Periodically polarized lithium niobate film and preparation method thereof

Abstract

The invention relates to a preparation method of a periodically poled lithium niobate film. The preparation method of the periodically poled lithium niobate film comprises the following steps: providing a low-resistance silicon substrate; forming a lithium niobate single crystal film on the low-resistance silicon substrate; wherein the thickness range of the lithium niobate single crystal thin film is 10 nanometers to 10 microns; forming a polarized electrode on the surface, deviating from the low-resistance silicon substrate, of the lithium niobate single crystal film; and preparing a reversal domain structure by adopting an external electric field polarization method to obtain the reversal domain structure penetrating through the lithium niobate single crystal film in the thickness direction. According to the embodiment of the invention, complete polarization inversion of the lithium niobate film can be realized, so that the inversion domain structure penetrates through the whole film thickness in depth, and the submicron periodically polarized lithium niobate film can be prepared.

Description

Technical Field

[0001] The invention relates to the field of optoelectronic materials, and in particular to a periodically polarized lithium niobate film and a preparation method thereof. Background Art

[0002] Periodically poled lithium niobate (PPLN) prepared by domain inversion of lithium niobate single crystal thin films is widely used in optical fields such as nonlinear frequency conversion, optical parametric oscillators, and electro-optical Bragg reflection gratings. In addition, the conductive domain walls generated by polarization inversion also show broad application prospects in nanoelectronics fields such as memristors, transistors, and PN junctions.

[0003] However, the current method for preparing the inversion domain structure makes it difficult to achieve complete polarization reversal of lithium niobate films, that is, the inversion domain structure cannot penetrate the entire film thickness in depth and is a surface domain structure.

[0004] Therefore, how to achieve complete polarization reversal of lithium niobate films is an urgent problem to be solved. Summary of the invention

[0005] Based on this, the embodiments of the present application provide a periodically poled lithium niobate film and a preparation method thereof, which can achieve complete polarization reversal of the lithium niobate film, so that the inversion domain structure runs through the entire film thickness in depth.

[0006] In order to achieve the above objectives, in a first aspect, some embodiments of the present application provide a method for preparing a periodically poled lithium niobate film, comprising the following steps:

[0007] Providing low-resistance silicon substrate;

[0008] A lithium niobate single crystal film is formed on a low-resistance silicon substrate; wherein the thickness of the lithium niobate single crystal film ranges from 10 nm to 10 μm;

[0009] A polarization electrode is formed on the surface of the lithium niobate single crystal film facing away from the low-resistance silicon substrate;

[0010] An inversion domain structure is prepared by an external electric field polarization method, and an inversion domain structure penetrating the lithium niobate single crystal film in the thickness direction is obtained.

[0011] In some embodiments of the present application, forming a lithium niobate single crystal thin film on a low-resistivity silicon substrate includes the following steps:

[0012] An initial lithium niobate single crystal film is formed on a low-resistance silicon substrate; wherein the thickness of the initial lithium niobate single crystal film ranges from 5 μm to 50 μm;

[0013] The thickness of the initial lithium niobate single crystal film is reduced to obtain a lithium niobate single crystal film.

[0014] In some embodiments of the present application, reducing the thickness of the initial lithium niobate single crystal film to obtain the lithium niobate single crystal film comprises the following steps:

[0015] The initial lithium niobate single crystal film is thinned by a chemical mechanical polishing process at a preset rotation speed, and a lithium niobate single crystal film is obtained after continuous processing for a preset time.

[0016] In some embodiments of the present application, the preset rotation speed includes: 20rpm~40rpm.

[0017] In some embodiments of the present application, the preset time includes: 5 hours to 8 hours.

[0018] In some embodiments of the present application, forming a polarization electrode on a surface of a lithium niobate single crystal film facing away from a low-resistance silicon substrate includes:

[0019] Polarization electrodes are formed in sequence on the surface of the lithium niobate single crystal film that is away from the low-resistance silicon substrate by using ultraviolet lithography, magnetron sputtering and lift-off processes.

[0020] In some embodiments of the present application, the polarized electrode includes: a chromium metal electrode. The thickness of the chromium metal electrode ranges from 20 nm to 400 nm.

[0021] In some embodiments of the present application, before the polarization electrodes are sequentially formed on the surface of the lithium niobate single crystal film away from the low-resistance silicon substrate by using the ultraviolet lithography process, the magnetron sputtering process and the lift-off process, the following steps are also included:

[0022] The lithium niobate single crystal film is cleaned by using a piranha solution, wherein the piranha solution comprises: 98% H2SO4 and 30% H2O2 mixed in a volume ratio of 7:3.

[0023] In a second aspect, some embodiments of the present application further provide a periodically poled lithium niobate film, including: a lithium niobate single crystal film located on a low-resistance silicon substrate. The lithium niobate single crystal film has an inversion domain structure; the inversion domain structure runs through the lithium niobate single crystal film in the thickness direction; the thickness range of the lithium niobate single crystal film includes: 10nm to 10μm.

[0024] In some embodiments of the present application, the resistivity range of the low-resistance silicon substrate includes: less than 0.05Ω·cm.

[0025] The periodically poled lithium niobate film and the preparation method thereof provided in the present application may have / at least have the following advantages:

[0026] In the embodiment of the present application, low-resistance silicon is used as the substrate for polarizing the lithium niobate single crystal thin film, that is, the bottom electrode, and then an external electric field polarization method is used to prepare the inversion domain structure. In this way, the problems of incomplete polarization, high required voltage, and severe domain broadening phenomenon that often occur in the domain reversal of lithium niobate single crystal thin films can be effectively improved. In this way, complete polarization reversal can be achieved in the lithium niobate single crystal thin film on the low-resistance silicon substrate, that is, the inversion domain can run through the entire film thickness, and when the polarization period is in the micron range, the domain broadening phenomenon can be ignored relative to the inversion domain size. Based on this, high-quality, sub-micron periodically polarized lithium niobate thin films can also be prepared on low-resistance silicon substrates.

[0027] In addition, in the embodiment of the present application, the lithium niobate single crystal film is thinned to the order of hundreds of nanometers by a precise chemical mechanical polishing process, and then an external electric field polarization method is used to prepare an inversion domain structure. In this way, complete polarization reversal of a lithium niobate single crystal film of the order of hundreds of nanometers can be achieved on a low-resistance silicon substrate. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present disclosure, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present disclosure. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0029] FIG. 1( a ) is a graph showing the relationship between polarization current and polarization voltage of a lithium niobate film; FIG. 1( b ) is an optical microscope image of a lithium niobate film;

[0030] Figure 2 A schematic diagram of a process for preparing a periodically poled lithium niobate film provided in an embodiment of the present application;

[0031] Figure 3 Schematic diagram of the process of step S200 in a method for preparing a periodically poled lithium niobate film provided in an embodiment of the present application;

[0032] Figure 4(a) is a schematic diagram of the cross-sectional structure of step S220 in a method for preparing a periodically poled lithium niobate film provided in an embodiment of the present application; Figure 4(b) is an optical microscope image of the structure shown in step S220 provided in an embodiment of the present application.

[0033] Figure 5(a) is an optical microscope image of a periodically polarized electrode of a lithium niobate film provided in an embodiment of the present application; Figure 5(b) is a schematic diagram of an experimental device for polarizing a lithium niobate film provided in an embodiment of the present application; Figure 5(c) is a graph showing the relationship between polarization current and polarization voltage of a lithium niobate film provided in an embodiment of the present application over time; Figure 5(d) is a piezoelectric atomic force microscope phase image of a partial micrometer periodic polarization inversion domain of a lithium niobate film provided in an embodiment of the present application; Figure 5(e) is a piezoelectric atomic force microscope phase image of a partial submicrometer periodic polarization inversion domain of a lithium niobate film provided in an embodiment of the present application.

[0034] Figure 6 Optical microscope images of a periodically poled lithium niobate film provided in an embodiment of the present application at different times of hydrofluoric acid treatment.

[0035] Description of reference numerals:

[0036] 1- low-resistance silicon substrate; 2- lithium niobate single crystal thin film; 20- initial lithium niobate single crystal thin film. DETAILED DESCRIPTION

[0037] In order to facilitate understanding of the present disclosure, the present disclosure will be described more fully below with reference to the relevant drawings. Embodiments of the present disclosure are given in the drawings. However, the present disclosure can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present disclosure more thorough and comprehensive.

[0038] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which the present disclosure belongs. The terms used herein in the specification of the present disclosure are only for the purpose of describing specific embodiments and are not intended to limit the present disclosure.

[0039] It should be understood that when an element or layer is referred to as being "on, adjacent to, connected to or coupled to other elements or layers, it may be directly on, adjacent to, connected to or coupled to other elements or layers, or there may be intervening elements or layers. In contrast, when an element is referred to as being "directly on, directly adjacent to, directly connected to or directly coupled to other elements or layers, there may be no intervening elements or layers. It should be understood that, although the terms first, second, third, etc. may be used to describe various elements, components, regions, layers and / or parts, these elements, components, regions, layers and / or parts should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or part from another element, component, region, layer or part. Therefore, without departing from the teachings of the present invention, the first element, component, region, layer or part discussed below may be represented as a second element, component, region, layer or part.

[0040] Spatially relative terms such as "under," "beneath," "below," "under," "above," "above," and the like may be used herein to describe the relationship of an element or feature shown in the figures to other elements or features. It should be understood that, in addition to the orientations shown in the figures, spatially relative terms also include different orientations of the device in use and operation. For example, if the device in the accompanying drawings is flipped, an element or feature described as "under other elements" or "under it" or "under it" will be oriented as being "above" the other elements or features. Thus, the exemplary terms "under" and "under" may include both upper and lower orientations. In addition, the device may also include additional orientations (e.g., rotated 90 degrees or other orientations), and the spatial descriptors used herein are interpreted accordingly.

[0041] When used herein, the singular forms "a", "an", and "said / the" may also include plural forms, unless the context clearly indicates otherwise. It should also be understood that when the terms "consisting of" and / or "comprising" are used in this specification, the presence of the features, integers, steps, operations, elements and / or parts can be determined, but the presence or addition of one or more other features, integers, steps, operations, elements, parts and / or groups is not excluded. At the same time, when used herein, the term "and / or" includes any and all combinations of the relevant listed items.

[0042] As used herein, a "deposition" process includes, but is not limited to, physical vapor deposition (PVD), chemical vapor deposition (CVD), or atomic layer deposition (ALD).

[0043] Embodiments of the invention are described herein with reference to cross-sectional views which are schematic diagrams of ideal embodiments (and intermediate structures) of the invention, such that variations in the shapes shown due to, for example, manufacturing techniques and / or tolerances are anticipated. Thus, embodiments of the invention should not be limited to the specific shapes of the regions shown herein, but rather include deviations in shapes due to, for example, manufacturing techniques. For example, an implanted region shown as a rectangle typically has rounded or curved features and / or an implant concentration gradient at its edges rather than a binary change from an implanted region to a non-implanted region. Similarly, a buried region formed by implantation may result in some implantation in the region between the buried region and the surface through which the implantation is performed. Thus, the regions shown in the figures are schematic in nature, their shapes do not represent the actual shape of the region of the device, and do not limit the scope of the invention.

[0044] Periodically poled lithium niobate (PPLN) prepared by domain inversion of lithium niobate single crystal thin films is widely used in optical fields such as nonlinear frequency conversion, optical parametric oscillators, and electro-optical Bragg reflection gratings. In addition, the conductive domain walls generated by polarization inversion also show broad application prospects in nanoelectronics fields such as memristors, transistors, and PN junctions.

[0045] At present, the inversion domain structure is mainly prepared in lithium niobate by external electric field polarization method and scanning probe tip field induced domain inversion technology. However, the above two commonly used methods for preparing the inversion domain structure are difficult to achieve complete polarization inversion of lithium niobate film, that is, the inversion domain structure cannot penetrate the entire film thickness in depth and is a surface domain structure.

[0046] There are two main structures of commercially produced 100-nanometer lithium niobate single crystal films. The first structure is a 100-nanometer lithium niobate single crystal film (x / z cut)-silicon dioxide layer-lithium niobate / silicon substrate. The second structure is a 100-nanometer lithium niobate single crystal film (x / z cut)-metal layer-silicon dioxide layer-lithium niobate / silicon substrate.

[0047] This lithium niobate single crystal film prepared by ion slicing technology is also called lithium niobate on insulator (LNOI). Based on LNOI, through the two common preparation methods mentioned above, there are examples of preparing PPLN for nonlinear frequency conversion or preparing conductive domain walls for memristors and other functions.

[0048] For thin film polarization, the basic method of external electric field polarization is to apply an external electric field opposite to the spontaneous polarization direction through solid or liquid electrodes at both ends of the crystal polarization direction (i.e., the z direction). When the electric field strength exceeds the coercive field, the polarization direction of the crystal is changed, thereby achieving ferroelectric domain reversal. The external electric field polarization method is currently the most widely used and mature ferroelectric domain reversal technology, with the advantages of simple process and good repeatability. However, this method is limited by the accuracy of photolithography technology and the domain broadening effect, and there are certain challenges in the preparation of submicron-scale domain structures.

[0049] The scanning probe tip field-induced domain reversal technology is usually implemented using an atomic force microscope (AFM). Due to the nanometer-scale probe diameter, high precision and precise positioning, AFM has unique advantages in the preparation and characterization of nanoscale ferroelectric domain structures. Similar to the external electric field polarization method, when the electric field applied to the conductive probe tip exceeds the coercive field, the spontaneous polarization can be reversed. However, this method is not conducive to the preparation of large-area inversion domains, and it is still difficult to move towards practical industrial applications.

[0050] For x-cut lithium niobate single crystal films, the polarization direction is in the film plane, and the preparation of PPLN requires the application of an in-plane electric field. The inversion domain generated by the external electric field polarization method has a serious broadening phenomenon, the inversion domain shape is difficult to control, and the inversion domain depth is usually difficult to penetrate the entire film thickness of the polarization region. The inversion domain prepared by the scanning probe tip field induced domain inversion technology is also difficult to penetrate the entire film thickness, and although the AFM scanning accuracy is high, the scanning range is limited, making it difficult to prepare a large area of ​​inversion domain structure.

[0051] For z-tangent lithium niobate single crystal films, the polarization direction is outside the film surface, and the preparation of PPLN requires the application of an out-of-plane electric field. For the first structure, the required polarization voltage is relatively large, usually several hundred volts. When the polarization voltage is insufficient, incomplete polarization is prone to occur. In addition, the electric field distribution along the depth direction (i.e., z direction) of the longitudinal growth of the inversion domain is uneven, and the inversion domain is cone-shaped. At the same time, the preparation of submicron inversion domains has a lateral broadening phenomenon, and the preparation conditions need to be strictly controlled.

[0052] For the second structure, the scanning probe tip field induced domain inversion technology is currently often used to prepare the inversion domain. The inversion domain is cone-shaped along the longitudinal direction and is also difficult to penetrate the entire film thickness.

[0053] For example, for the structure of z-cut 100 nanometer lithium niobate single crystal film-metal layer-silicon dioxide layer-lithium niobate substrate, an inversion domain structure was prepared using the external electric field polarization method. It was found that the inversion domain polarization was incomplete and failed to completely penetrate the entire film thickness in depth, and was a surface domain structure.

[0054] Further, for example, a gold layer with a thickness of about 100 nm was deposited on the surface of the z-cut lithium niobate film as a polarization electrode by sequentially coating, UV lithography, magnetron sputtering metal and lift-off technology, wherein the electrode consists of a circle with a radius of 80 μm and a rectangle with a length of 30 μm and a width of 25 μm, and the rectangle serves as a contact point with the external circuit. The +z plane of the lithium niobate points to the top gold electrode. During the polarization process, the top gold electrode is connected to the voltage source provided by the electrometer Keithley6517B through a probe holder with a flexible tungsten needle (T-4-35), and the bottom Cr electrode is led out with conductive silver glue and connected to the electrometer through a probe holder with a hard tungsten needle (ST-20-10). During the polarization process, it was found that the polarization current continued to increase with the increase of the polarization voltage until the current was too large and the film was burned.

[0055] Please refer to Figure 1(a). Taking a 300nm thick lithium niobate film of the same composition as an example, the relationship between the polarization current and the polarization voltage is shown in Figure 1(a). The phenomenon observed in the experiment is significantly different from the typical behavior of domain inversion. Normally, the pulse current generated during the polarization process should appear with the movement of the domain wall and disappear when the domain inversion is completed. Therefore, a leakage channel, such as a conductive domain wall, may be formed under the electrode, causing the current to continue to increase with the increase of voltage.

[0056] Regarding the depth characterization of the inversion domain of the above-mentioned lithium niobate film. After removing the polarized electrode with a gold etching solution, the hydrofluoric acid corrosion method was used to explore the reversal of the inversion domain. The corrosion rate of lithium niobate in hydrofluoric acid is highly anisotropic and depends on the direction of spontaneous polarization. At room temperature, the corrosion rate of the -z plane of lithium niobate in hydrofluoric acid with a purity of about 40% can reach several hundred nanometers per hour, while the corrosion rate of the +z plane is almost 0. Therefore, this method can observe the ferroelectric domain structure by forming a contrast contrast between the positive and negative domains.

[0057] Specifically, the film was immersed in a 42% hydrofluoric acid solution at room temperature for about 30 minutes and then washed with deionized water. The optical microscope image of the treated film is shown in Figure 1(b). By comparison, the grayscale represented by the color of the film below the polarization electrode is different from that of the surrounding uninverted area, indicating that an inversion domain has been formed. However, the color in the inversion area is uneven, indicating that the inversion depth of the inversion domain is uneven. In addition, as the corrosion time increases further, the color in the inversion area no longer changes and the chromium electrode at the bottom is never exposed, indicating that the inversion domain depth fails to penetrate the entire film thickness and is a surface domain structure.

[0058] In addition, when using metal materials such as chromium, copper, and aluminum as polarization electrodes in addition to gold as the top polarization electrode, incomplete polarization and uneven inversion domain depth were still observed even in the thinner 100nm lithium niobate film. Therefore, how to achieve complete polarization reversal of lithium niobate film is an urgent problem to be solved.

[0059] Based on this, the embodiments of the present application provide a periodically poled lithium niobate film and a preparation method thereof, which can achieve complete polarization reversal of the lithium niobate film, so that the inversion domain structure runs through the entire film thickness in depth.

[0060] See also Figure 2 Some embodiments of the present application provide a method for preparing a periodically poled lithium niobate film, including steps S100 to S400.

[0061] S100, providing a low-resistance silicon substrate.

[0062] S200, forming a lithium niobate single crystal film on a low-resistance silicon substrate; wherein the thickness of the lithium niobate single crystal film ranges from 10 nm to 10 μm.

[0063] S300, forming a polarization electrode on the surface of the lithium niobate single crystal film facing away from the low-resistance silicon substrate.

[0064] S400, an inversion domain structure is prepared by an external electric field polarization method, and an inversion domain structure penetrating the lithium niobate single crystal film in the thickness direction is obtained.

[0065] In the embodiment of the present application, low-resistance silicon is used as the substrate for polarizing the lithium niobate single crystal thin film, that is, the bottom electrode, and then an external electric field polarization method is used to prepare the inversion domain structure. In this way, the problems of incomplete polarization, high required voltage, and severe domain broadening phenomenon that often occur in the domain reversal of lithium niobate single crystal thin films can be effectively improved. In this way, complete polarization reversal can be achieved in the lithium niobate single crystal thin film on the low-resistance silicon substrate, that is, the inversion domain can run through the entire film thickness, and when the polarization period is in the micron level, the inversion domain broadening phenomenon can be ignored. In addition, lithium niobate films with submicron periodic polarization can also be prepared.

[0066] In some embodiments, see Figure 3 Step S200 of forming a lithium niobate single crystal thin film on a low-resistance silicon substrate includes steps S210 to S220.

[0067] S210, forming an initial lithium niobate single crystal film on a low-resistance silicon substrate; wherein the thickness of the initial lithium niobate single crystal film ranges from 5 μm to 50 μm.

[0068] S220, reducing the thickness of the initial lithium niobate single crystal film to obtain a lithium niobate single crystal film.

[0069] In some embodiments, step S210 of forming an initial lithium niobate single crystal thin film on a low-resistivity silicon substrate includes: bonding an initial lithium niobate single crystal thin film on the low-resistivity silicon substrate.

[0070] In some embodiments, step S220 of reducing the thickness of the initial lithium niobate single crystal film to obtain the lithium niobate single crystal film comprises the following steps:

[0071] The initial lithium niobate single crystal film is thinned by a chemical mechanical polishing process at a preset rotation speed, and a lithium niobate single crystal film is obtained after continuous processing for a preset time.

[0072] In some embodiments, the preset rotation speed includes: 20 rpm to 40 rpm.

[0073] In some embodiments, the preset time includes: 5 hours to 8 hours.

[0074] In some embodiments, step S300 forms a polarized electrode on the surface of the lithium niobate single crystal film facing away from the low-resistance silicon substrate, comprising the following steps:

[0075] Polarization electrodes are formed in sequence on the surface of the lithium niobate single crystal film that is away from the low-resistance silicon substrate by using ultraviolet lithography, magnetron sputtering and lift-off processes.

[0076] In some embodiments, the polarized electrode includes a chromium metal electrode, and the thickness of the chromium metal electrode ranges from 20 nm to 400 nm.

[0077] It should be noted that in the above embodiments, the execution of each step in the method is not strictly limited in order, and these steps may not necessarily be executed in the order described, and there may be other execution methods. Moreover, at least a part of any step may include multiple sub-steps or multiple stages, and these sub-steps or stages are not necessarily executed at the same time, but may be executed at different times, and the execution order of these sub-steps or stages is not necessarily sequential, but may be executed in turn or alternately with other steps or at least a part of the sub-steps or stages of other steps. The method is limited to being able to achieve the preparation of the corresponding lithium niobate single crystal film.

[0078] Based on this, with respect to the preparation methods of lithium niobate single crystal thin films provided in some of the above embodiments, the following embodiments of this application illustrate some methods as some possible implementation methods of the above preparation methods.

[0079] In some embodiments, before step S300 uses ultraviolet lithography, magnetron sputtering and lift-off processes to sequentially form polarized electrodes on the surface of the lithium niobate single crystal film away from the low-resistance silicon substrate, it also includes step S250:

[0080] The lithium niobate single crystal film is cleaned by using a piranha solution, wherein the piranha solution comprises: 98% H2SO4 and 30% H2O2 mixed in a volume ratio of 7:3.

[0081] In order to more clearly illustrate the method for preparing the lithium niobate single crystal thin film provided in the above embodiment, Figure 4(a) to Figure 6 The preparation method is described in detail.

[0082] In step S100 , referring to FIG. 4( a ), a low-resistance silicon substrate 1 is provided.

[0083] In some embodiments, the resistivity range of the low-resistance silicon substrate 1 includes: less than 0.05Ω·cm. For example, the resistivity of the low-resistance silicon substrate 1 may be: 0.01Ω·cm, 0.02Ω·cm, 0.03Ω·cm, 0.04Ω·cm, or 0.05Ω·cm, etc.

[0084] In step S210, please continue to refer to Fig. 4(a), an initial lithium niobate single crystal film 20 is formed on the low-resistivity silicon substrate 1. The thickness of the initial lithium niobate single crystal film 20 ranges from 5 μm to 50 μm.

[0085] For example, the thickness of the initial lithium niobate single crystal thin film 20 may be 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm or 50 μm, etc.

[0086] In step S220, referring to FIG. 4(a) and FIG. 4(b), the initial lithium niobate single crystal film 20 is thinned by a chemical mechanical polishing process at a preset rotation speed, and after continuous processing for a preset time, a lithium niobate single crystal film 2 is obtained.

[0087] Here, it should be noted that currently, the lithium niobate single crystal film of micrometer scale on the low-resistance silicon substrate 1 can be commercially produced on the market, and its structure is: lithium niobate single crystal film (z-cut) with a thickness of micrometer scale (for example, 10 μm) - silicon substrate. Thus, the embodiment of the present application uses a precise chemical mechanical polishing process to reduce the thickness of the initial lithium niobate single crystal film 20 to the order of hundreds of nanometers.

[0088] In some embodiments, the preset rotation speed includes: 20 rpm to 40 rpm. For example, the preset rotation speed may be: 20 rpm, 25 rpm, 30 rpm, 35 rpm or 40 rpm, etc.

[0089] In some embodiments, the preset time includes: 5 hours to 8 hours. For example, the preset time can be: 5 hours, 5.5 hours, 6 hours, 6.5 hours, 7 hours, 7.5 hours or 8 hours, etc.

[0090] In some embodiments, the thickness of the lithium niobate single crystal film 2 ranges from 10 nm to 10 μm. For example, the thickness of the lithium niobate single crystal film 2 can be 10 nm, 50 nm, 100 nm, 200 nm, 300 nm, 400 nm, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, or 10 μm, etc.

[0091] In an embodiment of the present application, in order to obtain a lithium niobate film with a thickness of hundreds of nanometers, a chemical mechanical polishing process is used to thin the initial lithium niobate single crystal film 20 at a preset rotation speed. After continuous processing for a preset time, a lithium niobate single crystal film 2 with a thickness of hundreds of nanometers is finally obtained.

[0092] Please refer to FIG. 4( b ), the area enclosed by the dotted line is the remaining lithium niobate single crystal film 2. Since the lithium niobate single crystal film 2 and the low-resistance silicon substrate 1 form equal thickness interference, the grayscale change represents the change in film thickness. Since the initial lithium niobate single crystal film 20 is relatively thick (in the micrometer range), when it is thinned to hundreds of nanometers, the long processing process will lead to uneven film thickness distribution.

[0093] In step S250, the lithium niobate single crystal thin film is cleaned with a piranha solution, wherein the piranha solution comprises: a mixture of 98% H2SO4 and 30% H2O2 in a volume ratio of 7:3.

[0094] In step S300, polarization electrodes are sequentially formed on the surface of the lithium niobate single crystal film facing away from the low-resistance silicon substrate by using an ultraviolet photolithography process, a magnetron sputtering process and a lift-off process.

[0095] In some embodiments, the polarized electrode includes a chromium metal electrode, and the thickness of the chromium metal electrode ranges from 20 nm to 400 nm.

[0096] For example, the thickness of the chromium metal electrode may be 20 nm, 50 nm, 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm or 400 nm, etc.

[0097] In step S400, refer to Figure 5(a) to Figure 5(e) The inversion domain structure was prepared by external electric field polarization method, and an inversion domain structure that penetrated the lithium niobate single crystal film in the thickness direction was obtained.

[0098] Please refer to Figure 5(a), which is an optical microscope image of a typical periodically polarized electrode, in which the comb-tooth electrode is 33μm wide, 300μm long, and has a period of 60μm. The schematic diagram of the polarization experimental setup is shown in Figure 5(b). The relationship between the polarization current and polarization voltage over time is shown in Figure 5(c). The generation and disappearance of the pulse current marks the completion of the polarization reversal. Figure 5(d) shows the piezoelectric atomic force microscope phase image (PFM) of a partial inversion domain, in which the dark area is the inversion domain. The experiment found that the inversion domain width is about 33μm, which is approximately consistent with the width of the prepared comb-tooth electrode. In addition, high-quality, submicron periodically polarized lithium niobate films can also be prepared, as shown in the PFM phase image of a partial inversion domain shown in Figure 5(e), and its polarization period is 800nm.

[0099] See also Figure 6 , the depth of the inversion domain was explored using the hydrofluoric acid etching method. Under the same polarization experimental device and conditions, a circular inversion domain with a radius of 50μm was provided, and after removing the surface chromium electrode with chromium etching liquid, the sample was immersed in a 42% hydrofluoric acid solution at room temperature. As the etching time increased, the inversion domain area was gradually corroded, and finally the low-resistance silicon substrate 1 was exposed, indicating that the lithium niobate film 2 on the low-resistance silicon substrate 1 can achieve complete polarization reversal, and for the lithium niobate film with micron-scale periodic polarization, the inversion domain broadening effect is negligible relative to the inversion domain size.

[0100] Please continue reading Figure 4(a) to Figure 6 Some embodiments of the present application also provide a periodically poled lithium niobate film, comprising: a lithium niobate single crystal film 2 located on a low-resistance silicon substrate 1. The lithium niobate single crystal film 2 has an inversion domain structure. The inversion domain structure runs through the lithium niobate single crystal film in the thickness direction; the thickness range of the lithium niobate single crystal film includes: 10nm to 10μm.

[0101] In the embodiment of the present application, the periodically poled lithium niobate film adopts the above structure. The technical effect that can be achieved by the periodically poled lithium niobate film is the same as the technical effect that can be achieved by the preparation method of the periodically poled lithium niobate film in the aforementioned embodiment, and will not be described in detail here.

[0102] For example, the thickness of the lithium niobate single crystal film 2 can be: 10nm, 50nm, 100nm, 200nm, 300nm, 400nm, 500nm, 600nm, 700nm, 800nm, 900nm, 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, or 10μm, etc.

[0103] In some embodiments, the resistivity range of the low-resistance silicon substrate includes: less than 0.05 Ω·cm.

[0104] For example, the resistivity of the low-resistance silicon substrate 1 may be 0.01 Ω·cm, 0.02 Ω·cm, 0.03 Ω·cm, 0.04 Ω·cm or 0.05 Ω·cm, etc.

[0105] In the description of this specification, the technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features of the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0106] The above-described embodiments only express several implementation methods of the present disclosure, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the patent application. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present disclosure, and these all belong to the protection scope of the present disclosure. Therefore, the protection scope of the patent of the present disclosure shall be subject to the attached claims.

Claims

1. A method for preparing a periodically poled lithium niobate film, characterized in that: include: Providing low-resistance silicon substrate; Forming a lithium niobate single crystal film on the low-resistance silicon substrate; wherein the thickness of the lithium niobate single crystal film ranges from 10 nm to 10 μm; forming a polarization electrode on a surface of the lithium niobate single crystal thin film facing away from the low-resistance silicon substrate; An inversion domain structure is prepared by an external electric field polarization method, so as to obtain the inversion domain structure penetrating the lithium niobate single crystal film in the thickness direction.

2. The method for preparing a periodically poled lithium niobate thin film according to claim 1, characterized in that: The step of forming a lithium niobate single crystal thin film on the low-resistance silicon substrate comprises: Forming an initial lithium niobate single crystal film on the low-resistance silicon substrate; wherein the thickness of the initial lithium niobate single crystal film ranges from 5 μm to 50 μm; The thickness of the initial lithium niobate single crystal film is reduced to obtain a lithium niobate single crystal film.

3. The method for preparing a periodically poled lithium niobate thin film according to claim 2, characterized in that: The step of reducing the thickness of the initial lithium niobate single crystal film to obtain the lithium niobate single crystal film comprises: The initial lithium niobate single crystal film is thinned by a chemical mechanical polishing process at a preset rotation speed, and the lithium niobate single crystal film is obtained after continuous processing for a preset time.

4. The method for preparing a periodically poled lithium niobate thin film according to claim 3, characterized in that: The preset rotation speed includes: 20rpm~40rpm.

5. The method for preparing a periodically poled lithium niobate thin film according to claim 3, characterized in that: The preset time includes: 5 hours to 8 hours.

6. The method for preparing a periodically poled lithium niobate thin film according to claim 1, characterized in that: The forming of a polarization electrode on a surface of the lithium niobate single crystal film away from the low-resistance silicon substrate comprises: Polarization electrodes are sequentially formed on the surface of the lithium niobate single crystal film away from the low-resistance silicon substrate by adopting ultraviolet photolithography, magnetron sputtering and lift-off processes.

7. The method for preparing a periodically poled lithium niobate film according to claim 6, characterized in that: The polarization electrode includes: a chromium metal electrode; the thickness of the chromium metal electrode ranges from 20nm to 400nm.

8. The method for preparing a periodically poled lithium niobate thin film according to claim 6, characterized in that: Before the ultraviolet lithography process, the magnetron sputtering process and the lift-off process are used to sequentially form polarized electrodes on the surface of the lithium niobate single crystal film away from the low-resistance silicon substrate, the method further includes: The lithium niobate single crystal film is cleaned using a piranha solution, wherein the piranha solution comprises: 98% H2SO4 and 30% H2O2 mixed in a volume ratio of 7:

3.

9. A periodically poled lithium niobate film, characterized in that: include: A single crystal thin film of lithium niobate on a low-resistivity silicon substrate; The lithium niobate single crystal film has an inversion domain structure; the inversion domain structure penetrates the lithium niobate single crystal film in the thickness direction; the thickness range of the lithium niobate single crystal film includes: 10nm ~ 10μm.

10. The periodically poled lithium niobate film according to claim 1, characterized in that: The resistivity range of the low-resistance silicon substrate includes: less than 0.05Ω·cm.

Images