Method for determining damage threshold of high-power laser to optical thin film

Abstract

The invention provides a method for determining a damage threshold of high-power laser to an optical thin film. The method comprises the following steps: preparing a high-reflection film; testing the high-reflective film through an ultraviolet laser damage experiment light path to obtain a three-dimensional damage morphology distribution diagram; and performing linear fitting on the test result to obtain the damage threshold of the high-reflective film. According to the invention, the influence of different protective layers on improvement of the laser damage threshold of the film is studied.

Description

Technical Field

[0001] The invention belongs to the field of lasers, and in particular relates to a method for determining a damage threshold of an optical film caused by a high-power laser. Background Art

[0002] The photon energy in the ultraviolet band is relatively high, and the damage problem is more prominent than that in the visible light and near-infrared bands. The damage threshold of typical ultraviolet optical films limits the advancement of ultraviolet optical devices towards higher power; therefore, studying the damage problem of typical ultraviolet optical films and exploring their mechanism of action and the laws of occurrence and development not only has important scientific significance but also has important application value in the fields of national defense security and military-civilian integration. Summary of the invention

[0003] The purpose of the present invention is to provide a method for determining the damage threshold of an optical film caused by a high-power laser.

[0004] The technical solution of the present invention is as follows:

[0005] A method for determining a damage threshold of an optical film caused by a high-power laser, the method comprising:

[0006] Preparation of high reflective film;

[0007] The high-reflective film is tested by using an ultraviolet laser damage experimental optical path to obtain a three-dimensional damage morphology distribution map;

[0008] The test results are linearly fitted to obtain the damage threshold of the high-reflection film.

[0009] Furthermore, the high-reflection film is formed by alternately coating two materials with high and low refractive indexes, substrate / (HL) n H / air λ / 4 regular film system composition, H refers to the high refractive index material with λ / 4 optical thickness, and L refers to the low refractive index material with λ / 4 optical thickness.

[0010] Furthermore, the methods for testing the high-reflective film include 1-on-1 and S-on-1 methods.

[0011] Furthermore, the 1-on-1 method specifically means that when each irradiation point on the film is irradiated by only one laser pulse, the film is moved to the next unirradiated point, and then the laser energy density is increased to continue irradiation, regardless of whether the irradiation point is damaged.

[0012] Furthermore, the S-on-1 is specifically: irradiating the same point of the film with multi-pulse lasers of the same laser energy density, and then moving to the next unirradiated point for multi-pulse irradiation. The arrangement of each point in the experiment is the same as 1-on-1, and S is the number of pulses radiated at each point.

[0013] Furthermore, the three-dimensional damage morphology distribution map is displayed by a surface profilometer and an atomic force microscope.

[0014] Furthermore, the damage threshold adopts an evaluation method of zero-probability damage.

[0015] The present invention also provides an electronic device, including a memory and a processor, wherein the memory is used to store a computer program, and the processor runs the computer program to enable the electronic device to execute the above-mentioned method for determining the damage threshold of an optical film caused by a high-power laser.

[0016] The present invention also provides a computer-readable storage medium storing a computer program, wherein the computer program is executed by a processor to implement the above-mentioned method for determining the damage threshold of an optical film caused by a high-power laser.

[0017] Compared with the prior art, the present invention has the following advantages:

[0018] The present invention prepares a variety of ultraviolet laser high-reflection film elements that meet the experimental requirements through film system design, material screening and optical performance testing and analysis of DUV / VUV (248nm, 193nm) band thin film elements, among which the 193nm band ultraviolet high-reflection mirror has a reflectivity greater than 95%, and the 248nm band full-reflection mirror has a reflectivity greater than 99%. An experimental platform for ultraviolet optical thin film elements damaged by 193nm and 248nm ultraviolet lasers was built, and experimental research on damage of corresponding ultraviolet high-reflection films was carried out. The damage morphology and depth information of the samples were observed and analyzed using differential interference microscopy and step profilers, and the laser damage thresholds of ultraviolet thin film elements with different film systems were measured, and the laser damage mechanism of ultraviolet optical thin films was analyzed and discussed. The damage characteristics of 193nm high-reflection films coated with protective layers of different materials were compared and analyzed, and the influence of different protective layers on improving the laser damage threshold of thin films was studied. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The accompanying drawings generally illustrate various embodiments by way of example and not limitation, and together with the description and claims, serve to illustrate the embodiments of the invention. Where appropriate, the same reference numerals are used throughout the drawings to refer to the same or similar parts. Such embodiments are illustrative and are not intended to be exhaustive or exclusive embodiments of the present apparatus or method.

[0020] Figure 1 The schematic diagram of the structure of the 248nm HfO2 / SiO2 high-reflective film system with SiO2 protective layer is shown;

[0021] Figure 2 The schematic diagram of the structure of the 193nm Al2O3 / MgF2 high reflective film system of the present invention is shown;

[0022] Figure 3 The schematic diagram of the optical path of the ultraviolet laser damage experiment of the present invention is shown. DETAILED DESCRIPTION

[0023] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0024] The embodiment of the present invention provides a method for determining the damage threshold of an optical film caused by a high-power laser, which specifically includes:

[0025] 248nm high reflection film preparation

[0026] In the DUV248 nm band, considering the bandgap width, extinction coefficient, melting point and other factors, five coating materials, SiO2, MgF2, HfO2, Al2O3 and LaF3, were finally selected. The high-reflective film is made of two high and low refractive index materials alternately coated, consisting of substrate / (HL) n H / airλ / 4 regular film system composition, H refers to high refractive index material with λ / 4 optical thickness, L refers to low refractive index material with λ / 4 optical thickness. Using the five selected coating materials, three high-reflective films for 45° reflection, namely oxide film system (HfO2 / SiO2), fluoride film system (LaF3 / MgF2), oxide / fluoride film system (Al2O3 / MgF2) were prepared respectively; in addition, HfO2 / SiO2 high-reflective film with SiO2 protective layer was also prepared in the 248nm band.

[0027] 248nm HfO2 / SiO2 high reflective film coated with SiO2 protective layer, the material properties of which are shown in Table 1, the film structure is substrate / (HL) 11 H2L / air, e.g. Figure 1 As shown. The SiO2 protective layer high reflective film has 25 layers, including 1 SiO2 protective film, 12 HfO2 layers, and 11 SiO2 layers. The transmittance of the film at 248nm is 0.4%, and the reflectivity is 99.6%.

[0028] Table 1

[0029]

[0030] There are three kinds of high-reflective films, namely, oxide film system (HfO2 / SiO2) without protective layer, fluoride film system (LaF3 / MgF2), and oxide / fluoride film system (Al2O3 / MgF2). The transmittance of the three high-reflective films at 248nm is less than 1%, and the reflectivity is greater than 99%. Among them, the transmittance of the HfO2 / SiO2 high-reflective film without SiO2 protective layer is 0.2%. It can be seen that the protective layer has little effect on the reflectivity results.

[0031] 193nm high reflection film preparation

[0032] In the VUV193nm band, fewer coating materials are used, and only a small amount of oxide and fluoride materials can be used. From the absorption coefficient point of view, below 230nm, only Al2O3 and SiO2 meet the requirements of oxides, and some fluorides meet the absorption requirements of DUV / VUV bands, such as high refractive index LaF3, GdF3, YF3, and low refractive index materials MgF2 and AlF3.

[0033] According to the refractive index, bandgap and other characteristics of the thin film material, an oxide / fluoride film system (Al2O3 / MgF2) high-reflection film and a 193nm Al2O3 / MgF2 high-reflection film coated with a SiO2 protective layer and a MgF2 protective layer were prepared. The basic properties of the coating material of the 193nm Al2O3 / MgF2 high-reflection film are shown in Table 2. The film system structure is substrate / (HL) 15 H / air λ / 4 regular film, where H represents Al2O3 and L represents MgF2, such as Figure 2 As shown in the figure, the 193nm Al2O3 / MgF2 high reflective film has 31 layers, including 16 layers of Al2O3 and 15 layers of MgF2.

[0034] Table 2

[0035]

[0036] The transmission spectrum measured under the condition of 45° incident angle shows that the transmittance of the film at 193nm is 4.6% and the reflectivity is 95.4%.

[0037] At the same time, the project team also coordinated with the Shanghai Institute of Optics and Fine Mechanics to purchase the 193nm ultraviolet high-reflection mirror in the "Shenguang" system, whose surface is coated with LaF3 / AlF3 high-reflection film with SiO2 protective layer. The film system structure is substrate / (HL) n H2L1 / air, LaF3 as high refractive index material, AlF3 as low refractive index material. The transmission spectrum of the film was measured under the incident condition of 45°, and the transmittance of the film at 193nm was 6.5% and the reflectivity was 93.5%.

[0038] Schematic diagram of the optical path of the UV laser damage experiment Figure 3 As shown; among them, the laser pulse width output by the 193nmArF ultraviolet laser is about 25ns, and the maximum single pulse energy is 400mJ; the laser pulse width output by the 248nmKrF ultraviolet laser is about 13ns, and the maximum single pulse energy is 600mJ. The energy of the laser can be adjusted by adjusting the voltage.

[0039] In the experiment, the 1-on-1 and S-on-1 methods were used to test the damage threshold of the film. 1-on-1 means that each irradiation point on the film is irradiated by only one laser pulse. Regardless of whether the irradiation point is damaged, the film is moved to the next unirradiated point, and then the laser energy density is increased to continue irradiation. S-on-1 is multi-pulse damage, which is to irradiate the same point of the film with multiple pulses of laser energy density, and then move to the next unirradiated point for multi-pulse irradiation. The arrangement of each point in the experiment is the same as 1-on-1, and S is the number of pulses irradiated at each point. The damage threshold is evaluated using the zero-probability damage method.

[0040] According to the ISO-21254 international damage judgment standard, Nomarski differential interference optical microscope and scanning electron microscope were used to observe the damage morphology of thin film materials, and surface profilometer (i.e. step profiler) and atomic force microscope were used to display the morphological information of the damaged cross section (in the depth direction) to obtain a three-dimensional damage morphology distribution map.

[0041] The damage threshold of 193nm oxide / fluoride film system Al2O3 / MgF2 high reflection film was obtained by linear fitting of experimental data, and the damage threshold of oxide / fluoride film system (Al2O3 / MgF2) high reflection film, Al2O3 / MgF2 high reflection film coated with SiO2 protective layer and Al2O3 / MgF2 high reflection film coated with MgF2 protective layer was 0.828J / cm 2 , 0.854J / cm 2 , 1.325J / cm 2 ,

[0042] Laser damage experiments were conducted on 193nm LaF3 / AlF3 high-reflective film. The obtained experimental data were linearly fitted, and the damage threshold of LaF3 / AlF3 high-reflective film with SiO2 protective layer was obtained to be 1.817J / cm 2 It can be seen that in the 193nm VUV band, the damage threshold of the wide-bandgap fluoride film system is higher than that of the oxide / fluoride film system, and the film using the fluoride film system will have a stronger ability to resist laser damage.

[0043] Damage morphology of 193nm oxide / fluoride film system Al2O3 / MgF2 high reflective film

[0044] When the laser incident energy is slightly higher than the damage threshold, a tortoise-shell crack appears in the center of the sample irradiation area, accompanied by two small pit-like damages, which are formed by the melting of the film defect; with the increase of laser energy density, the damage shows obvious melting characteristics, the damaged area of ​​the sample increases, and the damage depth continues to deepen. In the periphery of the damaged spot, it can be observed that multiple layers of light and dark colors appear alternately, reflecting that the damage has reached the multi-layer interior of the high-reflective film; with the increase of laser energy density, the temperature at the defect increases, and the absorbed heat gradually diffuses to the surroundings, and the film material at the defect explodes and falls off, forming a large area of ​​broken spots. With the further increase of energy density, multiple point-like damages appear again in the melted area, forming a larger melted area, and the damage further develops to the lower layer. In the area where cracks appear at the edge of the laser spot, the damage begins to develop from cracks to fragmentation, and the degree of damage to the film is further deepened.

[0045] The single pulse laser energy density is 0.95 J / cm 2 When multiple pulses are applied, as the number of pulses increases, the sample damages the central area, the point damage caused by defect absorption increases, and the tortoise shell cracks tend to develop into fragmentation. Under 10 laser pulses, the film damage area expands, cracks appear at the edge of the spot, and multiple broken spots of different sizes appear in the center, and the damage will develop to the next layer.

[0046] It can be seen that with the increase of laser energy, the damage morphology of the 193nm Al2O3 / MgF2 high-reflective film gradually develops from the tortoise-shell cracks and pitting damage at the defects caused by thermal stress under the initial characteristics to large-area broken spots, showing a layered peeling damage structure. At the same time, obvious plasma flashes were observed in the experiment, which can also reflect the influence of high temperature and plasma shock waves on the area in the damage morphology.

[0047] Therefore, experimental studies have shown that the damage mechanism of 193nm ultraviolet laser on Al2O3 / MgF2 high-reflective film is mainly multi-photon ionization damage mechanism, thermal-mechanical coupling damage mechanism and thermal damage mechanism caused by defect absorption.

[0048] Damage threshold and damage morphology of 248nm oxide film HfO2 / SiO2 high reflective film coated with SiO2 protective layer

[0049] Laser damage experiments were conducted on 248nm oxide film HfO2 / SiO2 high reflective film. The experimental data were linearly fitted and the damage threshold of HfO2 / SiO2 high reflective film with SiO2 protective layer was obtained to be 3.086J / cm 2 ,

[0050] The energy density of a single pulse is 3.3 J / cm 2The damage morphology caused by 248nm laser on HfO2 / SiO2 high-reflective film coated with SiO2 protective layer. When the laser energy density is slightly higher than the damage threshold, an obvious rectangular damage spot appears on the surface of the sample, and a periodic stripe structure appears in the center of the irradiated area with higher energy. A large number of cracks appear in the upper part of the irradiated area. These cracks are formed after the molten material cools down. At the bottom of the irradiated area with lower energy, there is a point-like ablation damage caused by defect absorption. It can be seen that the damage to the film starts from defect-induced damage.

[0051] The energy density of a single pulse is 5 J / cm 2 The damage morphology and depth information of the 248nm laser on the HfO2 / SiO2 high-reflective film coated with SiO2 protective layer. The central area of ​​the damaged spot shows a step-like melting damage, and the outer film in a large area outside the spot is ablated and falls off; from the depth analysis, the central area of ​​the spot shows a mottled state with ups and downs, and the burr-like elevations are formed by the accumulation of debris formed by the splashing, cooling and solidification of the molten material when multiple film layers are cracked. The deepest bottom of the damage spot is 338nm, reaching the 10th layer of SiO2 film.

[0052] The single pulse energy density is 8.93 J / cm 2 The damage morphology and depth information caused by 248nm laser on HfO2 / SiO2 high-reflective film coated with SiO2 protective layer. With the increase of energy density, the damage spot shows a development trend from the surface to the inner layer, from a small area to a large area. The traces of melting and shedding of multiple layers of thin film are clearly observed on the periphery of the light spot; from the depth analysis, it can be seen that the bottom of the broken spot presents an irregular arc shape, which further confirms the inference about melting. The deepest damage depth of the film is 680nm, and the damage reaches the 19th layer, which is the HfO2 film layer. According to the damage morphology depth analysis diagram, it can be found that the damaged area of ​​the film is concave downward as a whole, and the layered burrs at the bottom are still formed by the cooling and solidification of the film melt.

[0053] It can be seen that under different energy densities, a spiral periodic stripe structure appears.

[0054] After 20 pulses, new ablation damage began to appear on the periodic stripe structure of the substrate, and radial cracks began to appear in the irradiated area. The deepest damage depth of the film reached 2946nm, and 25 layers of the film were completely broken down. At the same time, the substrate material melted and showed a periodic stripe structure.

[0055] Deep ultraviolet optical films play an important role in excimer lasers and deep ultraviolet lithography machines. In view of the requirements for optical properties and high damage threshold of films in practical applications, combined with the specific conditions of the laboratory, a series of work has been carried out, including damage threshold testing of deep ultraviolet high-reflection films, damage morphology observation, construction of a practical deep ultraviolet high-reflection film laser-induced damage model, and analysis of deep ultraviolet high-reflection film damage mechanism based on experimental results. The main work contents are as follows:

[0056] 248nmHfO2 / SiO2 and 193nmAl2O3 / MgF2 high-reflective films were prepared respectively, and the damage characteristics of the two films were studied.

[0057] In the 193nm VUV band, the damage threshold of the wide-bandgap fluoride film is 1.817J / cm 2 , which is higher than that of oxide / fluoride film system. The film using fluoride film system will have stronger anti-laser damage ability. And the damage mechanism of 193nm ultraviolet laser to Al2O3 / MgF2 high reflective film is mainly multi-photon ionization damage mechanism, thermal-mechanical coupling damage mechanism and thermal damage mechanism caused by defect absorption.

[0058] Laser damage experiments were conducted on 248nm oxide film HfO2 / SiO2 high reflective film. The experimental data were linearly fitted and the damage threshold of HfO2 / SiO2 high reflective film with SiO2 protective layer was obtained to be 3.086J / cm 2 The damage of the film all starts from defect-induced damage. Under different energy densities, a spiral periodic stripe structure appears.

[0059] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A method for determining the damage threshold of an optical film caused by a high-power laser, characterized in that: The method comprises: Preparation of high reflective film; The high-reflective film is tested by using an ultraviolet laser damage experimental optical path to obtain a three-dimensional damage morphology distribution map; The test results are linearly fitted to obtain the damage threshold of the high-reflection film.

2. The method for determining the damage threshold of an optical film caused by a high-power laser according to claim 1, characterized in that: The high-reflection film is made of two high and low refractive index materials alternately plated, consisting of substrate / (HL) n H / air λ / 4 regular film system composition, H refers to the high refractive index material with λ / 4 optical thickness, and L refers to the low refractive index material with λ / 4 optical thickness.

3. The method for determining the damage threshold of an optical film caused by a high-power laser according to claim 1, characterized in that: The methods for testing the high-reflection film are 1-on-1 and S-on-1.

4. The method for determining the damage threshold of an optical film caused by a high-power laser according to claim 3, characterized in that: The 1-on-1 method specifically means that when each irradiation point on the film is irradiated by only one laser pulse, the film is moved to the next unirradiated point regardless of whether the irradiation point is damaged, and then the laser energy density is increased to continue irradiation.

5. The method for determining the damage threshold of an optical film caused by a high-power laser according to claim 3, characterized in that: The S-on-1 method is as follows: irradiating the same point of the film with a multi-pulse laser of the same laser energy density, and then moving to the next unirradiated point for multi-pulse irradiation. The arrangement of each point in the experiment is the same as 1-on-1, and S is the number of pulses radiated at each point.

6. The method for determining the damage threshold of an optical film caused by a high-power laser according to claim 3, characterized in that: The three-dimensional damage morphology distribution map is displayed by a surface profilometer and an atomic force microscope.

7. The method for determining the damage threshold of an optical film caused by a high-power laser according to claim 3, characterized in that: The damage threshold is evaluated by a zero-probability damage method.

8. An electronic device, characterized in that: The electronic device comprises a memory and a processor, wherein the memory is used to store a computer program, and the processor runs the computer program to enable the electronic device to execute the method for determining the damage threshold of an optical film caused by a high-power laser as described in claims 1-7.

9. A computer-readable storage medium, characterized in that: The computer program is stored therein, and when the computer program is executed by a processor, the method for determining the damage threshold of an optical film caused by a high-power laser as described in claims 1 to 7 is implemented.

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