as the pyroelectric thin film layer material, but with opposite polarities.

[0037] This invention addresses the problem of charge accumulation at the bonding interface by creating conductive domain walls at the interface between the pyroelectric elimination layer and the pyroelectric thin film layer, thus forming a conductive layer with good conductivity and eliminating depolarization of single-domain piezoelectric thin films.

[0038] According to a preferred embodiment of the present invention, the isolation layer is a silicon dioxide or silicon nitride amorphous material, and its refractive index is less than that of the pyroelectric thin film layer material.

[0039] According to a preferred embodiment of the present invention, the substrate material is silicon, silicon carbide, sapphire, quartz, or fused silica.

[0040] The single-domain nanoscale pyroelectric single-crystal thin film of the present invention adds a thin pyroelectric elimination layer between the insulating pyroelectric thin film layer and the isolation layer. The material of the elimination layer is the same as that of the pyroelectric thin film layer, but the polarity of the surface normal direction is reversed. This achieves the effect of improving the charge accumulation of the pyroelectric crystal thin film. At the same time, the material of the elimination layer is the same as that of the pyroelectric thin film layer, and it has the same physical properties. It has no effect on the piezoelectricity, pyroelectricity, electro-optic coefficient, etc. of the thin film. The pyroelectric thin film layer in the structure of the present invention has the characteristic of high polarization intensity.

[0041] The beneficial effects of this invention are as follows:

[0042] 1. Due to the pyroelectric effect of pyroelectric crystal films, the annealing process causes a large amount of charge to accumulate near the bonding surface between the crystal and the substrate. By adding a thin film of the same material with opposite polarity near the surface of the bonding interface, the phenomenon of charge accumulation at the bonding interface is solved, the possibility of depolarization of single-domain piezoelectric films is eliminated, and the single-domain nature of pyroelectric films is guaranteed.

[0043] 2. This invention adds a pyroelectric elimination layer at the bonding interface between the pyroelectric thin film layer and the substrate layer. The pyroelectric elimination layer is characterized by being made of the same material as the pyroelectric thin film layer and having the opposite polarity to the normal direction of the pyroelectric thin film layer. Compared with the prior art method of fabricating electrodes on both sides of the composite thin film, this method does not introduce additional electrodes or other materials dissimilar to the thin film. The process is relatively simple, cost-effective, and stress-free, avoiding problems such as thin film detachment between the pyroelectric thin film layer and the electrodes.

[0044] 3. The lithium niobate single-crystal piezoelectric thin film prepared using this invention exhibits excellent single-domain properties, effectively preserving the piezoelectric coefficient of the lithium niobate bulk material. The piezoelectric coefficient d33 of the single-domain lithium niobate bulk material is approximately 28 pC / N. The piezoelectric coefficient of the lithium niobate single-crystal piezoelectric thin film prepared using this invention is 98%–100% of that of the bulk material, while the piezoelectric coefficient of the existing technology is only 10%–90% of that of the bulk material. The single-domain single-crystal thin film exhibits stable piezoelectric performance during use, the electromechanical coupling coefficient of the device does not decrease, and it has a wide bandwidth, low loss, and good consistency. Attached Figure Description

[0045] Figure 1 This is a flowchart illustrating the fabrication process of single-domain nanoscale pyroelectric single-crystal thin films.

[0046] Figure 2 A schematic diagram of the structure of a single-domain nanoscale pyroelectric single-crystal thin film;

[0047] Among them, 1, LN thin film b, 2, LN thin film a, 3, silicon dioxide layer, and 4, substrate. Detailed Implementation

[0048] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Specific details set forth in the following description are intended to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0049] Injection stop layer: Plasma carries a certain amount of charge, which can accelerate positive ions through an electric field and change their direction of motion through a magnetic field. This allows ions to be controlled to enter the wafer with a certain amount of energy. After being injected into the wafer, the ions will collide with the atoms and nuclei of the wafer and lose energy. When the energy is exhausted, the ions will stop at a certain depth in the wafer, which is the injection stop layer.

[0050] Example 1

[0051] The preparation method of single-domain nanoscale pyroelectric single-crystal thin films, the preparation process is as follows: Figure 1 As shown, the steps are as follows:

[0052] 1) Provide silicon substrate wafers with smooth surfaces, 4 inches in size and 0.5 mm thick;

[0053] 2) The smooth surface of the silicon substrate wafer is cleaned, and a silicon dioxide layer is prepared on it by thermal oxidation as an isolation layer; the thickness of the silicon dioxide layer is 500nm, and the deposited silicon dioxide is annealed to eliminate impurity ions in the deposited silicon dioxide layer to reduce absorption loss. The roughness of the silicon dioxide layer is less than 0.5nm.

[0054] 3) A pyroelectric thin-film wafer a is provided. Pyroelectric thin-film wafer a is a z-oriented lithium niobate wafer with a size of 4 inches. After cleaning, helium ions (He+) are implanted into pyroelectric thin-film wafer a using ion implantation. The helium ion implantation energy is 50 keV (corresponding to an implantation stop layer thickness of approximately 200 nm in the lithium niobate crystal), and the dose is 1 × 10⁻⁶. 16 ions / cm 2 The injection surface is a +Z polarization surface, forming a thin film wafer with a three-layer structure of thin film layer, separation layer and residual material layer;

[0055] 4) Using plasma bonding, the +Z polarization plane of the pyroelectric thin film wafer a implanted in step 3) is bonded to the silicon dioxide layer of the substrate wafer in step 1) for the first time to obtain bond body a; the bond body structure from top to bottom consists of a lithium niobate thin film (-Z polarization plane facing away from silicon dioxide), a silicon dioxide isolation layer, and a silicon substrate.

[0056] 5) The bond body a is kept at a constant temperature of 260℃ to achieve the first peeling of the pyroelectric thin film, forming a pyroelectric elimination layer a with the +Z polarization surface facing the silicon dioxide, that is: forming a pyroelectric elimination layer with the -Z polarization surface facing away from the silicon dioxide as the top layer. The pyroelectric elimination layer a is polished and thinned to 20nm with a surface roughness of <0.5nm to obtain the bond body a with the pyroelectric elimination layer.

[0057] 6) Another pyroelectric thin-film wafer b is provided. Pyroelectric thin-film wafer b is a z-oriented lithium niobate wafer with a size of 4 inches. Its smooth surface is cleaned, and helium ions (He+) are implanted into pyroelectric thin-film wafer b by ion implantation. The helium ion implantation energy is 400 keV, and the dose is 5 × 10⁻⁶. 16 ions / cm 2 The injection surface is a polarization surface of -Z, forming a thin film wafer with a three-layer structure of thin film layer, separation layer and residual material layer;

[0058] 7) The implantation surface (-Z polarization surface) of the pyroelectric thin film wafer b implanted by ions in step 6) is bonded a second time with the pyroelectric elimination layer -Z polarization surface (facing away from the silicon dioxide layer) of the pyroelectric elimination layer bond body a obtained in step 5) to obtain bond body b; the structure of bond body b from top to bottom is lithium niobate wafer b (+Z polarization surface facing away from the first bond body), pyroelectric elimination layer (-Z polarization surface facing away from silicon dioxide), silicon dioxide isolation layer, silicon substrate;

[0059] 8) The bond body b is kept at a constant temperature of 300℃ to achieve the second exfoliation of the pyroelectric thin film, forming a pyroelectric thin film layer with the +Z polarization plane facing away from the bond body b. At this time, the thickness of the pyroelectric thin film layer is approximately 1100 nm. Finally, the pyroelectric thin film layer is polished and thinned to 900 nm (50 nm - 20 μm) to obtain the second bond body with the pyroelectric thin film layer, forming the final single-domain nanoscale pyroelectric single crystal thin film. The structure of the single-domain nanoscale pyroelectric single crystal thin film is as follows: Figure 2 As shown.

Claims

1. A method for preparing a single-domain nanoscale pyroelectric single-crystal thin film, comprising the following steps: 1) Provide a substrate wafer, and after cleaning, prepare a layer of silicon dioxide as an isolation layer on the substrate wafer; 2) Provide a pyroelectric thin film wafer a. After cleaning, helium ions (He+) are implanted into the pyroelectric thin film wafer a by ion implantation. 3) The +Z polarization surface of the pyroelectric thin film wafer a implanted by ions in step 2) is bonded to the silicon dioxide layer of the substrate wafer in step 1) for the first time to obtain the bond body a; 4) The bond body a is kept at a constant temperature of 150℃~400℃ to achieve the first peeling of the pyroelectric thin film and form a pyroelectric elimination layer a with the +Z polarization surface facing the silicon dioxide. 5) Provide a pyroelectric thin film wafer b. After cleaning, helium ions (He+) are implanted into the pyroelectric thin film wafer b by ion implantation. 6) The -Z polarization surface of the pyroelectric thin film wafer b implanted by ions in step 5) is bonded a second time with the -Z polarization surface of the pyroelectric elimination layer of the bond body a obtained in step 4) to obtain the bond body b. 7) The bond body b is kept at a constant temperature of 150℃~400℃ to achieve the second peeling of the pyroelectric thin film, forming a pyroelectric thin film layer with the +Z polarization surface facing away from the bond body b. The pyroelectric thin film layer is polished to obtain a single-domain pyroelectric crystal thin film b.

2. The preparation method according to claim 1, characterized in that, In step 1), the substrate wafer is selected from silicon, silicon carbide, sapphire, quartz, or fused silica, with a size of 3 to 12 inches and a thickness of 0.2 to 1 mm. The smooth surface of the substrate wafer is cleaned. After cleaning, a silicon dioxide layer is prepared on the substrate wafer by thermal oxidation, PECVD, LPCVD, or CVD. The thickness of the silicon dioxide layer is 50 nm to 5 μm, and the roughness of the silicon dioxide layer is less than or equal to 0.5 nm.

3. The preparation method according to claim 1, characterized in that, In step 2), the pyroelectric thin film wafer a is lithium niobate, lithium tantalate or silicon carbide, with a size of 3 to 12 inches and a thickness of 0.2 to 1 mm. Preferably, the pyroelectric thin film wafer a is a z-tangential lithium niobate wafer with a size of 4 inches and an injection surface that is a +Z polarization surface.

4. The preparation method according to claim 1, characterized in that, In step 2), the helium ion implantation energy is 30 keV to 5 MeV, and the dose is 1 × 10⁻⁶. 15 ions / cm 2 ~1×10 17 ions / cm 2 .

5. The preparation method according to claim 1, characterized in that, In step 3), the bonding is performed using plasma bonding. The bond body a structure consists of, from top to bottom, a lithium niobate thin film with the -Z polarization surface facing away from silicon dioxide, a silicon dioxide isolation layer, and a silicon substrate.

6. The preparation method according to claim 1, characterized in that, In step 4), the pyroelectric elimination layer a with the +Z polarization surface facing silicon dioxide is polished and thinned to 5nm-100nm with a surface roughness of less than or equal to 0.5nm to obtain a bonded body a with a pyroelectric elimination layer.

7. The preparation method according to claim 1, characterized in that, In step 5), the pyroelectric thin-film wafer b is the same as the pyroelectric thin-film wafer a in step 2), but the ion implantation surface is the -Z polarization surface, which is opposite to the implantation surface of pyroelectric thin-film wafer a. The helium ion implantation energy is 30 keV to 5 MeV, and the dose is 1 × 10⁻⁶. 15 ions / cm 2 ~1×10 17 ions / cm 2 The injection energy and dose of helium ions may be the same as or different from those in step 2).

8. The preparation method according to claim 1, characterized in that, In step 6), the bonding is performed using plasma bonding. The implantation surface of the pyroelectric thin film wafer b, which is implanted by ion implantation, is bonded a second time to the pyroelectric elimination layer of the bond body a obtained in step 4) that is away from the silicon dioxide layer. In step 7), the pyroelectric thin film layer is polished to reduce the pyroelectric thin film layer to 50nm-20μm.

9. A single-domain nanoscale pyroelectric single-crystal thin film, prepared by the method described in any one of claims 1-8.

10. The single-domain nanoscale pyroelectric single-crystal thin film according to claim 9, characterized in that, The single-domain nanoscale pyroelectric single-crystal thin film is provided from bottom to top with a substrate layer, an isolation layer, a pyroelectric elimination layer, and a pyroelectric thin film layer. The pyroelectric thin film layer material is lithium niobate or lithium tantalate pyroelectric thin film layer. The pyroelectric elimination layer material is the same as the pyroelectric thin film layer material, but with opposite polarity. The isolation layer is amorphous material of silicon dioxide or silicon nitride, and its refractive index is lower than that of the pyroelectric thin film layer material. The substrate layer material is silicon, silicon carbide, sapphire, quartz, or fused silica.

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