Periodically polarized film preparation method

Abstract

The invention relates to the technical field of semiconductor elements, and provides a preparation method of a periodically polarized thin film, which comprises the step of providing a thin film structure with periodically arranged metal electrodes. The thin film structure comprises a thin film layer and a supporting structure which are stacked, the thin film layer is in bonding connection with the supporting structure, and the metal electrode is connected to the side, away from the supporting structure, of the thin film layer. And performing temperature change treatment on the thin film structure with the metal electrode so as to generate a pyroelectric field in the thin film layer. The pyroelectric field is used for driving the region, corresponding to the metal electrode, in the thin film layer to generate polarization inversion, so that the periodically polarized thin film with the periodic inversion domain structure is obtained. The preparation method provided by the invention is simple and convenient to operate and low in cost, and can inhibit the transverse broadening effect of the inversion domain.

Description

Technical Field

[0001] This application relates to the field of semiconductor device technology, and in particular to a method for preparing a periodically polarized thin film and a periodically polarized thin film. Background Technology

[0002] Periodically polarized thin films are core materials in photonic integrated devices and are widely used in optical communication and information processing as well as in the fields of sensors and detectors.

[0003] Currently, the mainstream method for preparing periodically polarized thin films is the external electric field method. This method requires the fabrication of periodic metal electrodes on the surface of the thin film using photolithography, and the application of high voltage to the electrodes through an external circuit. This high voltage electric field drives Li⁺ ions across the oxygen plane to achieve polarization reversal.

[0004] However, the above method requires the use of equipment such as a signal generator, high-voltage amplifier, and oscilloscope, which is costly and complex to operate. Furthermore, high voltage can easily lead to the lateral broadening effect of inversion domains, making it difficult to accurately control the duty cycle and thus affecting the phase matching accuracy. Summary of the Invention

[0005] This application provides a method for preparing periodically polarized thin films to solve the problems in related technologies where achieving polarization reversal of thin film structures through an external electric field method is costly and complex, and easily leads to lateral broadening of reversed domains, affecting phase matching accuracy.

[0006] To achieve the above objectives, this application adopts the following technical solution:

[0007] This application provides a method for preparing a periodically polarized thin film, comprising: providing a thin film structure having periodically arranged metal electrodes; the thin film structure including a stacked thin film layer and a support structure, the thin film layer and the support structure being bonded together, and the metal electrodes being connected to the side of the thin film layer away from the support structure; applying a temperature change treatment to the thin film structure having the metal electrodes to generate a pyroelectric field inside the thin film layer; the pyroelectric field being used to drive polarization reversal in the region of the thin film layer corresponding to the metal electrodes, thereby obtaining a periodically polarized thin film with a periodically reversed domain structure.

[0008] As an optional implementation, applying a temperature change treatment to a thin film structure with metal electrodes includes: heating the thin film structure and raising the temperature of the thin film layer from room temperature to a preset temperature within a first time period; maintaining the thin film layer at the preset temperature for a second time period to make the surface of the thin film layer electrically neutral; and cooling the thin film structure and lowering the temperature of the thin film layer from the preset temperature to room temperature within a third time period.

[0009] As an optional implementation, the thin film structure is heated, and the temperature of the thin film layer is raised from room temperature to a preset temperature within a first time period, including: raising the temperature of the thin film layer from room temperature to the preset temperature through a segmented heating curve; wherein, the segmented heating curve includes n heating sub-segments and n-1 isothermal sub-segments; the isothermal sub-segments are connected between two adjacent heating sub-segments; the sum of the heating duration of each heating sub-segment and the isothermal duration of each isothermal sub-segment is the first time period; the sum of the temperature differences of each heating sub-segment is the difference between the preset temperature and room temperature.

[0010] As an optional implementation, the periodic reverse domain structure includes a plurality of alternately arranged reverse domain regions and non-reverse domain regions; wherein the sum of the widths of a set of adjacent reverse domain regions and non-reverse domain regions is the polarization period, and the polarization period is greater than or equal to 1 μm and less than or equal to 20 μm.

[0011] As an optional implementation, the preset temperature is greater than or equal to 100°C and less than or equal to 300°C.

[0012] As an optional implementation, the cooling rate of the thin film layer during the third time period is greater than or equal to 1°C / min and less than or equal to 10°C / min.

[0013] As an optional implementation, the heating rate of the thin film layer during the first time period is greater than or equal to 1°C / min and less than or equal to 10°C / min.

[0014] As an optional implementation, providing a thin film structure with periodically arranged metal electrodes includes: forming a thin film structure; forming a periodically arranged photoresist pattern on the surface of the thin film layer away from the support structure; depositing metal on the surface of the thin film layer with the photoresist pattern; and stripping the photoresist pattern and the metal covering the photoresist pattern to obtain a thin film structure with periodically arranged metal electrodes.

[0015] As an optional implementation, the material of the metal electrode is any one of Au, Al, Cr, and Ti; and / or, the thickness of the metal electrode is greater than or equal to 100 nm and less than or equal to 1000 nm; and / or, the width of the metal electrode is greater than or equal to 0.4 μm and less than or equal to 10 μm.

[0016] As an optional implementation, before obtaining a periodically polarized thin film with a periodically reversed domain structure after polarization reversal occurs, the method further includes: removing the metal electrode formed on the surface of the thin film layer opposite to the support structure.

[0017] The method for preparing a periodically polarized thin film provided in this application includes: providing a thin film structure with periodically arranged metal electrodes. The thin film structure includes a stacked thin film layer and a support structure, the thin film layer and the support structure being bonded together, and the metal electrodes being connected to the side of the thin film layer opposite to the support structure. A temperature change treatment is applied to the thin film structure with the metal electrodes to generate a pyroelectric field inside the thin film layer. This pyroelectric field can then drive polarization reversal in the region of the thin film layer corresponding to the metal electrodes, resulting in a periodically polarized thin film with a periodically reversed domain structure.

[0018] At room temperature, the spontaneous polarization of a thin film structure generates a space charge field, which attracts free charges on the surface to compensate, thereby achieving a steady-state charge balance and making the thin film structure macroscopically electrically neutral. However, when subjected to temperature changes (such as heating or cooling), the spontaneous polarization intensity inside the thin film layer changes with the temperature. For example, the spontaneous polarization intensity (Ps) of the thin film layer decreases when heated and increases when cooled.

[0019] At this point, the original free charge compensation system on the surface of the thin film layer will become unbalanced, resulting in residual bound charges on the surface of the thin film layer that have not been fully compensated, generating a pyroelectric field.

[0020] The presence of periodic metal electrodes provides a high density of mobile free charges for the thin film layer, which can rapidly form a strong potential difference when heated or cooled, thereby generating a pyroelectric field of a certain intensity. When the intensity of the generated pyroelectric field is greater than the coercive field of the thin film layer and opposite to the spontaneous polarization direction of the thin film layer, the pyroelectric field can drive the spontaneous polarization direction of the corresponding region to reverse.

[0021] The region without the metal electrode covering relies solely on a small number of free ions from the environment for compensation, resulting in minimal charge change during heating or cooling. This insufficient charge difference prevents the formation of a sufficiently strong potential difference, thus maintaining the spontaneous polarization direction in this region. Therefore, the above method enables the fabrication of periodically polarized thin films with a periodic reverse domain structure.

[0022] This application utilizes a pyroelectric field generated by temperature changes to replace an external high-voltage electric field, thus eliminating the need for an external circuit to provide a polarization reversal voltage. Instead, it only needs to utilize the inherent pyroelectric properties of ferroelectric crystals and generate a pyroelectric field through temperature changes to achieve polarization reversal.

[0023] Compared to the externally applied electric field polarization method, the pyroelectric field in the fabrication of periodically polarized thin films via the pyroelectric effect exists only between the upper and lower surfaces of the film layer. In this case, since the pyroelectric field directly polarizes a film layer with a thickness ≤1 μm, only a voltage of tens of volts is required to achieve polarization reversal. Furthermore, the lower polarization reversal voltage can suppress the lateral broadening effect of the reverse domains, thereby enabling the fabrication of submicron periodic reverse domain structures. This allows for use in spontaneous parametric down-conversion processes for reverse propagation, generating narrow-linewidth quantum entangled light sources.

[0024] Meanwhile, the preparation process does not require signal generators, high-voltage amplifiers, oscilloscopes, or other equipment, nor does it require connecting the polarization electrode to an external circuit. It only requires temperature control equipment to regulate the temperature of the thin film structure, making the operation simple and reducing production costs.

[0025] Furthermore, the preparation method provided in this application offers a high degree of freedom in adjusting the intensity of the pyroelectric field. It can be adjusted by changing the amount of temperature change or by changing the amount of pyroelectric charge dissipation, thereby achieving precise control of the domain structure, improving the consistency of thin film performance, and enhancing the adaptability of the preparation method to different ferroelectric materials. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 This is a schematic diagram illustrating the principle of periodically polarized lithium niobate crystal using an external electric field method.

[0028] Figure 2 A process flow diagram of a method for preparing a periodically polarized thin film provided in this application embodiment;

[0029] Figure 3 This is a schematic diagram of a thin film structure provided in an embodiment of this application;

[0030] Figure 4 This is a schematic diagram of the temperature change processing provided in an embodiment of this application;

[0031] Figure 5 For ferroelectric lithium niobate crystals in Figure 4 A schematic diagram illustrating the principle of the internal pyroelectric effect under temperature change processing;

[0032] Figure 6 for Figure 4The temperature change curve corresponding to the temperature change processing provided in the document;

[0033] Figure 7 Another temperature change curve provided for an embodiment of this application;

[0034] Figure 8 A schematic diagram illustrating the formation of a lithium niobate single-crystal thin film provided in an embodiment of this application;

[0035] Figure 9 This is a schematic diagram illustrating the formation of another lithium niobate single-crystal thin film provided in an embodiment of this application.

[0036] Explanation of reference numerals in the attached figures:

[0037] 100 - Support structure; 110 - Substrate layer; 120 - Isolation layer; 130 - Capture layer;

[0038] 200 - Ferroelectric crystal wafer; 210 - Thin film layer; 220 - Implanted layer; 230 - Residual mass layer. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0040] Electromagnetic waves profoundly influence modern society, finding wide application in areas such as information transmission, television broadcasting, mobile telecommunications, and Wi-Fi data access. Electromagnetic waves in the tens of gigahertz (GHz) range can be generated using electronic oscillators, corresponding to radio waves and microwave bands; millimeter waves and terahertz (THz) waves can be generated using resonant tunnel diodes, with frequencies ranging from tens of GHz to several THz; and even higher optical frequencies can be generated using solid-state and gas lasers, reaching frequencies on the picohertz scale.

[0041] However, these methods typically rely on fixed energy levels of specific materials, resulting in limited spectral coverage and often narrow spectral bandwidth. To overcome this limitation, scientists have proposed the method of "nonlinear frequency mixing." This involves multi-wave mixing of optical frequencies using nonlinear optical effects to unlock previously unavailable electromagnetic frequencies as needed.

[0042] Phase matching is a core prerequisite and necessary condition for achieving high-efficiency nonlinear mixing. Methods for achieving phase matching include birefringence phase matching (BPM), mode phase matching (MPM), cyclic phase matching (CPM), and quasi-phase matching (QPM). Quasi-phase matching is widely used because it has no requirements on the direction of light transmission, polarization direction, or wavelength (theoretically, there are no matching limitations for any wavelength of light within the crystal's transmission range). It also provides a new approach to achieving phase matching for some crystals with excellent nonlinear properties but no birefringence (such as lithium tantalate).

[0043] Taking lithium niobate as an example of a nonlinear mixing material, periodic polarization is the most common and effective method for achieving quasi-phase matching on lithium niobate crystals.

[0044] In existing technologies, the external electric field polarization method is the most mainstream preparation technique for periodically polarized lithium niobate crystals or thin films. Figure 1 The principle of periodically polarized lithium niobate crystals using an external electric field method is demonstrated.

[0045] There are two polarization directions in lithium niobate crystals, due to Li + The position of the ion relative to the oxygen plane is determined when Li + When ions shift upwards along the +z direction towards the oxygen plane, it is called "upward polarization," while when they shift downwards along the -z direction towards the oxygen plane, it is called "downward polarization." For example... Figure 1 As shown in (a), the spontaneous polarization direction of the single-domain lithium niobate crystal is along the +z direction of the crystal, i.e., "upward polarization".

[0046] When an external electric field higher than the crystal coercivity is applied to a single-domain lithium niobate crystal, for example, by applying a high-voltage electric field to the single-domain lithium niobate crystal through electrodes, Li can be made to... + When ions overcome the potential energy barrier of the oxygen plane and cross it to reach another stable state on the oxygen plane, Li can... + Ions "downward polarization," achieving polarization reversal, such as... Figure 1 As shown in (b) of the diagram.

[0047] The process flow for realizing periodically polarized lithium niobate crystals using the external electric field method includes the following steps:

[0048] First, periodic metal electrodes are fabricated on the upper surface (+z plane) of the z-cut lithium niobate crystal using processes such as ultraviolet lithography, electrode deposition, and lift-off. A full-coverage electrode is similarly fabricated on the lower surface (-z plane) by metal deposition.

[0049] Then, a circuit is formed by connecting the upper and lower surface electrodes with wires, and external devices are connected. For example, a waveform generator can be connected to edit the required polarization voltage waveform, a high-voltage amplifier can be connected to amplify the polarization voltage, and a dual-channel oscilloscope can be connected to monitor the amplified polarization voltage signal and the current signal during the periodic polarization process in real time. Finally, periodic polarization is achieved by applying an external high voltage.

[0050] Compared to bulk dielectric lithium niobate crystals, the lithium niobate layer thickness in thin-film lithium niobate (TFLN) films is typically ≤1μm. Under the same coercive field, if the upper and lower electrodes are directly applied to the upper and lower surfaces of the lithium niobate film layer, the polarization reversal voltage of TFLN can be reduced from several kilovolts to tens of volts. This reduces the need for high-voltage amplifiers during periodic polarization, making operation more convenient and reducing equipment costs.

[0051] However, in practice, it is difficult to directly apply a voltage signal to the lower surface of the lithium niobate thin film. This requires adding a metal electrode to the lower surface of the lithium niobate layer and partially exposing it through a special process to allow connection to external circuits via wires. However, this not only increases the complexity of the process but also severely affects the yield of the TFLN thin film and disrupts the high refractive index contrast between the silicon dioxide and lithium niobate layers, making it impossible to form a strongly constrained optical waveguide structure.

[0052] Therefore, when using the external electric field method to periodically polarize TFLN thin films, the lower electrode is usually placed on the back side of the silicon substrate. Due to the influence of the silicon dioxide layer, the silicon substrate, and the bonding interface, the required inversion voltage is still as high as several hundred volts or even thousands of volts. This not only requires a high-voltage amplifier to amplify the voltage signal, increasing the cost and process complexity, but also the high polarization voltage may lead to a severe lateral broadening effect of the inversion domains, which is not conducive to the precise control of the duty cycle of the inversion domains.

[0053] In view of this, embodiments of this application provide a method for preparing a periodically polarized thin film, comprising: providing a thin film structure having periodically arranged metal electrodes. The thin film structure includes a stacked thin film layer and a support structure, the thin film layer and the support structure being bonded together, and the metal electrodes being connected to the side of the thin film layer opposite to the support structure. A temperature change treatment is applied to the thin film structure with the metal electrodes to generate a pyroelectric field within the thin film layer. This allows the pyroelectric field to drive polarization reversal in the region of the thin film layer corresponding to the metal electrodes, resulting in a periodically polarized thin film with a periodically reversed domain structure.

[0054] At room temperature, the spontaneous polarization of a thin film structure generates a space charge field, which attracts free charges on the surface to compensate, thereby achieving a steady-state charge balance and making the thin film structure macroscopically electrically neutral. However, when subjected to temperature changes (such as heating or cooling), the spontaneous polarization intensity inside the thin film layer changes with the temperature. For example, the spontaneous polarization intensity (Ps) of the thin film layer decreases when heated and increases when cooled.

[0055] At this point, the original free charge compensation system on the surface of the thin film layer will become unbalanced, resulting in residual bound charges on the surface of the thin film layer that have not been fully compensated, generating a pyroelectric field.

[0056] The presence of periodic metal electrodes provides a high density of mobile free charges for the thin film layer, which can rapidly form a strong potential difference when heated or cooled, thereby generating a pyroelectric field of a certain intensity. When the intensity of the generated pyroelectric field is greater than the coercive field of the thin film layer and opposite to the spontaneous polarization direction of the thin film layer, the pyroelectric field can drive the spontaneous polarization direction of the corresponding region to reverse.

[0057] The region without the metal electrode covering relies solely on a small number of free ions in the environment for compensation, resulting in minimal charge change during heating or cooling. This insufficient charge difference prevents the formation of a sufficiently strong potential difference, thus maintaining the original polarization direction in this region. Therefore, the above method enables the fabrication of periodically polarized thin films with a periodic reverse domain structure.

[0058] This application utilizes a pyroelectric field generated by temperature changes to replace an external high-voltage electric field, thus eliminating the need for an external circuit to provide a polarization reversal voltage. Instead, it only needs to utilize the inherent pyroelectric properties of ferroelectric crystals and generate a pyroelectric field through temperature changes to achieve polarization reversal.

[0059] Compared to the externally applied electric field polarization method, the pyroelectric field in the fabrication of periodically polarized thin films via the pyroelectric effect exists only between the upper and lower surfaces of the film layer. In this case, since the pyroelectric field directly polarizes a film layer with a thickness ≤1 μm, only a voltage of tens of volts is required to achieve polarization reversal. Furthermore, the lower polarization reversal voltage can suppress the lateral broadening effect of the reverse domains, thereby enabling the fabrication of submicron periodic reverse domain structures. This allows for use in spontaneous parametric down-conversion processes for reverse propagation, generating narrow-linewidth quantum entangled light sources.

[0060] Meanwhile, the preparation process does not require signal generators, high-voltage amplifiers, oscilloscopes, or other equipment, nor does it require connecting the polarization electrode to an external circuit. It only requires temperature control equipment to regulate the temperature of the thin film structure, making the operation simple and reducing production costs.

[0061] Furthermore, the preparation method provided in this application offers a high degree of freedom in adjusting the intensity of the pyroelectric field. It can be adjusted by changing the amount of temperature change or by changing the amount of pyroelectric charge dissipation, thereby achieving precise control of the domain structure, improving the consistency of thin film performance, and enhancing the adaptability of the preparation method to different ferroelectric materials.

[0062] The contents of this application will now be described in detail with reference to the accompanying drawings, so that those skilled in the art can have a clearer and more detailed understanding of the contents of this application.

[0063] Periodically polarized thin films are a class of functional thin films with ferroelectric materials (such as lithium niobate and lithium tantalate) as their core. The spontaneous polarization directions inside the thin film are arranged in a periodic alternating reverse pattern, forming a regular periodic reverse domain structure.

[0064] Among them, the parameters such as the period and duty cycle of the inverted domain structure can be precisely controlled, making it one of the key core materials in fields such as optical communication and signal processing, quantum information technology, laser technology and frequency conversion, precision measurement and sensing.

[0065] Figure 2 This is a process flow diagram of a method for preparing a periodically polarized thin film, provided in an embodiment of this application. Figure 3 This is a schematic diagram of a thin film structure provided in an embodiment of this application.

[0066] Reference Figure 2 and Figure 3 As shown in the figure, this application provides a method for preparing a periodically polarized thin film.

[0067] The preparation method includes:

[0068] S100. A thin film structure having periodically arranged metal electrodes is provided. The thin film structure includes a stacked thin film layer and a support structure, the thin film layer and the support structure are bonded together, and the metal electrodes are connected to the side of the thin film layer opposite to the support structure.

[0069] Thin film layer 210 is a ferroelectric material and serves as a carrier for spontaneous polarization, providing a material basis for polarization reversal. For example, thin film layer 210 can be a lithium niobate thin film layer, a lithium tantalate thin film layer, a lead zirconate titanate thin film layer, etc., and no limitation is made here. The thickness of thin film layer 210 is ≤1μm.

[0070] The bonding connection between the support structure 100 and the thin film layer 210 not only provides mechanical support for the extremely thin film layer 210, preventing warping, deformation, and cracking due to thermal expansion and contraction during temperature changes, but also enables uniform heat conduction. This allows temperature changes to be transmitted synchronously and uniformly throughout the entire thin film layer 210, avoiding uneven electric field distribution caused by local temperature gradients and ensuring the consistency of the domain structure.

[0071] By placing a metal electrode (not shown) on the upper surface of the thin film layer 210, the metal electrode can quickly accumulate or release free charges when the temperature changes, thereby forming a pyroelectric field with a certain intensity within the thin film layer 210.

[0072] The period, width, and spacing of the metal electrodes determine the spatial distribution period of the pyroelectric field, which in turn affects the period and duty cycle range of the inverted domain structure. Understandably, the periodic arrangement of the metal electrodes can be adjusted according to the specific fabrication requirements of the periodically polarized thin film; therefore, no limitations are imposed here.

[0073] S200. A temperature change treatment is applied to the thin film structure with metal electrodes to generate a pyroelectric field inside the thin film layer. The pyroelectric field is used to drive polarization reversal in the region corresponding to the metal electrode in the thin film layer, resulting in a periodically polarized thin film with a periodic reverse domain structure.

[0074] Understandably, the thin film structure with metal electrodes can be placed on a temperature-controlled device such as a heating plate or a hot / cold stage. The heating plate or hot / cold stage contacts the bottom of the support structure 100, with the upper surface of the thin film layer 210 facing upwards and exposed to the air atmosphere. The heating plate or hot / cold stage allows for precise and uniform control of the temperature change of the thin film structure, causing a controllable abrupt change in the spontaneous polarization intensity of the thin film layer 210. This disrupts the charge balance, deposits residual bound charges, and forms a pyroelectric field.

[0075] At room temperature, the spontaneous polarization of the thin film structure generates a space charge field, which attracts free charges on the surface of the thin film layer 210 to compensate for the polarization, thereby achieving a steady-state charge balance and making the thin film structure macroscopically electrically neutral. However, when subjected to temperature changes (such as heating or cooling), the spontaneous polarization intensity inside the thin film layer 210 changes with the temperature. For example, the spontaneous polarization intensity Ps of the thin film layer 210 decreases when heated and increases when cooled.

[0076] At this point, the original free charge compensation system on the surface of thin film layer 210 will become unbalanced, resulting in residual bound charges on the surface of thin film layer 210 that are not fully compensated, thus generating a pyroelectric field. The presence of periodic metal electrodes provides thin film layer 210 with a high density of transferable free charges, which can quickly form a strong potential difference when heated or cooled, thereby generating a pyroelectric field of a certain intensity.

[0077] When the electric field intensity in the region below the metal electrode exceeds the coercive field of the thin film structure at this temperature and is opposite to the initial spontaneous polarization direction of the thin film layer 210, the spontaneous polarization direction in that region can be reversed. Meanwhile, the electric field intensity in the region not covered by the metal electrode is lower than the coercive field, and the polarization direction remains in its initial state. Thus, a periodic reversed domain structure can be formed within the thin film layer 210.

[0078] In the above preparation method, the pyroelectric field generated by temperature change replaces the traditional externally applied high voltage electric field. Therefore, it is not necessary to provide polarization reversal voltage by external circuit. Instead, it is only necessary to utilize the inherent pyroelectric properties of ferroelectric crystals and generate a pyroelectric field through temperature change to achieve polarization reversal.

[0079] Compared to the externally applied electric field polarization method, in the fabrication of periodically polarized thin films via the pyroelectric effect, the pyroelectric field exists only between the upper and lower surfaces of the thin film layer 210. In this case, since the pyroelectric field directly polarizes the thin film layer 210 with a thickness ≤1μm, only a voltage of tens of volts is required to achieve polarization reversal. Furthermore, the lower polarization reversal voltage can suppress the lateral broadening effect of the reverse domains, thereby enabling the fabrication of submicron periodic reverse domain structures. This allows for use in the spontaneous parametric down-conversion process of reverse propagation, generating narrow-linewidth quantum entangled light sources.

[0080] Meanwhile, the preparation process does not require signal generators, high-voltage amplifiers, oscilloscopes, or other equipment, nor does it require connecting the polarization electrode to an external circuit. It only requires temperature control equipment to regulate the temperature of the thin film structure, making the operation simple and reducing production costs.

[0081] Furthermore, the preparation method provided in this application offers a high degree of freedom in adjusting the intensity of the pyroelectric field. It can be adjusted by changing the amount of temperature change or by changing the amount of pyroelectric charge dissipation, thereby achieving precise control of the domain structure, improving the consistency of thin film performance, and enhancing the adaptability of the periodically polarized thin film preparation method to different ferroelectric materials.

[0082] It should be noted that the above preparation method is applicable at least to lithium niobate single crystal thin films and lithium tantalate single crystal thin films with a film layer thickness ≤ 1 μm, as well as all tangential directions except X-cut and Y-cut, such as Z, Y36, Y42, Y64, Y128, etc.

[0083] The following uses a lithium niobate single-crystal thin film as an example to further illustrate the method for preparing periodically polarized thin films provided in the embodiments of this application.

[0084] Figure 4 This is a schematic diagram of the temperature change processing provided in an embodiment of this application. Figure 5 For ferroelectric lithium niobate crystals in Figure 4 A schematic diagram illustrating the principle of the internal pyroelectric effect under temperature change processing is provided. Figure 6 for Figure 4 The temperature change curve provided in the software corresponds to the temperature change processing.

[0085] in, Figure 5 (a) in the diagram corresponds to the internal changes of a lithium niobate crystal in the ferroelectric phase at room temperature; Figure 5 (b) in the diagram represents the internal changes of the ferroelectric lithium niobate crystal under heated conditions. Figure 5 (c) in the diagram corresponds to the internal changes of the ferroelectric lithium niobate crystal under isothermal conditions; Figure 5 (d) in the diagram corresponds to the internal changes of the ferroelectric lithium niobate crystal under cooling conditions. It can be understood that the pyroelectric effect occurring inside the lithium niobate single crystal thin film can be inferred by referring to the principle of pyroelectric effect inside the ferroelectric lithium niobate crystal.

[0086] Reference Figure 4 As shown, in S200, applying a temperature change treatment to the thin film structure with metal electrodes may include:

[0087] S201. The thin film structure is heated, and the temperature of the thin film layer is raised from room temperature to the preset temperature within a first time period.

[0088] Reference Figure 5 As shown, at room temperature, inside the ferroelectric lithium niobate crystal, Li + Ions and Nb 5+ The ions all exhibit a certain degree of displacement along the z-axis. Among them, Li... +The ion left the common face of the oxygen octahedron, Nb 5+ When ions leave the center of the oxygen octahedron, the symmetry on the z-axis is broken, the centers of positive and negative charges no longer overlap, an electric dipole moment is generated, and spontaneous polarization is formed.

[0089] This spontaneous polarization generates a space charge field, attracting free charges on the surface to compensate, thus resulting in overall electrical neutrality, such as... Figure 5 As shown in (a) of the diagram.

[0090] When the thin film structure is heated, the thermal energy of the ions increases, and Li + Ions and Nb 5+ Ions migrate towards the adjacent oxygen plane and the center of the oxygen octahedron, respectively, reducing the spontaneous polarization of the crystal and weakening the positive charge of the +z plane. This results in an excess of negative charge for charge balance, making the crystal planes negatively charged. Figure 5 As shown in (b) above. However, at this time, the direction of the generated pyroelectric field is the same as the spontaneous polarization direction of the lithium niobate crystal, and it cannot cause polarization reversal.

[0091] For example, as one implementation method, the above process can be carried out by... Figure 6 The heating curve L1 shown is achieved. At t=0, the initial temperature of the thin film structure is room temperature RT. At this time, the thin film structure can be continuously heated by a heating plate or a hot-cold stage to raise the temperature of the thin film layer 210 from room temperature RT to a preset temperature T0 within the first time period (0-t1 interval). At this time, the heating rate of the thin film layer 210 within the first time period is (T0-RT) / t1. The preset temperature T0 can be set as needed and is not limited here.

[0092] Since there are periodic metal electrodes on the surface of the thin film layer 210, and metal is a good conductor, excess compensation charge will be quickly transferred to the metal electrode in contact with it, thereby accumulating negative charge when heated, and the potential becomes negative.

[0093] It should be noted that the heating process can be continuous (as shown in L1) or segmented. No specific form of heating process is specified here.

[0094] S202. The thin film layer is kept at a preset temperature for a second time period so that the surface of the thin film layer is electrically neutral.

[0095] When the temperature is raised to a preset temperature T0 and kept constant, the surface free charges attracted by the space charge field will respond to the change in spontaneous polarization inside the crystal, and eventually return to electrical neutrality on the crystal surface, such as... Figure 5 As shown in (c) in the figure.

[0096] This process can be achieved through Figure 6The isothermal curve L2 shown is achieved. During the second time period (corresponding to the interval t1 to t2), the thin film layer 210 is kept at a constant temperature T0 to allow the surface of the thin film layer 210 to regain electrical neutrality. The isothermal time (t2-t1) here needs to be greater than the response time of the surface free charges to avoid the residual pyroelectric field partially compensating for the pyroelectric field generated during cooling, which would be in the opposite direction and thus reduce the strength of the pyroelectric field used for polarization reversal. During the isothermal process, a multimeter can be used to detect the potential difference between the upper surface of the TFLN thin film and the heating plate. When the potential difference drops to 0, it can be considered that the TFLN thin film as a whole has regained electrical neutrality.

[0097] S203. Cool the thin film structure and reduce the temperature of the thin film layer from the preset temperature to room temperature within the third time period.

[0098] When the thin film structure is cooled, the thermal energy of the ions decreases, and Li + Ions and Nb 5+ The ions are further away from the oxygen plane and the center of the oxygen octahedron, which enhances the spontaneous polarization of the crystal, and the +z plane exhibits positive charge, such as... Figure 5 As shown in (d) in the figure.

[0099] During cooling, the periodic metal electrodes on the surface of thin film layer 210 accumulate positive charges, causing their potential to become positive and thus generating a pyroelectric field with the -z plane of the crystal. The direction of this pyroelectric field is opposite to the spontaneous polarization direction of the lithium niobate crystal. During cooling, polarization reversal occurs when the intensity of the pyroelectric field exceeds the coercive field of the lithium niobate crystal.

[0100] This process can be achieved through Figure 6 The cooling curve L3 shown is achieved. During the third time period (corresponding to the interval t2 to t3), the thin film layer 210 cools back from the preset temperature T0 to room temperature RT. At this time, the cooling rate of the thin film layer 210 during the third time period is (T0-RT) / (t3-t2). During this process, a pyroelectric field with the opposite polarization direction to the spontaneous polarization direction of the lithium niobate crystal and a field strength greater than the coercive field can be generated in the thin film layer 210 to drive polarization reversal in the region below the metal electrode.

[0101] In this way, through the combined temperature change treatment of heating, isothermal and cooling, the region corresponding to the metal electrode in the thin film layer 210 can achieve polarization reversal during the cooling process, while the part not covered by the metal electrode still maintains the original polarization direction, thereby obtaining a periodically polarized thin film with a periodic reverse domain structure at room temperature.

[0102] In one embodiment, the periodic reverse domain structure may include a plurality of alternately arranged reverse domain regions and non-reverse domain regions. The sum of the widths of a set of adjacent reverse domain regions and non-reverse domain regions constitutes the polarization period, which is greater than or equal to 1 μm and less than or equal to 20 μm.

[0103] For example, the polarization period can be 1μm, 2μm, 4μm, 6μm, 8μm, 10μm, 12μm, 14μm, 16μm, 18μm, 20μm, or any combination of both, to meet the polarization period requirements of practical nonlinear optical mixing designs.

[0104] As one implementation method, the preset temperature can be greater than or equal to 100°C and less than or equal to 300°C.

[0105] For example, the preset temperature T0 can be 100℃, 120℃, 150℃, 180℃, 200℃, 220℃, 250℃, 280℃, 300℃, or any range of both. This setting balances the polarization reversal voltage and the maximum operating temperature of the thin film.

[0106] Furthermore, by changing the preset temperature T0, the amount of temperature change can be adjusted, thereby regulating the intensity of the pyroelectric field and determining the magnitude of the pyroelectric voltage.

[0107] At this point, by adjusting the value of the preset temperature T0, other temperature parameters (such as the cooling rate) can be kept constant, that is, by using the temperature change as a variable, different pyroelectric field strengths can be obtained.

[0108] For example, the heating rate and cooling rate can both be set to 5℃ / min, the second time period (constant temperature time) can be set to 2hrs, and the preset temperature T0 can be set to 100℃, 150℃, 180℃, 200℃ and 250℃ to obtain different pyroelectric field strengths.

[0109] Upon heating, the spontaneous polarization of the TFLN thin film weakens due to the increased thermal energy, and negative charges are released from the +z surface. Upon cooling, spontaneous polarization strengthens, and the direction of the pyroelectric field is opposite to that of spontaneous polarization. When the electric field strength exceeds the coercive field, polarization reversal occurs in the region below the electrode. Furthermore, the greater the temperature change (T0-RT), the higher the pyroelectric field strength. However, it should be noted that excessively high voltage may cause excessive lateral broadening, thus requiring adaptive adjustments to the pyroelectric field.

[0110] Therefore, by adjusting the temperature change, the intensity of the pyroelectric field can be flexibly controlled, thereby precisely regulating the duty cycle of the inverted domains. Using a higher temperature change (such as setting the preset temperature T0 to 250℃) can enhance the pyroelectric field intensity, shorten the polarization time, and suppress the lateral widening of the inverted domains, achieving a submicron-level periodic structure. It also enhances the degree of process freedom.

[0111] In one embodiment, the cooling rate of the thin film layer 210 during the third time period can be greater than or equal to 1°C / min and less than or equal to 10°C / min.

[0112] For example, the cooling rate of the thin film layer 210 during the third time period can be 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min, or any combination of the above.

[0113] The polarization reversal process occurs in the third time period (corresponding to the interval from t2 to t3). Although the magnitude of the pyroelectric voltage is determined by the change in temperature, extending the cooling time, i.e. reducing the cooling rate, can increase the dissipation of pyroelectric charges into the surrounding environment, thereby reducing the intensity of the pyroelectric field and thus achieving regulation of the intensity of the pyroelectric field.

[0114] For example, the heating rate can be set to 5℃ / min, the preset temperature T0 can be set to 200℃, the third time period can be set to 2hrs, and the cooling rate can be set to 10℃ / min, 8℃ / min, 5℃ / min, 2℃ / min and 1℃ / min respectively. By extending the cooling time, the dissipation of pyroelectric charge is increased, thereby obtaining different pyroelectric field strengths.

[0115] Therefore, by adjusting the dissipation of pyroelectric charge, the intensity of the pyroelectric field can be flexibly controlled, thereby precisely regulating the duty cycle of the inverted domain.

[0116] In one embodiment, the heating rate of the thin film layer 210 during the first time period can be greater than or equal to 1°C / min and less than or equal to 10°C / min.

[0117] For example, the heating rate of the thin film layer 210 during the first time period can be 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min, or any combination of the above.

[0118] It is understandable that, since no polarization reversal occurs during the heating process, the heating rate only relates to how quickly the preset temperature T0 is reached and the overall process time required.

[0119] However, since TFLN film is a composite multilayer material, each layer has a different coefficient of thermal expansion. If drastic temperature changes are used, large stresses or strains may be generated. Therefore, by setting the heating rate within the above range, the film structure can be prevented from cracking during the heating and cooling process.

[0120] Figure 7 Another temperature change curve provided for an embodiment of this application.

[0121] In another embodiment, S401, heating the thin film structure and raising the temperature of the thin film layer from room temperature to a preset temperature within a first time period, may include:

[0122] The temperature of the thin film layer is increased from room temperature to a preset temperature using a step-by-step heating curve. This step-by-step heating curve consists of n heating segments and n-1 isothermal segments (n>1).

[0123] The isothermal segment is connected between two adjacent heating segments. The sum of the heating duration of each heating segment and the isothermal duration of each isothermal segment constitutes the first time period. The sum of the temperature differences of each heating segment is the difference between the preset temperature and the room temperature.

[0124] Heating can be achieved through gradual, step-by-step heating. For example, refer to... Figure 7 As shown, n can be equal to 3, and the segmented heating curve can include 3 heating segments and 2 isothermal segments. An isothermal segment is placed between two adjacent heating segments to allow the thin film layer 210 to respond to polarization abrupt changes caused by heating. Similarly, the potential difference between the upper surface of the thin film layer 210 and the heating plate can be measured using a multimeter. When the potential difference drops to 0, it can be considered that the thin film layer 210 has returned to electrical neutrality overall.

[0125] The sum of the temperature differences in each heating segment is the temperature change T0 - RT. For example, Figure 7 The temperature difference in the first heating segment is A1, the temperature difference in the second heating segment is A2, and the temperature difference in the third heating segment is A3. A1 + A2 + A3 = T0 - RT. The heating duration of each heating segment and the isothermal duration of each isothermal segment constitute the first time period.

[0126] It should be noted that during the segmented heating process, the temperature difference between each heating segment can be the same or different, and the heating rate of each segment can also be the same or different, which can be set according to specific needs. Furthermore, the number of heating segments can be reasonably set according to the temperature change, as long as n is an integer greater than 1.

[0127] At this point, the first time period corresponds to the interval 0-t1'. The second time period corresponds to the interval t1'-t2. The third time period still corresponds to the interval t2-t3. It is understandable that... Figure 6 Compared to continuous heating processes, due to Figure 7 The step-by-step heating process used in the process, when Figure 6 and Figure 7 When the same heating rate is used, Figure 7 The value of t1' in the equation is clearly greater than 1 / 2. Figure 6 The t1 value is given. Since the overall temperature change T0-RT has not changed, the t2 values ​​for both remain the same, thus not affecting the subsequent third time period.

[0128] Because of the significant difference in thermal expansion coefficients between the thin film layer 210 and the supporting structure 100, continuous rapid heating may lead to a sharp accumulation of interfacial stress, causing problems such as cracking of the thin film layer 210 or delamination from the supporting structure 100. By adopting a segmented heating method, the isothermal period between adjacent heating segments allows the thermal expansion of each layer to gradually reach equilibrium, releasing interfacial stress and ensuring the integrity of the thin film structure. Simultaneously, this setting also reduces the duration of the second time period, decomposing the original isothermal response time into the previous heating process, avoiding an excessively long second time period when the temperature change is large.

[0129] Figure 8 This is a schematic diagram illustrating the formation of a lithium niobate single-crystal thin film according to an embodiment of this application. Figure 9 This is a schematic diagram illustrating the formation of another lithium niobate single-crystal thin film provided in an embodiment of this application.

[0130] In one implementation, S100, providing a thin film structure having periodically arranged metal electrodes may include:

[0131] S101, forming a thin film structure.

[0132] In some embodiments, refer to Figure 8 As shown, the formation process of the thin film structure is explained using a lithium niobate single crystal thin film as an example. The formation of the thin film structure can specifically include:

[0133] A substrate 110 is provided, and an isolation layer 120 (such as a silicon dioxide layer) is deposited on top of the substrate 110 to obtain a support structure 100.

[0134] A ferroelectric crystal wafer 200 is provided, and the ferroelectric crystal wafer 200 is subjected to ion implantation to form an implanted wafer. The implanted wafer includes a residual mass layer 230, an implantation layer 220, and a thin film layer 210 stacked together.

[0135] The thin film layer 210 is bonded to the isolation layer 120 to form a bonded body, and the implantation layer 220 and the residual mass layer 230 are removed by annealing and peeling to obtain the thin film structure.

[0136] For example, the bonded body is subjected to annealing heat treatment, and the heat treatment process can be in the range of 180℃-600℃, with a heat treatment duration of 1-100 hours. During the heat treatment process, bubbles are formed in the implanted layer 220, for example, H ions form hydrogen gas, He ions form helium gas, etc. As the heat treatment progresses, the bubbles in the implanted layer 220 merge together, and finally the implanted layer 220 cracks, separating the residual mass layer 230 from the thin film layer 210, thereby peeling the residual mass layer 230 off the bonded body and forming the thin film layer 210 on the top surface of the treated substrate.

[0137] It should be noted that no limitation is made on the ion implantation method here, and any ion implantation method in the prior art can be used to perform ion implantation on the ferroelectric crystal wafer 200. Furthermore, the implanted ions can be any ions that can generate gas through heat treatment, such as hydrogen ions, helium ions, nitrogen ions, oxygen ions, or argon ions.

[0138] Furthermore, this application can use any bonding method in the prior art to achieve the bonding of the thin film layer 210 and the isolation layer 120. For example, the process surfaces of the thin film layer 210 and the isolation layer 120 can be surface activated, and then the two activated surfaces can be bonded to obtain a bonded body.

[0139] In other embodiments, reference is made to Figure 9 As shown, a trapping layer 130 can be formed between the substrate layer 110 and the isolation layer 120 to reduce the radio frequency loss of the thin film.

[0140] At this point, the formation of a thin film structure can specifically include,

[0141] A substrate layer 110 is provided, and a trapping layer 130 is formed on top of the substrate layer 110. An isolation layer 120 is deposited on top of the trapping layer 130 to obtain a support structure 100. At this time, the support structure 100 includes the isolation layer 120, the trapping layer 130 and the substrate layer 110 stacked together.

[0142] The material of the trapping layer 130 can be selected from at least one of polycrystalline silicon, amorphous silicon, or polycrystalline germanium. The trapping layer 130 can be formed by depositing polycrystalline silicon, depositing amorphous silicon, depositing polycrystalline germanium, etching the substrate layer 110, or implanting the substrate layer 110 to generate implantation damage.

[0143] A ferroelectric crystal wafer 200 is provided, and the ferroelectric crystal wafer 200 is subjected to ion implantation to form an implanted wafer. The implanted wafer includes a residual mass layer 230, an implantation layer 220, and a thin film layer 210 stacked together.

[0144] The thin film layer 210 is bonded to the isolation layer 120 to form a bonded body, and the implantation layer 220 and the residual mass layer 230 are removed by annealing and peeling to obtain the thin film structure.

[0145] Because the trapping layer 130 has a certain density of lattice defects, it can trap the carriers existing between the isolation layer 120 and the substrate layer 110, and prevent these carriers from causing carrier aggregation at the interface between the isolation layer 120 and the substrate layer 110, thereby reducing the radio frequency loss of the thin film.

[0146] It is understood that the provided ferroelectric crystal wafer 200 can be a single crystal wafer, a single crystal doped wafer, or a bonded combination of single crystal wafers and single crystal doped wafers.

[0147] When the ferroelectric crystal wafer 200 is a bond between a single crystal wafer and a single crystal doped wafer, for example, a lithium niobate single crystal wafer and a lithium niobate doped wafer arranged in parallel can be bonded to prepare a composite TFLN thin film, and the formation process can refer to the above process.

[0148] S102. A periodically arranged photoresist pattern is formed on the surface of the thin film layer away from the support structure.

[0149] For example, forming a periodically arranged photoresist pattern may specifically include:

[0150] A layer of positive ultraviolet photoresist is coated onto the upper surface of the cleaned thin film layer 210 using a spin coater. The thickness of the photoresist is generally greater than 1 μm, and the thickness of the photoresist is at least 1.5 times the thickness of the metal electrode.

[0151] The UV photoresist is pre-baked using a heating plate or oven to evaporate the organic solvent components and cure the photoresist. The pre-baking temperature and time are set according to different photoresists.

[0152] A photomask with a periodically arranged pattern is provided. The sample is exposed using an ultraviolet lithography machine. Ultraviolet light shines through the transparent window on the photomask onto the photoresist, causing the photoresist to denature and thus be dissolved by the developer in the subsequent development process. The photoresist that is not exposed to ultraviolet light due to the pattern on the photomask is retained after development.

[0153] High-temperature baking is used to harden the patterned photoresist formed after development, so that the photoresist can adhere more firmly to the wafer surface and enhance the photoresist's resistance to etching. The hardening temperature is usually slightly higher than the pre-baking temperature.

[0154] S103. Deposit metal on the surface of a thin film layer with a photoresist pattern;

[0155] A metal electrode layer is deposited on the surface of a periodically arranged photoresist pattern using methods such as electron beam evaporation and magnetron sputtering.

[0156] S104. Strip the photoresist pattern and the metal covering the photoresist pattern to obtain a thin film structure with periodically arranged metal electrodes.

[0157] Metal electrode liftoff is performed using specialized resist removers such as acetone and NMP. The metal deposited on the photoresist surface is removed as the photoresist dissolves, while the metal electrode directly deposited on the surface of thin film layer 210 is retained, thus forming a periodic metal electrode pattern consistent with the pattern on the photomask.

[0158] In some embodiments, the material of the metal electrode can be any one of Au, Al, Cr, and Ti. This allows the metal electrode to have good electrical conductivity.

[0159] The thickness of the metal electrode can be greater than or equal to 100 nm and less than or equal to 1000 nm. For example, the thickness of the metal electrode can be 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, 1000 nm, or any combination thereof. This ensures the conductivity of the metal electrode.

[0160] Considering the potential lateral broadening effect of reverse domains, the width of the metal electrode can be set to 40%~50% of the polarization period, that is, the width of the metal electrode is greater than or equal to 0.4μm and less than or equal to 10μm.

[0161] In some embodiments, before obtaining a periodically polarized thin film with a periodically reversed domain structure after polarization reversal occurs, the preparation method further includes:

[0162] Remove the metal electrode formed on the surface of the thin film layer opposite to the support structure.

[0163] After forming the reverse domain structure within the thin film layer 210, the metal electrode can be removed using wet etching technology to obtain a periodically polarized thin film. The reverse domain structure can be non-destructively inspected using piezoelectric microscopy (PFM).

[0164] It should be noted that the embodiments referred to in the specification, such as "one embodiment," "embodiment," "exemplary embodiment," and "some embodiments," may include specific features, structures, or characteristics, but not every embodiment necessarily includes that specific feature, structure, or characteristic. Furthermore, such phrases do not necessarily refer to the same embodiment. Moreover, when a specific feature, structure, or characteristic is described in connection with an embodiment, implementing such a feature, structure, or characteristic in conjunction with other embodiments, whether explicitly described or not, is within the knowledge scope of those skilled in the art.

[0165] Generally speaking, terms should be understood at least in part by their use in context. For example, at least in part by context, the term "one or more" as used in the text can be used to describe any feature, structure, or characteristic of the singular meaning, or a combination of features, structures, or characteristics of the plural meaning. Similarly, at least in part by context, terms such as "one" can be understood to convey either singular or plural usage.

[0166] It should be readily understood that the terms “on,” “above,” and “on top of” in this application should be interpreted in the broadest possible sense, such that “on” means not only “directly on something” but also “on something” with an intermediate feature or layer therebetween, and that “above” or “on top of” means not only “on something” but also “on something” without an intermediate feature or layer therebetween (i.e., directly on something).

[0167] Furthermore, for ease of explanation, spatially relative terms such as "below," "below," "under," "above," and "above" may be used to describe the relationship of one element or feature relative to other elements or features as shown in the figures. Spatially relative terms are intended to encompass different orientations of the device in use or operation other than those shown in the figures. The device may have other orientations (rotated 90° or in other orientations), and the spatially relative descriptive terms used herein may be interpreted accordingly.

[0168] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A method for producing a periodically poled thin film, characterized by comprising the steps of: include: ​ A thin film structure having periodically arranged metal electrodes is provided; the thin film structure includes a stacked thin film layer and a support structure, the thin film layer and the support structure are bonded together, and the metal electrodes are connected to the side of the thin film layer opposite to the support structure; A temperature change treatment is applied to the thin film structure having the metal electrode to generate a pyroelectric field inside the thin film layer; The pyroelectric field is used to drive polarization reversal in the region of the thin film layer corresponding to the metal electrode, thereby obtaining a periodically polarized thin film with a periodic reverse domain structure.

2. The method of claim 1, wherein the method further comprises: The temperature change treatment applied to the thin film structure having the metal electrode includes: The thin film structure is heated, and the temperature of the thin film layer is raised from room temperature to a preset temperature within a first time period. The thin film layer is held at the preset temperature for a second time period to make the surface of the thin film layer electrically neutral. The thin film structure is cooled down, and the temperature of the thin film layer is reduced from a preset temperature to room temperature within a third time period.

3. The method for preparing periodically polarized thin films according to claim 2, characterized in that, The process of heating the thin film structure and raising the temperature of the thin film layer from room temperature to a preset temperature within a first time period includes: The temperature of the thin film layer is increased from room temperature to a preset temperature through a step-by-step heating curve; The segmented heating curve includes n heating segments and n-1 isothermal segments; the isothermal segments are connected between two adjacent heating segments. The sum of the heating duration of each heating segment and the isothermal duration of each isothermal segment is the first time period; The sum of the temperature differences of each of the heating segments is the difference between the preset temperature and the room temperature.

4. The method for preparing periodically polarized thin films according to claim 1, characterized in that, The periodic reverse domain structure includes multiple alternately arranged reverse domain regions and non-reverse domain regions; The sum of the widths of a group of adjacent inverted domain regions and non-inverted domain regions is the polarization period, which is greater than or equal to 1 μm and less than or equal to 20 μm.

5. The method for preparing periodically polarized thin films according to claim 2, characterized in that, The preset temperature is greater than or equal to 100℃ and less than or equal to 300℃.

6. The method for preparing periodically polarized thin films according to claim 2, characterized in that, The cooling rate of the thin film layer during the third time period is greater than or equal to 1°C / min and less than or equal to 10°C / min.

7. The method for preparing periodically polarized thin films according to claim 2, characterized in that, The heating rate of the thin film layer during the first time period is greater than or equal to 1℃ / min and less than or equal to 10℃ / min.

8. The method for preparing periodically polarized thin films according to claim 1, characterized in that, Thin film structures providing periodically arranged metal electrodes include: Forming a thin film structure; A periodically arranged photoresist pattern is formed on the surface of the thin film layer opposite to the support structure; Metal is deposited on the surface of the thin film layer having the photoresist pattern; The photoresist pattern and the metal covering the photoresist pattern are stripped to obtain a thin film structure with periodically arranged metal electrodes.

9. The method for preparing periodically polarized thin films according to claim 1, characterized in that, The material of the metal electrode is any one of Au, Al, Cr, and Ti; And / or, the thickness of the metal electrode is greater than or equal to 100 nm and less than or equal to 1000 nm; And / or, the width of the metal electrode is greater than or equal to 0.4 μm and less than or equal to 10 μm.

10. The method for preparing periodically polarized thin films according to claim 9, characterized in that, Before obtaining a periodically polarized thin film with a periodically reversed domain structure after polarization reversal occurs, the method further includes: Remove the metal electrode formed on the surface of the thin film layer opposite to the support structure.

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