Low-absorption laser film and method of making same

Abstract

The application relates to the technical field of laser film preparation, in particular to a low-absorption laser film and a preparation method thereof. The method comprises the following steps: sequentially performing cleaning, drying and surface activation treatment on a substrate to obtain a pretreated substrate; placing the substrate in a vacuum chamber of an evaporation film coating device, adjusting the distance between the substrate and an evaporation source to 50-200 cm, and performing vacuum pumping; alternately evaporating high-refractive-index materials and low-refractive-index materials to deposit and form a multilayer alternately stacked initial film on the surface of the substrate; after the initial film is deposited, the vacuum degree is kept unchanged, high-purity oxygen is introduced, and the initial film is subjected to bombardment oxidation treatment by using an oxygen ion beam to complement oxygen vacancies and compact the film layer, thereby obtaining a multilayer dielectric film; and vacuum annealing and cooling are performed at 300-500 DEG C to obtain a finished product. The method has the advantages that a synergistic process of evaporation film coating combined with ion beam post-oxidation is adopted, the film layer is not damaged, the weak absorption of the film is lower than 10 ppm, and the method is suitable for the production of high-power laser elements.

Description

Technical Field

[0001] This invention relates to the field of laser thin film preparation technology, and in particular to a low-absorption laser thin film and its preparation method. Background Technology

[0002] As a core optical component in high-power laser systems and ultra-high precision measurement systems, laser thin films directly determine the output power, stability, and lifespan of the laser system due to their absorption loss. This is one of the key bottlenecks limiting the development of high-power laser technology towards higher power and higher precision. Low-absorption laser thin films require extremely low light absorption coefficients in the target laser wavelength band, while also possessing good optical homogeneity, high density, strong adhesion, and excellent resistance to laser damage to meet the demands of extreme working environments.

[0003] Currently, the mainstream methods for preparing laser thin films include electron beam evaporation deposition, ion beam sputtering deposition, and magnetron sputtering deposition. Among these, electron beam evaporation deposition is widely used in the large-scale production of laser thin films due to its advantages such as moderate equipment cost, controllable deposition rate, high film purity, and ease of multilayer film stacking. However, traditional electron beam evaporation deposition technology has significant drawbacks: during the high-vacuum evaporation process, the film material is prone to insufficient oxidation, leading to microscopic defects such as oxygen vacancies and defect states within the film. These defects significantly increase the absorption loss of the film to laser light. At the same time, the evaporated and deposited film usually has low packing density and high surface roughness, further aggravating light scattering and absorption, making it difficult to reduce the weak absorption of the film to the sub-ppm level, which cannot meet the requirements of high-end, high-power laser systems.

[0004] To address the aforementioned issues, existing technologies have proposed plasma-assisted evaporation deposition (PALD), which involves bombarding the deposition surface with an ion beam during the thin film evaporation deposition process to improve film density and reduce defects. However, this technology still has shortcomings: on the one hand, ion beam bombardment during deposition can easily damage incompletely formed atomic layers, introducing new structural defects; on the other hand, the synergistic effect between the ion beam and evaporation particles is difficult to control precisely, easily leading to uneven film composition and failing to fundamentally solve the absorption loss problem caused by insufficient oxidation. Furthermore, existing technologies also employ subsequent heat treatment for oxidation completion, but high-temperature heat treatment can easily lead to grain growth and decreased surface smoothness, increasing light scattering loss and potentially causing thermal stress cracking between the film and the substrate, affecting the film's stability and lifespan.

[0005] Therefore, developing a method for preparing low-absorption laser thin films that can precisely control the degree of oxidation, reduce microscopic defects, lower absorption loss, and simultaneously ensure the integrity of the film structure and the stability of its performance has become an urgent technical problem to be solved in the field of laser thin films. Summary of the Invention

[0006] To address the aforementioned problems, this invention provides a low-absorption laser thin film and its preparation method.

[0007] The primary objective of this invention is to provide a method for preparing a low-absorption laser thin film, comprising the following steps: S1. The substrate is sequentially cleaned, dried, and surface activated to obtain a pretreated substrate; S2. Place the pretreated substrate inside the vacuum chamber of the evaporation coating equipment, adjust the distance between the substrate and the evaporation source to 50~200cm, and evacuate the vacuum. S3. High-refractive-index materials and low-refractive-index materials are alternately vapor-deposited to form a multilayered, stacked initial thin film on the substrate surface; S4. After the initial thin film deposition is completed, the vacuum level is kept constant, high-purity oxygen is introduced, and the initial thin film is bombarded with oxygen ion beam to oxidize it, fill the oxygen vacancies in the film layer and compact the film layer to obtain a multilayer dielectric film. S5. Vacuum annealing is performed at 300~500℃, followed by cooling to obtain a low-absorption laser thin film.

[0008] Preferably, in step S1, the drying method is baking, the baking temperature is 30~80℃, and the baking time is 30~60min; the surface activation treatment is bombarded with a low-energy argon ion beam for 5~15min.

[0009] Preferably, in step S1, the cleaning is multi-frequency segmented ultrasonic cleaning, including 12 segmented ultrasonic tanks, with tanks 1-3 using a frequency of 40kHz, tanks 4-8 using a frequency of 80kHz, and tanks 9-12 using a frequency of 1MHz. The substrate is one of quartz, sapphire, CaF2, Ge, Si, or ZnSe.

[0010] Preferably, in step S3, the high refractive index material is selected from at least one of Hf, HfO2, Ta2O5, TiO2, and Nb2O5, and the low refractive index material is selected from any one of SiO2, YbF3, and MgF2; the substrate temperature is controlled at 30~300℃ throughout the deposition process, and the constant temperature holding time is 30~300min.

[0011] Preferably, the evaporation rate of the high refractive index material is controlled at 0.01~0.5 nm / s, and the evaporation rate of the low refractive index material is controlled at 0.1~2.0 nm / s.

[0012] Preferably, the initial film stacking structure is SUB / (k1Hk2L). n / A、SUB / (k1Hk2Hk3L) n / A or SUB / (k1Hk2Lk3Hk4L) n One of / A, where SUB represents the pretreated substrate, A represents the air side, H represents the high refractive index film, L represents the low refractive index film, k1, k2, k3, and k4 are the optical thickness coefficients of the corresponding film at one-quarter of the reference wavelength, and n is the number of alternating stacking cycles of high and low refractive index films; n ranges from 1 to 30.

[0013] Preferably, in step S4, the high-purity oxygen is oxygen with a purity of ≥99.99%, the oxygen flow rate is 20~150 sccm, the oxidation treatment time is 5~30 min, and the substrate temperature is maintained at 30~300℃ during the oxidation stage.

[0014] Preferably, in step S5, the holding time for vacuum annealing is 10~40 min; and the cooling rate is 5~10℃ / min.

[0015] The second objective of this invention is to provide a low-absorption laser thin film, which is prepared using the aforementioned method for preparing a low-absorption laser thin film, comprising a substrate and a multilayer dielectric film; the multilayer dielectric film consists of alternating stacked high-refractive-index layers and low-refractive-index layers, with a total film thickness of 0.8~5μm; The material of the high refractive index film is selected from at least one of Hf, HfO2, Ta2O5, TiO2, and Nb2O5, and the material of the low refractive index film is selected from any one of SiO2, YbF3, and MgF2.

[0016] Compared with the prior art, the present invention can achieve the following beneficial effects: (1) The synergistic process of evaporation coating combined with ion beam oxidation is different from the traditional ion beam assisted evaporation. The ion beam is applied to the deposited initial film, which can replenish oxygen atoms through oxygen ion beam, fill oxygen vacancies in the film layer, realize full oxidation of the film layer, and reduce defect state absorption from the source. At the same time, the bombardment effect of low energy ion beam can compact the film layer, reduce surface roughness, reduce light scattering loss, significantly reduce the absorption loss of the film, and reduce the weak absorption of the film to below 10 ppm.

[0017] (2) By precisely controlling the ion beam voltage, current, incident angle and oxidation time, combined with annealing, the internal stress of the film can be effectively eliminated, micro-defects can be repaired, and problems such as grain growth and surface flatness reduction caused by high temperature heat treatment can be avoided. This ensures the high density, good optical uniformity and strong adhesion of the film, and significantly improves the laser damage threshold of the film, which can meet the high-end application requirements of high-power laser systems.

[0018] (3) The process is simple and highly controllable. It does not require complex equipment modification. It can be achieved by adding an ion source to the existing electron beam evaporation coating equipment. The production cost is moderate and it is easy to scale up production. At the same time, the film structure can be flexibly adjusted to meet the laser requirements of different wavelength ranges and has a wide range of applications.

[0019] (4) The selected high and low refractive index materials have the characteristics of low absorption and high damage threshold. Combined with ion beam oxidation and annealing treatment, the stoichiometry and microstructure of the film are further optimized, so that the film has excellent wear resistance, water resistance and environmental stability while having low absorption characteristics, thus extending its service life. Attached Figure Description

[0020] Figure 1 This is a flowchart of a method for preparing a low-absorption laser thin film according to an embodiment of the present invention.

[0021] Figure 2 This is a schematic diagram of the chamber structure of a low-absorption laser thin film fabrication device according to an embodiment of the present invention.

[0022] Figure label: 1. Base; 2. Initial thin film; 3. Ion source; 4. Electron gun. Detailed Implementation

[0023] In the following description, embodiments of the invention will be described with reference to the accompanying drawings. In the description below, the same modules are denoted by the same reference numerals. Where the same reference numerals are used, their names and functions are also the same. Therefore, their detailed description will not be repeated.

[0024] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and do not constitute a limitation thereof.

[0025] This invention provides a method for preparing a low-absorption laser thin film, specifically comprising the following steps: S1. Substrate pretreatment: Select an optical substrate and perform cleaning, drying and surface activation treatments in sequence to obtain a pretreated substrate; Specifically, the optical substrate is one of quartz, sapphire, CaF2, Ge, Si or ZnSe, and the surface roughness Ra of the substrate after pretreatment is ≤1nm, and the surface finish is better than 20 / 10 (meeting industry standards). The cleaning process employs multi-tank ultrasonic cleaning (i.e., multi-frequency segmented ultrasonic cleaning), specifically 12 segmented ultrasonic tanks. Tanks 1-3 use a frequency of 40kHz, tanks 4-8 use a frequency of 80kHz, and tanks 9-12 use a frequency of 1MHz. After slow draining in tank 13, hot air drying is performed in tank 14 to remove microscopic particles and oil stains adhering to the substrate surface. Drying is carried out using hot air at a temperature of 30-80℃ for 30-60 minutes to ensure no moisture residue remains on the substrate surface. Surface activation treatment uses low-energy argon ion beam bombardment for 5-15 minutes to remove the oxide layer and adsorbed impurities on the substrate surface, improving the adhesion between the substrate and the film layer.

[0026] S2. Vacuum Chamber Preparation: Place the pretreated substrate inside the vacuum chamber of the evaporation coating equipment and fix it on the umbrella-shaped fixture. Adjust the distance between the substrate and the evaporation source to 50~200cm. Start the vacuum pumping system to evacuate the chamber, raising the background vacuum level to 8.0×10⁻⁶. -4 ~1.0×10 -5 Pa removes residual water vapor and air impurities from the chamber, preventing the introduction of additional defects into the film layer during the deposition process.

[0027] S3. Evaporation and deposition of film: High refractive index evaporation material and low refractive index evaporation material are respectively loaded into the electron gun crucible; the electron gun is started and the electron gun voltage is adjusted to 7~10kV to heat the material in the crucible so that it vaporizes into gaseous particles. The gaseous particles are continuously deposited on the substrate surface to prepare an initial film with multiple alternating stacks. Specifically, the high refractive index material is selected from at least one of Hf, HfO2, Ta2O5, TiO2, and Nb2O5, and the low refractive index material is selected from any one of SiO2, YbF3, and MgF2; the evaporation rate of the high refractive index material is controlled at 0.01~0.5 nm / s, and the evaporation rate of the low refractive index material is controlled at 0.1~2.0 nm / s. When the high refractive index material is elemental hafnium, the evaporation rate is limited to 0.01~0.1 nm / s, and high-purity oxygen with a purity ≥99.99% is simultaneously introduced into the chamber at a flow rate of 20~200 sccm to achieve the initial oxidation of the metal hafnium during the deposition stage. When high refractive index materials are directly selected from oxide powders (HfO2, Ta2O5, TiO2, Nb2O5), the evaporation rate is controlled at 0.1~0.5 nm / s, and no additional oxygen is required during the deposition process.

[0028] Specifically, the substrate temperature is controlled at 30~300℃ throughout the deposition process, and the constant temperature holding time is 30~300min; The initial thin film stack structure is denoted as SUB / (k1Hk2L). n / A、SUB / (k1Hk2Hk3L) n / A or SUB / (k1Hk2Lk3Hk4L) n / A, where SUB represents the pretreated substrate, A represents the air side, H represents the high-refractive-index film, L represents the low-refractive-index film, k1, k2, k3, and k4 are the optical thickness coefficients of the corresponding film layers at one-quarter of the reference wavelength, and n is the number of alternating stacking cycles of high and low refractive-index films; n ranges from 1 to 30, and the process-adapted reference laser wavelength range is 1000 to 1100 nm; the initial thin film can be prepared using any one of the following three standardized film system structures: (1) SUB / k1Hk2L / A, k1 takes the value of 0.20~0.50, k2 takes the value of 0.85~1.95, preferably k1=0.35, k2=1.32; (2) SUB / k1Hk2Hk3L / A, k1 takes the value 0~4, k2 takes the value 0.20~0.50, k3 takes the value 0.85~1.95, preferably k1=1.32, k2=0.35, k3=1.32; (3) SUB / k1Hk2Lk3Hk4L / A, k1 takes the value of 0.10~0.24, k2 takes the value of 1.0~2.4, k3 takes the value of 0.18~0.32, k4 takes the value of 0.8~2.0, preferably k1=0.17, k2=1.7, k3=0.25, k4=1.4.

[0029] S4. Post-ion beam oxidation treatment: After the initial thin film deposition is completed, the existing vacuum level of the vacuum chamber is kept constant, and high-purity oxygen is introduced into the chamber. The ion source in the chamber is activated to generate a directional oxygen ion beam to bombard the initial thin film on the substrate surface for oxidation treatment. The oxidation treatment lasts for 5 to 30 minutes to fill the oxygen vacancies in the film layer and compact the film layer to obtain a multilayer dielectric film. During the oxidation stage, the substrate temperature is maintained at 30 to 300°C. Specifically, high-purity oxygen is oxygen with a purity of ≥99.99%, and the oxygen flow rate is 20~150 sccm.

[0030] See Figure 2 The chamber structure of the low-absorption laser thin film preparation equipment is shown. A rotating tooling umbrella frame is set in the chamber, and the substrate 1 is fixed on the umbrella frame. An ion source 3 is arranged at the center of the bottom of the chamber, and electron guns 4 are symmetrically arranged on both sides. The electron guns 4 are used to evaporate high and low refractive index coating materials to deposit an initial thin film 2 on the surface of the substrate 1. After the initial thin film 2 is deposited, the ion source 3 outputs a directional oxygen ion beam to bombard the already formed thin film on the surface of the substrate 1, and then performs an oxidation treatment. Unlike traditional synchronous ion beam assisted deposition, the electron gun deposition process and the ion source oxidation process are executed in a time-separated manner. The oxygen ion beam can replenish oxygen atoms into the film layer, fully filling the oxygen vacancies generated during the deposition stage, so that the film material reaches the standard stoichiometry, eliminating laser absorption loss caused by defect states from the root. At the same time, the continuous bombardment of low-energy oxygen ions can compact the loose film layer, improve the overall density of the film layer, reduce the surface roughness of the film, reduce light scattering loss, and the bombardment is applied to the formed film without damaging the overall structure of the film layer, thus achieving dual optimization of low absorption and film structure integrity.

[0031] S5. In-situ annealing and cooling: Sequentially shut off the ion source and the high-purity oxygen supply line to maintain a high vacuum environment in the vacuum chamber. Heat the substrate to 300~500℃ for vacuum annealing, and hold for 10~40 minutes. After annealing, shut off the vacuum pumping system and slowly introduce high-purity nitrogen into the chamber until the chamber returns to normal pressure. Cool down the substrate to room temperature, open the vacuum chamber and remove the sample to finally obtain the target low-absorption laser thin film. Specifically, the cooling rate is 5~10℃ / min; The annealing process eliminates deposition stress inside the film and repairs micro-lattice defects, further improving the optical uniformity and long-term stability of the thin film.

[0032] The low-absorption laser thin film obtained by the above preparation method comprises a multilayer dielectric film with an optical substrate and a substrate surface stacked alternately, and the total thickness of the film layers is 0.8~5μm. The film has an optical transmittance of ≥99.9% in the 1000~1100nm near-infrared laser band and a weak absorption loss of ≤10ppm. It has the characteristics of high density, strong film-substrate adhesion, excellent resistance to laser damage threshold, water resistance, wear resistance and good environmental stability, and is suitable for use in various high-power laser systems and ultra-high precision laser measurement equipment.

[0033] Example 1: This embodiment provides a method for preparing a low-absorption laser thin film, which specifically includes the following steps: S1. Substrate pretreatment: Quartz substrate is selected and multi-frequency ultrasonic cleaning is used; hot air drying temperature is 55℃ and drying time is 45min; surface activation treatment is performed by low-energy argon ion beam bombardment for 10min; after pretreatment, the substrate Ra < 1nm and the surface smoothness is better than 20 / 10. S2. Vacuum Chamber Preparation: Adjust the distance between the substrate and the evaporation source to 120cm, and evacuate the chamber to a background vacuum of 1×10⁻⁶. -4 Pa; S3. Evaporation deposition of film: High refractive index material hafnium (Hf) and low refractive index material SiO2 are selected; electron gun voltage is 8.5 kV; hafnium evaporation rate is 0.05 nm / s, high-purity oxygen is introduced simultaneously at a flow rate of 110 sccm; SiO2 evaporation rate is 1.0 nm / s; the substrate temperature is controlled at 160℃ throughout the deposition process, and the holding time is 160 min; film system scheme (1) is selected, and the stacking period is n=1; S4. Post-ion beam oxidation treatment: High-purity oxygen was introduced at a flow rate of 85 sccm, the oxygen ion beam bombardment time was 18 min, and the substrate was kept at a constant temperature of 160℃. S5. In-situ annealing and cooling: Annealing temperature 400℃, holding temperature for 25 min; nitrogen cooling rate 7.5℃ / min, cool to room temperature and take out the finished product. The total thickness of the finished film is 2.6μm, the optical transmittance in the 1000~1100nm near-infrared laser band is ≥99.9%, and the film's weak absorption loss is <10ppm.

[0034] Example 2: This embodiment provides a method for preparing a low-absorption laser thin film, which specifically includes the following steps: S1. Substrate pretreatment: Sapphire substrate was selected and multi-frequency ultrasonic cleaning was used; hot air drying temperature was 50℃ and drying time was 40min; surface activation treatment was performed by low-energy argon ion beam bombardment for 8min; after pretreatment, the substrate Ra < 1nm and the surface finish was better than 20 / 10. S2. Vacuum Chamber Preparation: Adjust the distance between the substrate and the evaporation source to 100cm, and evacuate the chamber to a background vacuum of 5×10⁻⁶. -5 Pa; S3. Evaporation deposition of film: High refractive index material hafnium oxide powder HfO2 and low refractive index material SiO2 are selected; electron gun voltage 8kV; HfO2 evaporation rate 0.3nm / s, no additional oxygen is required during the deposition process; SiO2 evaporation rate 1.1nm / s; the substrate temperature is controlled at 140℃ throughout the deposition process, and the holding time is 150min; film system scheme (3) is selected, and the stacking period n=1; S4. Post-ion beam oxidation treatment: High-purity oxygen was introduced at a flow rate of 70 sccm, and the oxygen ion beam bombardment time was 15 min, with the substrate kept at a constant temperature of 140℃. S5. In-situ annealing and cooling: Annealing temperature 380℃, holding temperature for 20min; nitrogen cooling rate 6℃ / min, cool to room temperature and take out the finished product. The total thickness of the finished film is 2.2μm, the optical transmittance in the 1000~1100nm near-infrared laser band is ≥99.9%, and the film's weak absorption loss is <10ppm.

[0035] It should be understood that the various forms of processes shown above can be used to reorder, add, or delete steps. For example, the steps described in this invention disclosure can be executed in parallel, sequentially, or in different orders, as long as the desired result of the technical solution disclosed in this invention can be achieved, and this is not limited herein.

[0036] The specific embodiments described above do not constitute a limitation on the scope of protection of this invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A method for preparing a low-absorption laser thin film, characterized in that: Includes the following steps: S1. The substrate is sequentially cleaned, dried, and surface activated to obtain a pretreated substrate; S2. Place the pretreated substrate inside the vacuum chamber of the evaporation coating equipment, adjust the distance between the substrate and the evaporation source to 50~200cm, and evacuate the vacuum. S3. High-refractive-index materials and low-refractive-index materials are alternately vapor-deposited to form a multilayered, stacked initial thin film on the substrate surface; S4. After the initial thin film deposition is completed, the vacuum level is kept constant, high-purity oxygen is introduced, and the initial thin film is bombarded with oxygen ion beam to oxidize it, fill the oxygen vacancies in the film layer and compact the film layer to obtain a multilayer dielectric film. S5. Vacuum annealing is performed at 300~500℃, followed by cooling to obtain a low-absorption laser thin film.

2. The method for preparing a low-absorption laser thin film according to claim 1, characterized in that: In step S1, the drying method is drying, the drying temperature is 30~80℃, and the drying time is 30~60min; the surface activation treatment is low-energy argon ion beam bombardment, and the bombardment time is 5~15min.

3. The method for preparing a low-absorption laser thin film according to claim 2, characterized in that: In step S1, the cleaning is multi-frequency segmented ultrasonic cleaning, which includes 12 segmented ultrasonic tanks. Tanks 1-3 use a frequency of 40kHz, tanks 4-8 use a frequency of 80kHz, and tanks 9-12 use a frequency of 1MHz. The substrate is one of quartz, sapphire, CaF2, Ge, Si, or ZnSe.

4. The method for preparing a low-absorption laser thin film according to claim 1, characterized in that: In step S3, the high refractive index material is selected from at least one of Hf, HfO2, Ta2O5, TiO2, and Nb2O5, and the low refractive index material is selected from any one of SiO2, YbF3, and MgF2; the substrate temperature is controlled at 30~300℃ throughout the deposition process, and the constant temperature holding time is 30~300min.

5. The method for preparing a low-absorption laser thin film according to claim 4, characterized in that: The evaporation rate of the high refractive index material is controlled at 0.01~0.5 nm / s, and the evaporation rate of the low refractive index material is controlled at 0.1~2.0 nm / s.

6. The method for preparing a low-absorption laser thin film according to claim 4, characterized in that: The initial thin film has a stacked structure of SUB / (k1Hk2L). n / A、SUB / (k1Hk2Hk3L) n / A or SUB / (k1Hk2Lk3Hk4L) n One of / A, where SUB represents the pretreated substrate, A represents the air side, H represents the high refractive index film, L represents the low refractive index film, k1, k2, k3, and k4 are the optical thickness coefficients of the corresponding film at one-quarter of the reference wavelength, and n is the number of alternating stacking cycles of high and low refractive index films; n ranges from 1 to 30.

7. The method for preparing a low-absorption laser thin film according to claim 1, characterized in that: In step S4, the high-purity oxygen is oxygen with a purity of ≥99.99%, and the oxygen flow rate is 20~150 sccm; the oxidation treatment time is 5~30 min; and the substrate temperature is maintained at 30~300℃ during the oxidation stage.

8. The method for preparing a low-absorption laser thin film according to claim 1, characterized in that: In step S5, the holding time for vacuum annealing is 10~40 min; the cooling rate is 5~10℃ / min.

9. A low-absorption laser thin film, prepared by the method for preparing a low-absorption laser thin film according to claim 1, characterized in that: It includes a substrate and a multilayer dielectric film; the multilayer dielectric film consists of alternating layers of high refractive index and low refractive index, with a total thickness of 0.8~5μm; The material of the high refractive index film is selected from at least one of Hf, HfO2, Ta2O5, TiO2, and Nb2O5, and the material of the low refractive index film is selected from any one of SiO2, YbF3, and MgF2.

Images