Preparation method of high-quality titanium dioxide film with controllable refractive index

Abstract

A preparation method of a high-quality titanium dioxide film with a controllable refractive index comprises the following steps that (1) a pulse laser light path is adjusted, and uniform scanning of a laser beam on the surface of a target material is achieved through a two-dimensional light beam scanning galvanometer; (2) preparing a titanium dioxide film: placing the substrate and the target material in a deposition cavity of vacuum deposition equipment, adjusting the distance between the target material and the substrate, setting the rotating speed of the target material and the heating temperature of the substrate, setting pulse laser parameters, bombarding the target material through collinear pulse string laser, and depositing generated plasma plume on the substrate to form the titanium dioxide film; and (3) annealing treatment of the titanium dioxide film: carrying out high-temperature oxygen introduction annealing treatment on the deposited titanium dioxide film. According to the method, the roughness of the titanium dioxide film is greatly improved in a collinear pulse string laser output mode, so that the high-quality titanium dioxide film with a smoother surface is obtained; the refractive index of the titanium dioxide film is flexibly regulated and controlled by changing the annealing temperature and time of high-temperature oxygen introduction.

Description

Technical Field

[0001] The invention relates to a method for preparing a high-quality titanium dioxide film with controllable refractive index, belonging to the technical field of titanium dioxide film preparation. Background Art

[0002] Titanium dioxide (TiO2) thin films are widely used in various fields due to their excellent optical, chemical and electronic properties. As an optical coating, titanium dioxide thin films have a high refractive index and good light transmittance, which can improve the reflectivity and anti-reflection properties of lenses, optical windows and mirrors. In the manufacture of solar cells, titanium dioxide thin films are often used as electron transport layers due to their superior electrochemical stability and moderate band gap width. Among them, the different refractive index characteristics of titanium dioxide thin films can change the path length of light inside the film, thereby determining different photoelectric conversion efficiencies. In biosensors and chemical sensors, titanium dioxide thin films can be used to detect various chemicals and biomarkers. The refractive index of titanium dioxide determines the sensitivity of the sensor to light and affects its detection capabilities.

[0003] The refractive index of titanium dioxide film is significantly affected by the preparation method. This is because different preparation techniques will lead to differences in crystal quality, density, crystal size, etc., which in turn affect the refractive index. For example, when preparing titanium dioxide film by hydrothermal synthesis, the crystal phase and microstructure of the film can be carefully regulated by changing the reaction temperature, time, pH value and other reaction conditions, thereby obtaining titanium dioxide films with different refractive indices. When preparing titanium dioxide film by sol-gel method, sol is generated by hydrolysis and condensation reaction, and then gel is formed by heat treatment, and finally a low-refractive-index titanium dioxide film with a porous structure is prepared at low temperature. When preparing titanium dioxide film by magnetron sputtering, changing the deposition parameters such as sputtering power, substrate temperature and atmosphere can adjust the crystal structure and density of the film to obtain a high-refractive-index titanium dioxide film with higher crystal quality. However, these methods have many control parameters, are not easy to operate, and have deviations in the chemical composition of the film when preparing titanium dioxide films with different refractive indices.

[0004] Pulsed laser deposition (PLD) is an advanced thin film preparation technology that is widely used to prepare high-quality single-crystal or polycrystalline TiO thin films. PLD technology precisely controls the evaporation amount and evaporation rate of the target material by adjusting parameters such as laser energy, pulse width and frequency, thereby affecting the thickness, crystal structure, density, etc. of the TiO thin film, and then affecting the refractive index of the film. Compared with other methods, pulsed laser deposition technology has higher process flexibility, more precise film quality control and fast deposition efficiency, making it easier to prepare titanium dioxide films with different refractive indices to meet various specific technical and industrial application requirements. Traditional pulsed laser deposition technology mainly uses excimer nanosecond laser as the excitation source. During the ablation of the target material, the electron-lattice relaxation time is tens of picoseconds, which is much smaller than the pulse width of the ablation laser. At this time, the lattice will be affected by the thermal effect, producing large particles, making the film rough and affecting the performance of the film. In order to improve the deposition effect, some PLD depositions use picosecond ultrafast lasers. In contrast, the shorter pulses of picosecond lasers reduce thermal damage, make the structure of the film more uniform, and reduce the formation of defects. However, there are still problems such as a certain number of particles on the film surface, poor film crystallinity, and low film deposition efficiency and refractive index. Summary of the invention

[0005] In view of the shortcomings of the existing titanium dioxide film preparation technology, the present invention proposes a method for preparing a high-quality titanium dioxide film with controllable refractive index based on ultrafast pulse train laser deposition technology. This method greatly improves the roughness of the film and reduces the size of surface particles, and can obtain a high-quality titanium dioxide film with a smoother surface. Through high-temperature oxygen annealing treatment, the refractive index of the titanium dioxide film can be flexibly controlled.

[0006] To achieve the above purpose, the method for preparing a high-quality titanium dioxide film with controllable refractive index in the present invention comprises the following steps:

[0007] (1) Build a pulsed laser optical path and control the focused laser beam to scan uniformly on the target surface;

[0008] The construction of the pulse laser optical path includes: expanding and collimating the output pulse laser so that the laser beam is irradiated on the target material in the deposition chamber, and then adjusting the spot size so that the high-energy laser is focused on the surface of the target material, resulting in ablation and evaporation of the target surface material to generate high-temperature plasma.

[0009] The controlling of the focused laser beam to uniformly scan the surface of the target material refers to the laser beam being subjected to the action of a two-dimensional beam scanning galvanometer to achieve uniform scanning of the focused high-energy laser on the surface of the target material.

[0010] (2) Preparation of titanium dioxide thin films;

[0011] Place the substrate and target material in the deposition chamber of the vacuum deposition equipment, adjust the distance between the target material and the substrate, set the target material rotation speed and substrate heating temperature, set the pulse laser parameters, bombard the target material with a co-linear pulse train laser, and the generated plasma plume is deposited on the substrate to form a titanium dioxide film.

[0012] The substrate may be made of materials such as single crystal silicon, silicon dioxide or ceramics; the target material may be a high-purity titanium dioxide target material with a mass fraction greater than 99.99%.

[0013] The substrate and target are pre-treated before use to remove impurities (dust, grease, etc.) on the substrate and target.

[0014] The distance between the target material and the substrate is adjusted to 8 cm-12 cm, which can meet the requirements of laser ablation of titanium dioxide target material.

[0015] The target material rotation speed is 5-15 rpm; the substrate heating temperature is 400-600°C.

[0016] The pulse laser parameters are: a laser wavelength of 532 nm, a pulse frequency of more than 50 KHz, and a laser pulse number of at least 2 pulses.

[0017] The deposition on the substrate is to set the substrate to rotate continuously after pre-deposition, start co-linear pulse train laser deposition, and the pre-deposition time is 3-5 minutes.

[0018] (3) Annealing treatment of titanium dioxide film;

[0019] The titanium dioxide film is subjected to high temperature oxygen annealing treatment.

[0020] The high temperature oxygen annealing temperature is 400-500° C., and the annealing time is 60-120 min.

[0021] The oxygen supply is the supply of high-purity oxygen.

[0022] The above method uses an ultrafast high-repetition-rate pulsed laser in a collinear pulse train output mode to control the number and size of particles on the surface of a titanium dioxide film prepared by pulsed laser deposition from the source of particle generation and the plasma transmission stage, and then performs a high-temperature oxygen annealing treatment on the obtained film. By changing the annealing temperature and time, the density and crystallinity of the film are improved, the refractive index of the titanium dioxide film is regulated, and a high-quality titanium dioxide film with a controllable refractive index is obtained.

[0023] The present invention proposes for the first time to use a combination of a collinear pulse train and a high-temperature oxygen annealing process to improve the flatness and refractive index of a titanium dioxide film. Compared with the prior art, the present invention has the following beneficial effects:

[0024] 1. Use pulsed laser to ablate titanium dioxide target under vacuum conditions, and obtain high-quality titanium dioxide film with smoother surface by adjusting the laser output additional sub-pulses with an interval of tens of nanoseconds.

[0025] 2.Using ultrafast pulse laser (such as picosecond laser), the single pulse energy is low, so each pulse of laser interacts with the material and can only stimulate a few particles, which hinders the generation of large particles. High pulse frequency (greater than 50KHz) can stimulate more particles and ensure a higher deposition rate.

[0026] 3. The output mode of the collinear pulse train can output additional sub-pulses within a very short time interval, which can secondary ablate the large particles generated by the previous pulse, further enhance the ionization degree of the plasma, and heat and decompose the nano-particles, thereby greatly improving the roughness of the film and reducing the size of the surface particles.

[0027] 4. With the help of high-temperature oxygen annealing process, the refractive index of titanium dioxide film can be flexibly controlled by changing the annealing temperature and time. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is the SEM image of the titanium dioxide film prepared under the five-pulse train conditions in Example 2.

[0029] Figure 2 This is the SEM image of the titanium dioxide film prepared under the single pulse train conditions in Example 3.

[0030] Figure 3 This is the refractive index curve of titanium dioxide film before and after annealing and at different annealing times under seven-pulse train preparation conditions.

[0031] Figure 4 This is a statistical diagram of the roughness of titanium dioxide films prepared under different pulse numbers. DETAILED DESCRIPTION

[0032] Example 1

[0033] In this embodiment, a titanium dioxide film is prepared by a three-pulse train, and the specific steps are as follows.

[0034] 1. Picosecond pulse laser deposition optical path adjustment

[0035] Before film deposition begins, adjust the laser beam path collimation and laser spot size of the picosecond laser.

[0036] Turn on the picosecond laser pulsed laser deposition optical path indicator laser, that is, a 650nm helium-neon laser, which is coaxially transmitted with the picosecond pulse laser. Use a Galilean laser beam expander to expand the laser beam to obtain a collimated expanded beam with a diameter of 6mm, and then adjust the spot size through a focusing lens to obtain a focused spot of 78.6μm. Finally, by adjusting the beam two-dimensional scanning galvanometer (two deflection lenses can be used to scan the beam in the XY plane), the laser can be uniformly scanned on the target surface.

[0037] 2. Titanium dioxide film preparation

[0038] (1) Substrate and target pretreatment: A single crystal silicon substrate and a rutile titanium dioxide target with a diameter of 25.4 mm, a thickness of 5 mm, and a mass fraction of 99.999% were ultrasonically cleaned for 15 minutes in the order of acetone cleaning, anhydrous ethanol cleaning, and deionized water cleaning, and finally dried with clean nitrogen gas.

[0039] (2) Install the substrate on the sample holder and then fix it on the rotating bracket in the deposition chamber. The target is fixed on the rotatable target holder. Adjust the distance between the target and the substrate to 8 cm. After closing all the windows and valves in the deposition chamber, prepare to start vacuum deposition.

[0040] (3) In the control panel of the vacuum deposition equipment, open the mechanical pump and the bypass valve, wait for the pressure in the deposition chamber to drop to less than 10Pa, close the bypass valve, open the front pole valve and the molecular pump, wait for the molecular pump speed to increase to 27000, open the gate valve, and set the regulating valve to the fully open state. Open the deposition chamber electrode switch, wait for the deposition chamber pressure to drop to 10 -4 Pa or less. Set the target rotation speed to 10 r / min and the substrate heating to 400°C.

[0041] (4) Turn on the picosecond laser, set the picosecond laser output wavelength to 532 nm, the single pulse energy to 150 μJ, the pulse frequency to 100 kHz, and the number of laser pulses to three pulses.

[0042] (5) After 5 minutes of pre-deposition, the baffle of the deposition chamber was opened, and the substrate was set to rotate continuously at a speed of 25 r / min. Pulsed laser deposition was started and the deposition was completed after 20 minutes.

[0043] (6) Turn off the coating chamber switch. Close the gate valve and molecular pump in turn, wait for the molecular pump speed to reach zero, and close the front pole valve and mechanical pump. Open the vent valve and wait for the pressure in the chamber to return to atmospheric pressure. Open the deposition chamber, take out the deposited titanium dioxide film sample, and wait for the deposited sample to cool to room temperature.

[0044] 3. Annealing of titanium dioxide film

[0045] Place the prepared titanium dioxide film sample in a quartz boat, and place the quartz boat in an annealing furnace. Set the annealing furnace temperature to 500°C, the heating time to 60 minutes, and the annealing time to 60 minutes. Turn on the vacuum pump of the annealing furnace, and turn off the vacuum pump when the air pressure in the annealing furnace drops below 1Pa. Introduce oxygen into the annealing furnace at an oxygen flow rate of 1L / min until the air pressure in the annealing furnace reaches 1Pa. Turn on the annealing furnace switch and start the annealing process. After the annealing is completed, wait for the temperature in the furnace to cool to room temperature, and then take out the titanium dioxide film sample.

[0046] Example 2

[0047] In this embodiment, a titanium dioxide film is prepared by a five-pulse train, and the specific steps are as follows.

[0048] 1. Adjust the laser light path collimation and laser spot size

[0049] Turn on the pulsed laser deposition optical path indicator laser, use the Galileo laser beam expander to expand the laser, obtain a collimated expanded light with a diameter of 8 mm, and then adjust the spot size through the focusing lens to obtain a focused spot of 45.76 μm. Finally, adjust the two-dimensional scanning galvanometer of the light beam to achieve uniform scanning of the laser on the target surface.

[0050] 2. Titanium dioxide film preparation

[0051] (1) A single crystal silicon substrate and a rutile titanium dioxide target with a diameter of 25.4 mm, a thickness of 5 mm, and a mass fraction of 99.999% were ultrasonically cleaned in acetone, anhydrous ethanol, and deionized water for 15 minutes each, and finally dried with clean nitrogen gas.

[0052] (2) Install the substrate on the sample holder and then fix it on the rotating bracket in the deposition chamber. The target is fixed on the rotatable target holder. Adjust the distance between the target and the substrate to 10 cm. After closing all the windows and valves in the deposition chamber, prepare to start the vacuum experiment.

[0053] (3) In the equipment control panel, open the mechanical pump and the bypass valve, and wait for the pressure in the vacuum chamber to drop to less than 10Pa. Close the bypass valve, open the front valve and molecular pump, wait for the molecular pump speed to increase to 27000, open the gate valve, and set the regulating valve to the fully open state. Open the coating chamber switch and wait for the pressure in the chamber to drop to 10 -4 The target rotation speed was set to 5 r / min and the substrate heating was set to 500°C.

[0054] (4) Turn on the laser, set the laser output wavelength to 532 nm, the single pulse energy to 120 μJ, the pulse frequency to 80 kHz, and the number of laser pulses to five pulses.

[0055] (5) After 5 minutes of pre-deposition, the baffle of the deposition chamber was opened, and the substrate was set to rotate continuously at a speed of 25 r / min. Pulsed laser deposition was started and the deposition was completed after 15 minutes.

[0056] (6) Turn off the coating chamber switch. Close the gate valve and molecular pump in turn, wait for the molecular pump speed to reach zero, and close the front pole valve and mechanical pump. Open the vent valve and wait for the pressure in the chamber to return to atmospheric pressure. Open the deposition chamber and take out the deposited titanium dioxide film sample. Wait for the deposited sample to cool to room temperature.

[0057] 3. Annealing of titanium dioxide film

[0058] Place the prepared titanium dioxide film sample in a quartz boat, and place the quartz boat in an annealing furnace. Set the annealing furnace temperature to 450°C, the heating time to 50 minutes, and the annealing time to 120 minutes. Turn on the vacuum pump of the annealing furnace, and turn off the vacuum pump when the air pressure in the annealing furnace drops below 1Pa. Introduce oxygen into the annealing furnace at a flow rate of 1.5L / min until the air pressure in the annealing furnace reaches 1Pa. Turn on the annealing furnace switch and start the annealing process. After the annealing is completed, wait for the temperature in the furnace to cool to room temperature, and then take out the titanium dioxide film sample.

[0059] Figure 1 The surface SEM morphology of the five-pulse titanium dioxide thin film sample prepared in this example is given.

[0060] Example 3

[0061] This embodiment serves as a comparative example of Embodiment 2, in which a titanium dioxide film is prepared by a single pulse. The difference from Embodiment 2 is that in step (4) of preparing the titanium dioxide film, the number of laser pulses is set to a single pulse, and the rest of the process is the same as that of Embodiment 2.

[0062] Figure 2 The surface SEM morphology of the single pulse titanium dioxide film sample prepared in this embodiment (comparative example) is given. Figure 1 The comparison of the results under the five-pulse preparation conditions shows that the titanium dioxide film prepared under the pulse train has fewer surface particles and a smoother film.

[0063] Example 4

[0064] This embodiment prepares a titanium dioxide film by a seven-pulse train, and the specific steps are as follows.

[0065] 1. Adjust the laser light path collimation and laser spot size

[0066] Turn on the pulsed laser deposition optical path indicator laser, use the Galileo laser beam expander to expand the laser, obtain a collimated expanded light with a diameter of 7 mm, and then adjust the spot size through the focusing lens to obtain a focused spot of 59.16 μm. Finally, adjust the beam two-dimensional scanning galvanometer to achieve uniform scanning of the laser on the target surface.

[0067] 2. Titanium dioxide film preparation

[0068] (1) A single crystal silicon substrate and a rutile titanium dioxide target with a diameter of 25.4 mm, a thickness of 5 mm, and a mass fraction of 99.999% were ultrasonically cleaned in acetone, anhydrous ethanol, and deionized water for 15 minutes each, and finally dried with clean nitrogen gas.

[0069] (2) Install the substrate on the sample holder and then fix it on the rotating bracket in the deposition chamber. The target is fixed on the rotatable target holder. Adjust the distance between the target and the substrate to 12 cm. After closing all the windows and valves in the deposition chamber, prepare to start the vacuum experiment.

[0070] (3) In the equipment control panel, open the mechanical pump and the bypass valve, and wait for the pressure in the vacuum chamber to drop to less than 10Pa. Close the bypass valve, open the front valve and molecular pump, wait for the molecular pump speed to increase to 27000, open the gate valve, and set the regulating valve to the fully open state. Open the coating chamber switch and wait for the pressure in the chamber to drop to 10 -4 The target rotation speed was set to 15 r / min and the substrate heating was set to 600°C.

[0071] (4) Turn on the laser, set the laser output wavelength to 532 nm, the single pulse energy to 80 μJ, the pulse frequency to 50 kHz, and the number of laser pulses to seven pulses.

[0072] (5) After 4 minutes of pre-deposition, the baffle of the deposition chamber was opened, and the substrate was set to rotate continuously at a speed of 25 r / min. Pulsed laser deposition was started and the deposition was completed after 40 minutes.

[0073] (6) Turn off the coating chamber switch. Close the gate valve and molecular pump in turn, wait for the molecular pump speed to reach zero, and close the front pole valve and mechanical pump. Open the vent valve and wait for the pressure in the chamber to return to atmospheric pressure. Open the deposition chamber, take out the deposited titanium dioxide film sample, and wait for the deposited sample to cool to room temperature.

[0074] 3. Annealing of titanium dioxide film

[0075] Place the prepared titanium dioxide film sample in a quartz boat, and place the quartz boat in an annealing furnace. Set the annealing furnace temperature to 500°C, the heating time to 55 minutes, and the annealing time to 60 minutes. Turn on the vacuum pump of the annealing furnace, and turn off the vacuum pump when the air pressure in the annealing furnace drops below 1Pa. Introduce oxygen into the annealing furnace at a flow rate of 2L / min until the air pressure in the annealing furnace reaches 1Pa. Turn on the annealing furnace switch and start the annealing process. After the annealing is completed, wait for the temperature in the furnace to cool to room temperature, and then take out the titanium dioxide film sample.

[0076] Example 5

[0077] This embodiment is to prepare a titanium dioxide film by a seven-pulse train. The difference from Embodiment 4 is that the annealing time during the annealing process of the titanium dioxide film is 90 minutes, and the rest of the process is the same as that of Embodiment 4.

[0078] Figure 3 The refractive index curves of the titanium dioxide films prepared under the seven-pulse train conditions obtained in Example 4 and Example 5 at different annealing times are given. Annealing can improve the density and crystal quality of the film, thereby improving the refractive index of the film. Different annealing temperatures and times have different effects on the refractive index of the film, and the refractive index of the film can be adjusted by the annealing time.

[0079] Figure 4 Statistical graphs of the surface roughness values ​​of titanium dioxide films prepared under single-pulse, three-pulse, five-pulse and seven-pulse conditions in Example 3, Example 1, Example 2 and Example 4 are given. By comparison, it can be found that with the increase of the number of pulses, the surface roughness value of the titanium dioxide film decreases from 97.1nm under single-pulse to 4.53nm under seven-pulse, indicating that high-quality titanium dioxide films with smoother surfaces can be prepared by the co-linear pulse train output mode.

Claims

1. A method for preparing a high-quality titanium dioxide film with controllable refractive index, characterized in that: The following steps are involved: (1) Build a pulsed laser optical path and control the focused laser beam to scan uniformly on the target surface; (2) Preparation of titanium dioxide thin films; The substrate and the target are placed in a deposition chamber of a vacuum deposition device, the distance between the target and the substrate is adjusted, the target rotation speed and the substrate heating temperature are set, and the pulse laser parameters are set. The target is bombarded by a co-linear pulse train laser, and the generated plasma plume is deposited on the substrate to form a titanium dioxide film; (3) Annealing treatment of titanium dioxide film; The titanium dioxide film is subjected to high temperature oxygen annealing treatment.

2. The method for preparing a high-quality titanium dioxide film with controllable refractive index according to claim 1, characterized in that: The step (1) of constructing the pulse laser optical path includes: expanding and collimating the output pulse laser so that the laser beam is irradiated on the target material, and then adjusting the spot size so that the high-energy laser is focused on the surface of the target material, ablating the material on the surface of the target material to evaporate and generate high-temperature plasma.

3. The method for preparing a high-quality titanium dioxide film with controllable refractive index according to claim 1, characterized in that: In the step (1), controlling the focused laser beam to scan uniformly on the target surface means that the laser beam is used through the two-dimensional beam scanning galvanometer to achieve uniform scanning of the target surface in the deposition chamber by the incident laser beam.

4. The method for preparing a high-quality titanium dioxide film with controllable refractive index according to claim 1, characterized in that: In the step (2), the substrate is made of materials such as single crystal silicon, silicon dioxide or ceramics; and the target material is a high-purity titanium dioxide target material with a mass fraction greater than 99.99%.

5. The method for preparing a high-quality titanium dioxide film with controllable refractive index according to claim 1, characterized in that: In the step (2), the substrate and the target are pretreated before use to remove impurities on the substrate and the target.

6. The method for preparing a high-quality titanium dioxide film with controllable refractive index according to claim 1, characterized in that: In the step (2), the distance between the target and the substrate is adjusted to 8 cm-12 cm, which can meet the requirements of laser ablation of titanium dioxide target.

7. The method for preparing a high-quality titanium dioxide film with controllable refractive index according to claim 1, characterized in that: In the step (2), the target rotation speed is set to 5-15 rpm so that the focused high-energy laser can evenly scan the target surface; the substrate heating temperature is 400°C-600°C.

8. The method for preparing a high-quality titanium dioxide film with controllable refractive index according to claim 1, characterized in that: The pulse laser parameters in step (2) are: a laser wavelength of 532 nm, a pulse frequency of 50 kHz or more, and a laser pulse number of at least 2 pulses.

9. The method for preparing a high-quality titanium dioxide film with controllable refractive index according to claim 1, characterized in that: In the step (2), the deposition on the substrate is performed by setting the substrate to rotate continuously after pre-deposition to perform co-linear pulse train laser deposition; the pre-deposition time is 3-5 minutes.

10. The method for preparing a high-quality titanium dioxide film with controllable refractive index according to claim 1, characterized in that: The high temperature of the high temperature oxygen annealing treatment in step (3) is 400-500° C., the annealing time is 60-120 min, and the oxygen annealing is the introduction of high-purity oxygen.

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