Magnetron sputtering process for improving content of vanadium dioxide in film and film coating device

Abstract

The invention discloses a magnetron sputtering process and a coating device for improving the content of vanadium dioxide in films, a first film and a second film are deposited on the surface of a substrate through two times of magnetron sputtering, and oxygen flows of process gases used twice are different, so that an oxygen-enriched vanadium oxide film and an oxygen-deficient vanadium oxide film are prepared; the surface-coated substrate is put into an annealing cavity, the temperature in the annealing cavity is maintained at 150-350 DEG C, oxygen atoms can obtain enough energy to overcome lattice potential barriers and start to move at the temperature, the oxygen atoms can spontaneously diffuse from a high-concentration oxygen-enriched layer to a low-concentration oxygen-deficient layer, and finally uniform distribution of oxygen elements in the thin film is achieved; and finally, a film with relatively high vanadium dioxide content and uniform components is formed.

Description

Technical Field

[0001] This application relates to the technical field of wafer coating, in particular to a magnetron sputtering process and coating device for increasing the vanadium dioxide content in a thin film. Background Art

[0002] Vanadium dioxide (VO2) thin films have unique metal-insulator phase transition characteristics and are widely used in many fields. Since the magnetron sputtering technology can uniformly deposit materials on a substrate to form a continuous thin film, it can effectively ensure the consistency of the physical properties of the thin film, is beneficial to the thermal and electrical sensitivity responses of vanadium oxide, and can precisely adjust the thickness and composition of the thin film, which is conducive to achieving specific phase transition characteristics of VO. Therefore, the magnetron sputtering technology is often used to prepare vanadium dioxide thin films.

[0003] During the process of preparing vanadium oxide thin films by magnetron sputtering, due to the characteristic of vanadium element having multiple valence states, vanadium and oxygen can form compounds with various valence state combinations, such as vanadium monoxide (VO), vanadium sesquioxide (V2O3), vanadium dioxide (VO2), and vanadium pentoxide (V2O5).

[0004] In the traditional magnetron sputtering process, it is difficult to achieve uniform oxygen distribution by controlling the oxygen partial pressure in a single coating chamber, resulting in the composition of the thin film being prone to deviate from the target value, ultimately affecting the phase transition temperature stability of the thin film. Summary of the Invention

[0005] The purpose of this application is to overcome the deficiencies existing in the prior art and provide a magnetron sputtering process and coating device for increasing the vanadium dioxide content in a thin film.

[0006] This application provides a magnetron sputtering process for increasing the vanadium dioxide content in a thin film, including: S1. In the coating chamber, using a pure vanadium target, introducing a first mixed gas, and forming a first layer of thin film on the substrate surface by magnetron sputtering; S2. In the coating chamber, using a pure vanadium target, introducing a second mixed gas, and forming a second layer of thin film on the first layer of thin film by magnetron sputtering; both the first mixed gas and the second mixed gas include Ar and O2, but the flow rates of O2 in the two are different, such that one of the first layer of thin film and the second layer of thin film is an oxygen-rich layer and the other is an oxygen-deficient layer; the content ratio of oxygen atoms to vanadium atoms in the oxygen-rich layer is greater than 2; the content ratio of oxygen atoms to vanadium atoms in the oxygen-deficient layer is less than 2; S3. Placing the substrate after two coatings into an annealing chamber, and keeping the substrate at 150 - 350 °C for 5 - 45 min to promote the diffusion of oxygen atoms from the oxygen-rich layer to the oxygen-deficient layer, so as to form a vanadium dioxide thin film with uniform composition.

[0007] Further, the flow rate of O2 in S1 is 2 - 30 sccm, the flow rate of O2 in S2 is 1 - 20 sccm, and the flow rate of O2 in S1 is higher than that in S2; after two depositions, the first layer of film on the substrate is an oxygen-rich layer, and the second layer of film is an oxygen-deficient layer.

[0008] Further, in S1 and S2, the flow rate of Ar is 15 - 100 sccm, the DC voltage power of the target is 100 - 1500 W, and the chamber pressure is 2 - 30 mTorr.

[0009] Further, in S1 and S2, the temperature of the susceptor is 80 - 300 °C.

[0010] Further, in S3, during annealing, an inert gas is introduced into the annealing chamber to prevent secondary oxidation on the film surface at high temperature; alternatively, S3 uses vacuum annealing, and during annealing, the pressure in the annealing chamber is less than 10 -3 Torr, which can reduce the interference of gas molecules on the oxygen diffusion path.

[0011] Further, after two depositions, the film thicknesses of both the first layer of film and the second layer of film on the substrate are 200 - 400 Å.

[0012] Further, S1 is carried out in the first deposition chamber, S2 is carried out in the second deposition chamber. Both the first deposition chamber and the second deposition chamber are equipped with a target and a susceptor. The first deposition chamber is also equipped with a first grid, and the second deposition chamber is also equipped with a second grid. The grid surfaces of the first grid and the second grid are arranged in a complementary form, and the grid bar structure of the first grid is interchanged with the space position of the second grid; through S1, a wavy first layer of film is formed on the substrate surface; the second layer of film formed by S2 is complementary to the first layer of film in terms of peaks and valleys, making the film formed after two depositions tend to be flat; during annealing, the oxygen atoms in the oxygen-rich layer gradually transfer to the vacancies in the oxygen-deficient layer. Since the contact surface between the first layer of film and the second layer of film is wavy, the channels for oxygen atom diffusion are increased, which can reduce the time required for diffusion, thereby reducing the process time required for annealing.

[0013] Further, in S1, in the first deposition chamber, the temperature of the susceptor is set at 150 °C, the chamber pressure is 3.4 mTorr, the Ar flow rate is 35 sccm, the O2 flow rate is 3 sccm, the DC voltage power of the target is 165 W, and the deposition time is 860 s; in S2, in the second deposition chamber, the temperature of the susceptor is set at 150 °C, the chamber pressure is 3.4 mTorr, the Ar flow rate is 35 sccm, the O2 flow rate is 1 sccm, the DC voltage power of the target is 150 W, and the deposition time is 800 s; in S3, in the annealing chamber, the chamber temperature is set at 300 °C, and the annealing duration is 10 min.

[0014] The present application also provides a coating device for implementing the above-mentioned magnetron sputtering process for increasing the vanadium dioxide content in the thin film.

[0015] Further, the coating device includes: a first coating chamber, in which a first stage and a first target are provided; a second coating chamber, in which a second stage and a second target are provided; a first grid, disposed in the first coating chamber and located between the first stage and the first target, the grid surface of the first grid is formed by a plurality of grid bar structures laid at intervals in the horizontal direction; a second grid, disposed in the second coating chamber and located between the second stage and the second target, the grid surface of the second grid is also formed by a plurality of grid bar structures laid at intervals in the horizontal direction; the distribution of the grid bar structures in the first grid is staggered from the distribution of the grid bar structures in the second grid; during the first coating, due to the selective shielding of the first grid, a first layer of thin film with a wavy and uneven surface is formed on the substrate surface; during the second coating, due to the selective shielding of the second grid, a second layer of thin film with opposite peaks and valleys and opposite undulations is formed on the first layer of thin film; the shape of the second layer of thin film is complementary to that of the first layer of thin film, so that the thin film formed after two coatings tends to be flat.

[0016] The present application provides a magnetron sputtering process for increasing the vanadium dioxide content in a thin film. By magnetron sputtering twice, a first layer of thin film and a second layer of thin film are deposited on the substrate surface. The oxygen flow rates of the process gases used twice are different to prepare a vanadium oxide thin film in a rich oxygen state and a vanadium oxide thin film in a lack of oxygen state; the substrate with the surface coated is placed in an annealing chamber, and the temperature in the annealing chamber is maintained at 150 - 350 °C. At this temperature, oxygen atoms can obtain enough energy to overcome the lattice potential barrier and start to move. Oxygen atoms will spontaneously diffuse from the rich oxygen layer with a high concentration to the lack of oxygen layer with a low concentration, and finally achieve a uniform distribution of oxygen elements in the thin film, forming a high-content vanadium dioxide thin film with a uniform composition.

[0017] The present application also provides a coating device for implementing the above-mentioned magnetron sputtering process for increasing the vanadium dioxide content in the thin film; the coating device includes a coating chamber and an annealing chamber. By controlling the oxygen flow rate introduced into the coating chamber, two vanadium oxide thin films with different oxygen contents can be formed on the substrate surface. Subsequently, the substrate is annealed in the annealing chamber, and using the oxygen concentration difference between the two thin films, oxygen atoms diffuse from the rich oxygen layer to the lack of oxygen layer, and finally a thin film with a relatively high vanadium dioxide content and a uniform composition is formed. Description of the Drawings

[0018] Figure 1 It is a schematic structural diagram of a first coating chamber provided by the present application; Figure 2 It is a schematic structural diagram of a second coating chamber provided by the present application. Detailed Embodiments

[0019] To make the above objects, features, and advantages of the present application more apparent and understandable, the following provides a detailed description of the specific embodiments of the present application with reference to the accompanying drawings. Many specific details are set forth in the following description to facilitate a full understanding of the present application. However, the present application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without departing from the connotation of the present application. Therefore, the present application is not limited by the specific embodiments disclosed below.

[0020] The present application provides a magnetron sputtering process for increasing the vanadium dioxide content in a thin film, including: S1. In a coating chamber, using a pure vanadium target, introducing a first mixed gas, and forming a first layer of thin film on the surface of a substrate by magnetron sputtering; S2. In the coating chamber, using a pure vanadium target, introducing a second mixed gas, and forming a second layer of thin film on the first layer of thin film by magnetron sputtering; Both the first mixed gas and the second mixed gas include Ar (argon) and O2 (oxygen), but the flow rate of O2 in the two is different, such that one of the first layer of thin film and the second layer of thin film is an oxygen-rich layer and the other is an oxygen-deficient layer; In the oxygen-rich layer, the content ratio of oxygen atoms to vanadium atoms is greater than 2; In the oxygen-deficient layer, the content ratio of oxygen atoms to vanadium atoms is less than 2; S3. Placing the substrate after two coatings into an annealing chamber, and keeping the substrate at 150 - 350 °C for 5 - 45 min to promote the diffusion of oxygen atoms from the oxygen-rich layer to the oxygen-deficient layer, so as to form a vanadium dioxide thin film with uniform composition.

[0021] Specifically, a carrier stage and a target are provided in the coating chamber, and the target is the "material source" for magnetron sputtering. The target is made of pure vanadium material, and usually requires a purity of more than 99.95% to ensure that no impurity elements (such as Fe, Al, C, etc.) are introduced during the sputtering process, avoiding affecting the stoichiometry and functional characteristics of the thin film.

[0022] The substrate can be made of materials such as silicon nitride, sapphire, silicon, silicon oxide, etc. The substrate needs to ensure a clean surface, and appropriate surface cleaning pretreatment can be carried out before coating.

[0023] During the magnetron sputtering process, the carrier stage is used to place and fix the substrate to ensure its stability during sputtering and avoid displacement or vibration resulting in uneven coating.

[0024] At the start of S1, the target is powered on, and at the same time, a mixed gas containing Ar and O2 is introduced into the coating chamber; Ar serves as the sputtering gas and will be ionized into argon ions (Ar + ) under the action of an electric field, Ar +Under the acceleration of an electric field, a pure vanadium target is bombarded, causing vanadium atoms (V) to sputter out from the surface of the target. At the same time, O2 participates in the reaction and combines with the sputtered vanadium atoms to form a vanadium oxide thin film on the substrate surface.

[0025] It should be added that in the magnetron sputtering process, argon and oxygen have different functions. O2 is the key element participating in the chemical reaction and generating the vanadium oxide thin film. It can combine with the sputtered vanadium atoms and is an indispensable substance for forming the vanadium dioxide thin film. And Ar mainly plays a role in physical sputtering during the film coating process. As an inert gas, Ar has a stable chemical property and will not chemically react with the target or reaction substances. Under the action of an electric field, Ar is ionized into Ar + , Ar + Under the acceleration of an electric field, it bombards the pure vanadium target at high speed, causing vanadium atoms to sputter out from the surface of the target and providing an atomic source for the formation of the thin film. By mixing Ar and O2, it can not only provide a material basis for the formation of the thin film through the sputtering action of Ar, but also utilize O2 to participate in the chemical reaction and generate the target product.

[0026] Due to the different flow rates of O2 in the first mixed gas and the second mixed gas, the oxygen content in the first layer of thin film and the second layer of thin film formed is different. The layer with a high oxygen content is used as the oxygen-rich layer, and the layer with a low oxygen content is used as the oxygen-deficient layer.

[0027] Specifically, by making the flow rate of the introduced O2 higher, a vanadium oxide thin film in an oxygen-rich state (the oxygen atom content is higher than the stoichiometric ratio, that is, V:O < 2) can be formed. Similarly, by making the flow rate of the introduced O2 lower, a vanadium oxide thin film in an oxygen-deficient state (the oxygen atom content is lower than the stoichiometric ratio, that is, V:O > 2) can be formed.

[0028] By controlling the flow rate of O2, two layers of thin films with different oxygen contents are formed. During annealing, oxygen atoms can diffuse from the high-concentration region (oxygen-rich layer) to the low-concentration region (oxygen-deficient layer), and finally achieve overall compositional homogenization.

[0029] More specifically, after successively obtaining the first layer of thin film and the second layer of thin film through the magnetron sputtering process, the substrate with the surface coated is placed in an annealing chamber. The temperature in the annealing chamber is maintained at 150 - 350 °C. At this temperature, oxygen atoms can obtain enough energy to overcome the lattice potential barrier and start to move. Since the number of oxygen atoms in the oxygen-rich layer is large, and there are oxygen vacancies in the oxygen-deficient layer, according to the diffusion principle, oxygen atoms will spontaneously diffuse from the oxygen-rich layer with a high concentration to the oxygen-deficient layer with a low concentration, and finally achieve a uniform distribution of oxygen elements in the thin film, forming a vanadium dioxide thin film with a uniform composition.

[0030] According to Fick's first law, atoms or molecules will diffuse from a region of high concentration to a region of low concentration to reduce the free energy of the system. Therefore, in this application, due to the concentration gradient between the oxygen-rich layer (high oxygen concentration) and the oxygen-deficient layer (low oxygen concentration), by utilizing the thermal energy provided by annealing to activate the migration of oxygen atoms, the oxygen atoms can be promoted to diffuse into the oxygen-deficient layer. During the annealing process, the system tends to thermodynamic equilibrium, and the oxygen content in the two thin films becomes uniform through diffusion of oxygen atoms.

[0031] According to the Arrhenius equation, the diffusion coefficient D has an exponential relationship with temperature T:

[0032] where Q is the activation energy for diffusion and R is the gas constant.

[0033] It can be seen that increasing the annealing temperature can effectively improve the diffusion rate of atoms.

[0034] In a specific embodiment, the annealing temperature is 300 °C. At this time, the diffusion coefficient of oxygen atoms in vanadium oxide is about 10 - 14 cm² / s, which is sufficient to complete atomic migration.

[0035] In solid materials, atoms are arranged periodically to form a regular lattice structure. However, atoms are not stationary but constantly vibrate around their equilibrium positions, and this vibration is called lattice vibration. Generally, the higher the temperature, the greater the amplitude of atomic vibration. When the first thin film and the second thin film are annealed at a high temperature, the thermal energy will enhance the lattice vibration. The strong vibration will make the chemical bonds between atoms easier to break, thereby weakening the binding force between atoms. At the same time, the vibration provides enough energy for atoms to break away from their original positions and migrate to other positions. Therefore, at high temperatures, more vacancies and dislocations are activated, further providing paths for diffusion. Oxygen atoms quickly migrate from the oxygen-rich layer to the oxygen-deficient layer through defect channels such as vacancies, grain boundaries, and dislocations, and finally achieve the homogenization of the thin film composition.

[0036] Furthermore, the diffusion distance x and time t satisfy:

[0037] That is, the diffusion distance is proportional to the square root of time, and the diffusion coefficient D determines the diffusion rate.

[0038] Since the diffusion distance increases with the square root of time, not linearly. For example, if the annealing time is extended by 4 times, the diffusion distance only increases by 2 times. Therefore, during the annealing stage, it is necessary to balance the time cost and the diffusion effect.

[0039] Taking the annealing of vanadium dioxide (VO2) thin film as an example for explanation.

[0040] The diffusion coefficient D≈10 -14  cm 2 / s (at 300 °C); The annealing time t = 10 min = 600 s; Diffusion distance ; That is, oxygen atoms diffuse approximately 24.5 nm within 10 minutes, which matches the film thickness (such as 300 Å = 30 nm), verifying the process feasibility of this application.

[0041] Based on the above principle, in practical applications, the required annealing time and annealing temperature can be deduced based on the target diffusion depth (such as the composition homogenization of VO2 film).

[0042] Simply put, controlling the annealing time within 5 - 45 min and the annealing temperature within 150 - 350 °C can ensure that oxygen atoms fully diffuse to the equilibrium state.

[0043] After annealing, the atomic percentages of V and O in the film are confirmed by XPS composition analysis (X-ray Photoelectron Spectroscopy). The results show that the overall oxygen content in the film after annealing tends to the stoichiometric ratio of VO2, that is, V:O≈1:2, and the oxygen concentration difference between the oxygen-rich layer and the oxygen-deficient layer disappears. This indicates that the average chemical composition of the film is close to VO2.

[0044] Furthermore, the resistance-temperature curve of the film is obtained by combining four-probe resistance measurement with a temperature control system to verify its metal-insulator phase transition characteristics. The test results show that the film undergoes a metal-insulator phase transition at 68 °C (in line with the typical characteristics of VO2). At the same time, the resistance change ΔR > 10 3 , and the width of the thermal hysteresis loop ΔT≈5 °C, demonstrating improved composition uniformity and excellent phase transition characteristics of the VO2 film.

[0045] In summary, annealing promotes the diffusion of oxygen atoms from the oxygen-rich layer to the oxygen-deficient layer through thermodynamic driving (concentration gradient) and kinetic acceleration (high temperature increases the diffusion rate), combined with the assistance of lattice defect channels, ultimately achieving the homogenization of the film composition.

[0046] The magnetron sputtering process provided by this application first forms an overlapping oxygen-rich vanadium oxide film and oxygen-deficient vanadium oxide film on the substrate surface by controlling the oxygen flow rate during two coating processes, and then uses annealing to make oxygen atoms diffuse, which not only balances the film composition and is beneficial to the consistency of film physical properties, but also increases the content of vanadium dioxide in the film, facilitating the preparation of a high-purity vanadium dioxide film.

[0047] In one embodiment, the flow rate of O2 in S1 is 2 - 30 sccm, the flow rate of O2 in S2 is 1 - 20 sccm, and the flow rate of O2 in S1 is higher than that in S2; after two depositions, the first layer of film on the substrate is an oxygen-rich layer, and the second layer of film is an oxygen-deficient layer.

[0048] Setting the O2 flow rate in S1 to 2 - 30 sccm and the O2 flow rate in S2 to 1 - 20 sccm is determined based on a large number of experiments and research on the principle of the magnetron sputtering process.

[0049] During the process of preparing vanadium oxide thin films by magnetron sputtering, the oxygen flow rate has a great influence on the composition and structure of the thin films. If the oxygen flow rate is too low, insufficient vanadium oxide can be formed, making it difficult to obtain the target product; if the flow rate is too high, it may lead to over-oxidation, generating vanadium oxides with other valence states, which affects the purity and performance of the vanadium dioxide thin films.

[0050] Using a relatively high O2 flow rate (2 - 30 sccm) in S1 can enable the sputtered vanadium atoms to combine with more oxygen atoms to form an oxygen-rich vanadium oxide thin film. This provides an abundant oxygen source for the subsequent annealing process, ensuring that during annealing, there are sufficient oxygen atoms diffusing from the oxygen-rich layer to the oxygen-deficient layer, which is beneficial to achieving the homogenization of the overall composition of the thin film and ultimately increasing the content of vanadium dioxide in the thin film.

[0051] Using a relatively low O2 flow rate (1 - 20 sccm) in S2 can form an oxygen-deficient vanadium oxide thin film, creating a concentration difference condition for the diffusion of oxygen atoms. During annealing, the oxygen atoms in the oxygen-rich layer will spontaneously diffuse to the oxygen-deficient layer. This diffusion process driven by the concentration difference will make the distribution of oxygen atoms more uniform, contributing to controlling the composition of the thin film and forming high-quality vanadium dioxide thin films.

[0052] In other embodiments, it is also possible to make the first layer of film an oxygen-deficient layer and the second layer of film an oxygen-rich layer, and the specific O2 flow rate can also be adjusted according to process requirements. This application does not limit the specific parameters of the film deposition in S1 and S2, as long as it can form overlapping oxygen-deficient vanadium oxide thin films and oxygen-rich vanadium oxide thin films on the substrate surface.

[0053] Optionally, in S1 and S2, the flow rate of Ar is 15 - 100 sccm.

[0054] When the argon gas flow rate is in the range of 15 - 100 sccm, a stable plasma environment can be ensured in the coating chamber. It is easy to understand that when the argon gas flow rate is too low, the number of argon ions generated is insufficient, and the target cannot be bombarded sufficiently, resulting in a small amount of sputtered vanadium atoms, thus slowing down the coating rate and affecting the production efficiency. When the argon gas flow rate is too high, excessive argon ions bombard the target, which may cause the surface temperature of the target to be too high, resulting in excessive thermal stress on the target and even damaging the target. At the same time, it may also lead to plasma instability, affecting the uniformity and quality of the thin film, and even diluting the reaction gas, affecting the oxygen binding rate.

[0055] In the range of 15 - 100 sccm, a sufficient amount of argon ions can be generated, stably sputter vanadium atoms from the surface of the target, and deposit them on the substrate, ensuring the continuous and stable progress of the coating process.

[0056] Optionally, in S1 and S2, the DC voltage power of the target is 100 - 1500 W.

[0057] The DC voltage power of the target directly affects the energy and quantity of argon ions during the magnetron sputtering process. It is easy to understand that when the power is low, the energy of argon ions is insufficient, and it is difficult to sputter enough vanadium atoms from the target. As the power increases, the energy and quantity of argon ions increase, the amount of sputtered vanadium atoms increases, and the coating rate increases. However, too high power will cause the surface temperature of the target to rise sharply, which may lead to overheating deformation or even damage of the target, and will also cause changes in the structure and properties of the thin film, such as increased film stress and poor crystallization quality.

[0058] The range of 100 - 1500 W is determined after comprehensively considering factors such as coating rate, target life, and thin film quality.

[0059] Within this power range, it can not only ensure that enough vanadium atoms are sputtered out to guarantee production efficiency, but also enable the sputtered vanadium atoms to have appropriate energy to be evenly deposited on the substrate surface and form a good crystalline structure, which helps to improve the purity and performance stability of the vanadium dioxide thin film.

[0060] Optionally, in S1 and S2, the chamber pressure is 2 - 30 mTorr.

[0061] The chamber pressure has an important influence on the plasma characteristics and atomic transport process during magnetron sputtering. Understandably, when the pressure is too low, the number of argon molecules is small, and the number of ionized argon ions is also small, which is not conducive to the sputtering of vanadium atoms. At the same time, the collision probability of atoms with gas molecules during the transport process decreases, resulting in poor film uniformity. When the pressure is too high, the gas molecule concentration is large, and argon ions collide frequently with gas molecules, which will cause a large amount of energy loss of argon ions before reaching the target, thus reducing the sputtering efficiency. It may also cause excessive reactions between sputtered atoms and gas molecules before reaching the substrate, affecting the composition and quality of the film.

[0062] A range of 2 - 30 mTorr can balance these factors.

[0063] Within this pressure range, stable plasma discharge can be maintained, ensuring sufficient generation of argon ions and achieving efficient sputtering of vanadium atoms. At the same time, appropriate pressure allows for an appropriate number of collisions between sputtered atoms and gas molecules during the transport process, helping them to deposit evenly on the substrate surface, thereby improving the film uniformity and density and enhancing the overall quality of the vanadium dioxide film.

[0064] Optionally, in S1 and S2, the stage temperature is 80 - 300 °C.

[0065] Specifically, a temperature adjustment device (such as a resistance wire, coil, thermocouple, etc.) is provided inside the stage. During the magnetron sputtering process, the temperature adjustment device controls the temperature of the stage, enabling the stage to have a certain temperature to facilitate the deposition of the film on the substrate surface.

[0066] The stage temperature affects the diffusion of atoms on the substrate surface and the film growth process. Understandably, when the temperature is too low, the diffusion rate of atoms on the substrate surface is slow, and the vanadium atoms and oxygen atoms sputtered onto the substrate are difficult to move to the appropriate lattice positions, which is not conducive to the formation of a good film structure and may lead to large internal stresses and imperfect crystallization in the film. When the temperature is too high, although the atomic diffusion rate increases, some negative effects may be triggered, such as the film peeling off due to the too large difference in thermal expansion coefficients between the film and the substrate, or the intensification of component diffusion in the film, making it difficult to precisely control the film composition.

[0067] The temperature range of 80 - 300 °C takes into account the requirements of atomic diffusion and film stability.

[0068] Within this temperature range, the diffusion of atoms on the substrate surface can be promoted, enabling vanadium atoms and oxygen atoms to migrate and arrange better on the substrate surface, forming a vanadium dioxide film with good crystallization and stable structure. At the same time, the appropriate temperature can optimize the bonding force between the film and the substrate, improve the film adhesion, reduce film defects, further enhance the film performance, and ensure that the vanadium dioxide film can stably exhibit its characteristics in subsequent applications.

[0069] In one embodiment, when preparing the vanadium oxide thin film in an oxygen-deficient state, by "high power + low pressure", the kinetic energy of vanadium atoms can be enhanced and the collision probability of oxygen can be reduced, thereby inhibiting the oxygen adsorption reaction to facilitate the precise control of the oxygen content in the thin film.

[0070] Specifically, high power (such as 1000 - 1500 W) means a larger voltage gradient between the target and the plasma, and argon ions can be accelerated to higher energies (such as 500 - 1000 eV). When high-energy argon ions bombard the target, the kinetic energy transferred to vanadium atoms increases significantly, such that the initial sputtering velocity of vanadium atoms can reach 10 3 -10 4 m / s (much higher than the velocity at low power). Vanadium atoms with high kinetic energy have a shorter residence time during flight, and the probability of collision with oxygen molecules is reduced. After high-energy vanadium atoms reach the substrate, they may directly embed into the lattice or form a dense structure, thereby reducing the surface oxygen adsorption sites.

[0071] In addition, at low pressure (such as 2 - 10 mTorr), the density of gas molecules (Ar, O2) in the coating chamber decreases significantly. According to the ideal gas law, pressure is proportional to the number density of molecules. Therefore, low pressure means fewer gas molecules per unit volume. The mean free path of vanadium atoms (the average moving distance between two collisions) increases. At low pressure, the number of collisions between vanadium atoms and oxygen molecules during the path from the target to the substrate decreases. Oxygen molecule adsorption requires physical or chemical adsorption with vanadium atoms, and the reduced collision probability directly inhibits the oxygen adsorption process.

[0072] In summary, high power ensures that vanadium atoms detach from the target with high kinetic energy. At the same time, low pressure reduces the gas resistance and collision frequency in the flight path. Under the combined action of the two, vanadium atoms can reach the substrate more efficiently and are not easily captured by oxygen, which is beneficial to reducing the formation of vanadium oxide.

[0073] Similarly, reducing the DC voltage power of the target and / or increasing the chamber pressure can promote the formation of vanadium oxide to facilitate the obtaining of an oxygen-rich vanadium oxide thin film.

[0074] In actual application, the appropriate Ar flow rate, target DC voltage power, chamber pressure, and stage temperature can be selected according to the actual requirements of oxygen deficiency and oxygen enrichment (the stoichiometric ratio of V to O), and the specific parameters of the process are not limited in this application.

[0075] Optionally, in S3, during annealing, an inert gas is introduced into the annealing chamber to prevent secondary oxidation on the surface of the thin film at high temperature.

[0076] During the high-temperature annealing process, the vanadium dioxide thin film has high activity and is prone to reacting with oxygen in the air, resulting in secondary oxidation on the film surface. Introducing inert gases (such as argon, nitrogen, etc.) can form a protective gas layer around the film, isolating the film from oxygen in the air. Inert gases have stable chemical properties and will not undergo chemical reactions with the vanadium dioxide thin film at the annealing temperature, effectively preventing secondary oxidation and ensuring that the composition and properties of the film are not affected.

[0077] Specifically, after placing the substrate coated twice into the annealing chamber, an appropriate amount of inert gas is introduced into the annealing chamber. Control the flow rate and pressure of the inert gas to ensure that it can form a uniform and effective protective gas layer on the film surface.

[0078] Optionally, S3 uses vacuum annealing. During annealing, the pressure in the annealing chamber is made less than 10 -3 Torr, which can reduce the interference of gas molecules on the oxygen diffusion path.

[0079] During the annealing process, oxygen atoms diffuse from the oxygen-rich layer to the oxygen-deficient layer. The presence of gas molecules will collide with oxygen atoms, changing the diffusion direction and speed of oxygen atoms, interfering with the oxygen diffusion path, and ultimately resulting in a decrease in diffusion efficiency and even affecting the homogenization of the film composition. Pumping the annealing chamber to a vacuum state can effectively reduce the number of gas molecules, lower the probability of collision between oxygen atoms and other gas molecules, enable oxygen atoms to diffuse more smoothly from the oxygen-rich layer to the oxygen-deficient layer according to the concentration gradient, improve the diffusion efficiency, and achieve more precise composition control.

[0080] Specifically, place the substrate coated twice into the annealing chamber, and use a vacuum pump to evacuate the annealing chamber to reduce the pressure in the annealing chamber to a vacuum degree less than 10 -3 Torr. During the vacuum pumping process, closely monitor the pressure change to ensure that the target vacuum degree is reached. After reaching the vacuum state, raise the temperature of the annealing chamber to 150 - 350 °C and keep the substrate in it for 5 - 45 min. During the annealing process, continuously maintain the vacuum state to prevent external air from entering the annealing chamber.

[0081] Optionally, after two coatings, the film thicknesses of the first and second thin films on the substrate are both 200 - 400 Å.

[0082] The performance of the vanadium dioxide thin film is closely related to its thickness. If the film thickness is too small, it is prone to interface defects and may not fully exhibit the metal-insulator phase transition characteristics of vanadium dioxide. If the film thickness is too large, it will not only prolong the annealing time, increase production costs, but also may cause excessive internal stress in the film, easily resulting in defects such as cracks, affecting the stability and service life of the film.

[0083] In this application, a vanadium dioxide thin film with uniform composition is formed through the diffusion of oxygen atoms in the oxygen-rich layer and the oxygen-deficient layer. A suitable film thickness helps the oxygen atoms to fully diffuse during the annealing process. If the thin film is too thin, the space available for oxygen atom diffusion is limited, making it difficult to achieve a uniform composition distribution; if it is too thick, it will increase the distance and difficulty of oxygen atom diffusion, prolong the annealing time, and reduce the production efficiency. A film thickness of 200 - 400 Å provides a suitable diffusion path and space for oxygen atoms, enabling the oxygen atoms to diffuse from the oxygen-rich layer to the oxygen-deficient layer within a reasonable time, and ultimately achieving the homogenization of the thin film composition.

[0084] In a specific embodiment, the average film thicknesses of the first thin film and the second thin film on the substrate are both 300 Å.

[0085] In one implementation, S1 is carried out in the first coating chamber 10, and S2 is carried out in the second coating chamber 20. Both the first coating chamber 10 and the second coating chamber 20 are provided with a target and a stage. The first coating chamber 10 is further provided with a first grid 31, and the second coating chamber 20 is further provided with a second grid 32. The grid surfaces of the first grid 31 and the second grid 32 are arranged in a complementary form, and the grid bar structure of the first grid 31 is interchanged with the space position of the second grid 32; through S1, a wavy first thin film is formed on the substrate surface; the second thin film formed through S2 is complementary to the first thin film in terms of peaks and valleys, making the thin film formed after two coatings tend to be flat; during annealing, the oxygen atoms in the oxygen-rich layer gradually transfer to the vacancies in the oxygen-deficient layer. Since the contact surface between the first thin film and the second thin film is wavy, it increases the channels for oxygen atom diffusion, can reduce the time required for diffusion, and thus reduce the process time required for annealing.

[0086] For details, reference can be made to Figure 1 and Figure 2 , which illustrate two similar coating chambers. The first coating chamber 10 is provided with a first stage and a first target, and the first grid 31 is arranged between the first stage and the first target; the second coating chamber 20 is provided with a second stage and a second target, and the second grid 32 is arranged between the second stage and the second target. The grid surfaces of the grids face the target and the stage, and the grid surface is formed by a plurality of grid bar structures laid horizontally at intervals. The gap between any two adjacent grid bar structures is a space position.

[0087] It is easily understandable that the grid bar structure will hinder the movement of atoms. During the magnetron sputtering process, the vanadium atoms sputtered from the target can only be deposited downward through the space positions.

[0088] In S1, in the first coating chamber 10, the vanadium atoms sputtered from the target deposit onto the substrate through the space positions of the first grid 31. Since the space positions are separated by the grid bar structure, the deposition area of the vanadium atoms is blocked. The vanadium atoms deposited on the substrate mainly gather at the space positions and partially disperse to both sides where the grid bar structure is located, finally forming a first layer of film that is wavy and uneven, like a wave.

[0089] In S2, in the second coating chamber 20, since the space positions of the second grid 32 are the grid bar structures of the first grid 31, therefore, the deposition positions of the vanadium atoms will be complementary to those in S1. Specifically, at the wave peaks where more vanadium atoms are deposited in S1, the deposition amount of vanadium atoms in S2 will decrease, while at the wave valleys where fewer vanadium atoms are deposited in S1, the deposition amount of vanadium atoms in S2 will increase. The peaks and valleys of the two layers of film are complementary, making the surface of the film formed after two coatings tend to be flat, and finally showing a normal planar film state.

[0090] Adding a grid makes the contact surface between the two layers of film wavy, increasing the contact area between the two layers of film, providing more channels for the diffusion of oxygen atoms, facilitating the reduction of the diffusion time of oxygen atoms, and thus reducing the process time required for annealing (compared with ordinary films with a planar contact surface, the annealing time can be shortened by 40%). At the same time, it is beneficial to promote the diffusion of oxygen atoms between the oxygen-rich layer and the oxygen-deficient layer, thereby improving the compositional uniformity of the vanadium dioxide film.

[0091] In a specific embodiment, in S1, in the first coating chamber 10, the stage temperature is set at 150 °C, the chamber pressure is 3.4 mTorr, the Ar flow rate is 35 sccm, the O2 flow rate is 3 sccm, the DC voltage power of the target is 165 W, and the coating time is 860 s; in S2, in the second coating chamber 20, the stage temperature is set at 150 °C, the chamber pressure is 3.4 mTorr, the Ar flow rate is 35 sccm, the O2 flow rate is 1 sccm, the DC voltage power of the target is 150 W, and the coating time is 800 s; in S3, in the annealing chamber, the chamber temperature is set at 300 °C, and the annealing duration is 10 min.

[0092] Specifically, a clean substrate (such as silicon nitride) is selected and placed on the stage in the first coating chamber 10; the stage temperature is 150 °C, which helps the diffusion of atoms on the substrate surface, enabling the vanadium atoms and oxygen atoms sputtered onto the substrate to migrate and arrange better, and can improve the adhesion between the film and the substrate. Coating starts, and a DC voltage with a power of 165 W is applied to the target. At this power, the energy and quantity of argon ions are controllable, and the sputtering rate of vanadium atoms is guaranteed; argon gas with a flow rate of 35 sccm is introduced, and its function is to ionize under the action of an electric field to generate argon ions, which strike the pure vanadium target to sputter out vanadium atoms; at the same time, oxygen gas with a flow rate of 3 sccm is introduced to provide an appropriate amount of oxygen atoms for forming an oxygen-rich vanadium oxide film; the pressure in the first coating chamber 10 is controlled to be 3.4 mTorr, which can maintain stable plasma discharge, ensure the generation of sufficient argon ions to achieve efficient sputtering, and at the same time ensure that the sputtered atoms are evenly deposited on the substrate surface; sputtering continues for 860 s; during the coating process, the pure vanadium target sputters out vanadium atoms under the impact of argon ions, and the vanadium atoms combine with oxygen atoms and are deposited on the substrate surface through the gaps of the first grid 31, forming an oxygen-rich vanadium oxide film with an average film thickness of about 300 Å, and the film is wavy.

[0093] Transfer the substrate after one coating to the stage in the second coating chamber 20.

[0094] In the second coating chamber 20, the stage temperature is 150 °C, the chamber pressure is maintained at 3.4 mTorr, and the Ar flow rate is 35 sccm. These parameters are the same as those in the first coating to ensure the stability and coherence of the process. The O2 flow rate is adjusted to 1 sccm, which is lower than the O2 flow rate in the first coating, aiming to form an oxygen-deficient vanadium oxide film. The DC voltage power of the target is adjusted to 150 W, slightly lower than the power in the first coating, and the coating time is 800 s. During the second coating process, the vanadium atoms sputtered from the target combine with oxygen atoms and are deposited on the first layer of film through the spaces of the second grid 32, forming an oxygen-deficient vanadium oxide film with an average film thickness of about 300 Å, and this film is complementary to the first layer of film in terms of peaks and valleys.

[0095] Put the substrate after two coatings into the annealing chamber. The temperature of the annealing chamber is 300 °C, and this temperature can enable oxygen atoms to obtain sufficient energy to overcome the lattice potential barrier. The annealing duration is 10 min. To prevent secondary oxidation on the film surface at high temperatures, an inert gas (such as argon) can be selected to be introduced; or vacuum annealing can be used to reduce the interference of gas molecules on the oxygen diffusion path. During the annealing process, the oxygen atoms in the oxygen-rich layer obtain energy under the action of high temperature and start to diffuse into the vacancies in the oxygen-deficient layer. Since the contact surface between the two layers of film is wavy, it greatly increases the channels for oxygen element diffusion, making the diffusion speed of oxygen atoms faster. The final film has a uniform composition and a high vanadium dioxide content.

[0096] The present application also provides a coating device for implementing the magnetron sputtering process for increasing the content of vanadium dioxide in the thin film as described above.

[0097] Magnetron sputtering coating utilizes the combined action of an electric field and a magnetic field to ionize inert gases such as argon to form a plasma. Positive ions are accelerated under the action of the electric field to impact the target, sputtering target atoms out and depositing them on the substrate to form a thin film.

[0098] In the present application, by controlling the oxygen flow rate, two vanadium oxide thin films with different oxygen contents are formed on the substrate surface. Subsequently, through annealing treatment, using the oxygen concentration difference between the two thin films, oxygen atoms diffuse from the oxygen-rich layer to the oxygen-deficient layer, ultimately forming a thin film with a relatively high vanadium dioxide content and uniform composition.

[0099] In one embodiment, the coating device provided by the present application includes only one coating chamber. After the substrate enters the coating chamber, under appropriate carrier stage temperature, chamber pressure, and target DC voltage power, in cooperation with the first mixed gas, the first thin film is formed on the substrate surface. Subsequently, according to requirements, the carrier stage temperature, chamber pressure, and / or target DC voltage power are selectively adjusted, and the second mixed gas is introduced to form a second thin film on the first thin film.

[0100] The coating device provided by the present application further includes an annealing chamber, and a temperature control device (such as a heating rod, an infrared lamp tube, etc.) is provided in the annealing chamber. The temperature in the annealing chamber is adjusted to a preset value (such as 300 °C) through the temperature control device; the substrate that has been coated twice is placed into the annealing chamber and left for 5 - 45 minutes to facilitate the diffusion of oxygen atoms in the oxygen-rich layer to the oxygen-deficient layer, ultimately forming a thin film with uniform composition and a relatively high vanadium dioxide content.

[0101] Compared with the solution of using two coating chambers, using only one coating chamber can reduce the equipment purchase and maintenance costs, can reduce the upfront investment and later operation costs of production, and can also avoid the transfer of the substrate between different coating chambers, reducing the risks of contamination and damage that may occur during the transfer process.

[0102] In another embodiment, the coating device provided by the present application includes: a first coating chamber 10, in which a first carrier table and a first target are provided; a second coating chamber 20, in which a second carrier table and a second target are provided; a first grid 31, disposed in the first coating chamber 10 and located between the first carrier table and the first target, and the grid surface of the first grid 31 is formed by a plurality of grid bar structures laid at intervals in the horizontal direction; a second grid 32, disposed in the second coating chamber 20 and located between the second carrier table and the second target, and the grid surface of the second grid 32 is also formed by a plurality of grid bar structures laid at intervals in the horizontal direction; the distribution of the grid bar structures in the first grid 31 is staggered from the distribution of the grid bar structures in the second grid 32; during the first coating, due to the selective shielding of the first grid 31, a first layer of film with a wavy and uneven surface is formed on the surface of the substrate; during the second coating, due to the selective shielding of the second grid 32, a second layer of film with opposite peaks and valleys and opposite undulations is formed on the first layer of film; the shape of the second layer of film is complementary to that of the first layer of film, so that the film formed after two coatings tends to be flat.

[0103] Specifically, refer to Figure 1 , which illustrates a first coating chamber 10. The first coating chamber 10 provides a space for the first coating. Inside the first coating chamber 10, a first carrier table is provided at the bottom, and the first carrier table is used to place the substrate to be coated. A first target is provided at the top, and the first target is a pure vanadium target for providing vanadium atoms. The first target is located directly above the first carrier table.

[0104] Continue to refer to Figure 1 , the first grid 31 is installed in the first coating chamber 10 and located between the first carrier table and the first target. The grid surface of the first grid 31 is composed of a plurality of grid bar structures laid at intervals in the left - right direction, and these grid bar structures will block the deposition paths of some atoms.

[0105] Refer to Figure 2 , which illustrates a second coating chamber 20. The second coating chamber 20 provides a space for the second coating. Inside the second coating chamber 20, a second carrier table is provided at the bottom, and the second carrier table is also used to hold the substrate. A second target is provided at the top, and the second target is also a pure vanadium target for providing vanadium atoms for the second coating. The second target is located directly above the second carrier table.

[0106] Continue to refer to Figure 2 , the second grid 32 is installed in the second coating chamber 20 and located between the second carrier table and the second target. The grid surface of the second grid 32 is also composed of a plurality of grid bar structures laid at intervals in the left - right direction. The difference is that the distribution of the grid bar structures in the first grid 31 is just staggered from the distribution of the grid bar structures in the second grid 32 - when the grid surface of the first grid 31 and the grid surface of the second grid 32 are arranged side by side in the vertical direction, the horizontal projection of the grid bar structures in the first grid 31 is just located at the blank space positions of the second grid 32.

[0107] During film coating, a clean substrate is placed on the first carrier stage in the first coating chamber 10; during the magnetron sputtering process, under the action of an electric field and a magnetic field, vanadium atoms on the surface of the first target are sputtered out by argon ions; blocked by the first grid 31, only the metal atoms passing through the space positions can be deposited on the substrate surface, thereby forming a first layer of film with a wavy and uneven surface on the substrate.

[0108] After one-time film coating is completed, the substrate with the first layer of film deposited on its surface is transferred to the second carrier stage in the second coating chamber 20; during the magnetron sputtering process, the vanadium atoms sputtered from the second target can be deposited on the first layer of film only by passing through the space positions of the second grid 32; due to the staggered distribution of the grid bar structures of the first grid 31 and the second grid 32, the second layer of film formed by secondary film coating is opposite to the first layer of film in terms of peak and valley, and has an opposite undulation, and the two are complementary in shape, making the film on the substrate tend to be flat after two-time film coating.

[0109] After two-time film coating is completed, the substrate with two layers of film deposited is transferred to the annealing chamber; during the annealing process, oxygen atoms in the oxygen-rich layer will gradually transfer to the vacancies in the oxygen-deficient layer. Since the contact surface between the first layer of film and the second layer of film is wavy, this structure greatly increases the channels for oxygen atom diffusion, can promote the diffusion of oxygen atoms, thereby greatly shortening the time required for oxygen atom diffusion, and further reducing the process time required for annealing. Due to the reduction of the annealing time, the cycle of the entire film coating process is shortened, the production efficiency is improved, which is beneficial to large-scale industrial production.

[0110] The above embodiments only represent several implementation manners of the present application, and the description thereof is relatively specific and detailed, but it should not be construed as a limitation to the scope of the patent application. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present application, several deformations and improvements can still be made, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the appended claims.

Claims

1. A magnetron sputtering process for increasing the content of vanadium dioxide in a thin film, characterized in that, Including: S1. In the first coating chamber (10), using a pure vanadium target, introducing a first mixed gas, and forming a first layer of film on the substrate surface by magnetron sputtering; S2. In the second coating chamber (20), using a pure vanadium target, introducing a second mixed gas, and forming a second layer of film on the first layer of film by magnetron sputtering; Both the first mixed gas and the second mixed gas include Ar and O2, but the flow rate of O2 in the two is different, so that one of the first layer of film and the second layer of film is an oxygen-rich layer and the other is an oxygen-deficient layer; In the oxygen-rich layer, the content ratio of oxygen atoms to vanadium atoms is greater than 2; In the oxygen-deficient layer, the content ratio of oxygen atoms to vanadium atoms is less than 2; Both the first coating chamber (10) and the second coating chamber (20) are provided with a target and a stage. The first coating chamber (10) is further provided with a first grid (31), and the second coating chamber (20) is further provided with a second grid (32). The mesh surfaces of the first grid (31) and the second grid (32) are arranged in a complementary form, and the grid bar structure of the first grid (31) is interchanged with the space position of the second grid (32); Through S1, a wavy first layer of film is formed on the substrate surface; The second layer of film formed through S2 is complementary to the peaks and valleys of the first layer of film, so that the film formed after two coatings tends to be flat; S3. Place the substrate after two coatings into the annealing chamber, so that the substrate is maintained at 150 - 350 °C for 5 - 45 min, promoting the diffusion of oxygen atoms from the oxygen-rich layer to the oxygen-deficient layer, in order to form a vanadium dioxide film with uniform composition; During annealing, the oxygen atoms in the oxygen-rich layer gradually transfer to the vacancies in the oxygen-deficient layer. Since the contact surface between the first layer of film and the second layer of film is wavy, the channels for oxygen atom diffusion are increased, which can reduce the time required for diffusion, thereby reducing the process time required for annealing.

2. The magnetron sputtering process for increasing the content of vanadium dioxide in the thin film according to claim 1, characterized in that, The flow rate of O2 in S1 is 2 - 30 sccm, the flow rate of O2 in S2 is 1 - 20 sccm, and the flow rate of O2 in S1 is higher than that in S2; After two coatings, the first layer of film on the substrate is an oxygen-rich layer, and the second layer of film is an oxygen-deficient layer.

3. The magnetron sputtering process for increasing the vanadium dioxide content in the thin film according to claim 2, characterized in that, In S1 and S2, the flow rate of Ar is 15 - 100 sccm, the DC voltage power of the target is 100 - 1500 W, and the chamber pressure is 2 - 30 mTorr.

4. The magnetron sputtering process for increasing the vanadium dioxide content in the thin film according to claim 1, characterized in that, In S1 and S2, the stage temperature is 80 - 300 °C.

5. The magnetron sputtering process for increasing the vanadium dioxide content in the thin film according to claim 1, wherein, In S3, during annealing, an inert gas is introduced into the annealing chamber to prevent secondary oxidation on the film surface at high temperature; Alternatively, S3 uses vacuum annealing. During annealing, the pressure in the annealing chamber is made less than 10 -3 Torr, which can reduce the interference of gas molecules with the oxygen diffusion path.

6. The magnetron sputtering process for increasing the vanadium dioxide content in the thin film according to claim 1, wherein After two coatings, the film thicknesses of both the first layer of film and the second layer of film on the substrate are 200 - 400 Å.

7. The magnetron sputtering process for increasing the content of vanadium dioxide in the thin film according to claim 1, characterized in that, In S1, inside the first coating chamber (10), the stage temperature is set at 150 °C, the chamber pressure is 3.4 mTorr, the Ar flow rate is 35 sccm, the O2 flow rate is 3 sccm, the DC voltage power of the target is 165 W, and the coating time is 860 s; In S2, in the second coating chamber (20), the temperature of the carrier stage is set to 150 °C, the chamber pressure is 3.4 mTorr, the Ar flow rate is 35 sccm, the O2 flow rate is 1 sccm, the DC voltage power of the target is 150 W, and the coating time is 800 s; In S3, in the annealing chamber, the chamber temperature is set to 300 °C, and the annealing duration is 10 min.

8. A coating device for implementing the magnetron sputtering process for increasing the vanadium dioxide content in a thin film according to any one of claims 1-7, characterized in that, Comprising: A first coating chamber (10) provided with a first carrier stage and a first target in the first coating chamber (10); A second coating chamber (20) provided with a second carrier stage and a second target in the second coating chamber (20); A first grid (31) disposed in the first coating chamber (10) and located between the first carrier stage and the first target, and the grid surface of the first grid (31) is formed by a plurality of grid bar structures laid horizontally at intervals; A second grid (32) disposed in the second coating chamber (20) and located between the second carrier stage and the second target, and the grid surface of the second grid (32) is also formed by a plurality of grid bar structures laid horizontally at intervals; The distribution of the grid bar structures in the first grid (31) is staggered from the distribution of the grid bar structures in the second grid (32); During the first coating, due to the selective shielding of the first grid (31), a first layer of film with a wavy and uneven surface is formed on the substrate surface; During the second coating, due to the selective shielding of the second grid (32), a second layer of film with opposite peaks and valleys and opposite undulations is formed on the first layer of film; The second layer of film is complementary to the shape of the first layer of film, so that the film formed after two coatings tends to be flat.

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