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Abstract
This study reports that SiO2 was selected to fabricate broadband anti-reflection (AR) films on fused silica substrates by using glancing angle deposition and substrate rotation. Through accurate control of the graded index of the SiO2 layer, the average residual reflectance of the graded broadband AR film can achieve an average value of 0.59% across a spectral range of 400-1800nm. By comparing the performance, the broadband anti-reflection film with substrate speed has higher stability compared with the broadband anti-reflection film without substrate speed and a higher damage threshold than the traditional anti-reflection film.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In a variety of optical systems, such as the laser fusion apparatus at the National Ignition Facility (NIF) of Lawrence Livermore National Laboratories (LLNL) [1] and the Shenguang-II Laser Facility [2–4], any external reflections or scattered light from the lasing windows or system lenses, gratings, and debris shields add spatial noise to the beam, limiting the ability of the system to focus enough power. In these optical systems, a high-performance AR film is required to reduce reflections from both the detector and optics in a sensor system. There are two approaches to reduce the reflection across the interface of two different media: one is based on the interference of a multilayer stack, and the other is based on the graded index layer between the incident layer and the output layer [5]. Traditional broadband anti-reflection films are prepared using high- and low refractive index film stacks. Although the traditional method with multi-layer dielectric can reduce reflection, most of this type of film suffers from limited bandwidths as well as very restricted laser-induced damage resistance performance. In the case of gradient index optics, the reflectivity is minimized by the gradual change in the refractive index from ambient to substrate. These types of films have structures that are sometimes referred to as biomimetic “moth-eye” structures and have high-transmission spectral bandwidths [6–12].
In our previous research, broadband sculptured thin films were prepared by the glancing angle deposition (GLAD) method, employing silicon dioxide material [13]. Any ideal refractive index between the refractive index range of air and the material can be realized by controlling the porosity. However, for practical reasons, especially in the glancing angle deposition method, it is impossible to avoid inhomogeneous film thickness [14]. The film thickness at the end far from the evaporation source was thin, and the film thickness at the end near the evaporation source was thick. This reduces the practical applicability of films prepared by glancing angle deposition.
In this study, a high-performance SiO2 gradient refractive index broadband anti-reflection film with a wide band, high damage threshold, and uniform thickness was prepared using a rotating device and electron beam evaporation technology combined with GLAD technology. Its optical properties, such as residual reflectivity, thickness uniformity, and laser damage threshold, were tested, analyzed, and compared with the variable refractive index anti-reflection film without rotating speed and the traditional anti-reflection film.
2. Theory
According to the Fresnel function [15], the reflection from the incidence to the interface of the thin film is determined by the refractive index deviation. A medium thin film with a continuous gradient refractive index can reduce the refractive index mismatch of the interface and achieve high residual reflectivity.
The key to GLAD technology is to tilt the substrate at a certain angle, control the angle between the incidence of air flow and the normal direction of the substrate surface, and supplement with different substrate rotation modes. In this way, we can obtain films with anisotropic structures that are different from the traditional structure, so that the properties of thin films can fundamentally change. The shadow effect is one of the most important reasons why the GLAD film structure is different from the general film structure, as shown in Fig. 1.
Fig. 1. Diagram of shadowing effect with glancing angle deposition film (a) in the initial stage of deposition (b) in the process of deposition.
As shown in Fig. 1(a), when the evaporation beam is incident at a glancing angle with the growth of the film, the atomic clusters at the growth point block the growth of the film at similar positions. Glancing angle deposition is generally performed at room temperature. The diffusion and migration rates of the deposited atoms were very low, and atoms could not diffuse freely in the film. Thus, there will be no atoms in the blocked areas to deposit and form a cavity structure. As shown in Fig. 1(b), the unobstructed area forms an inclined columnar structure because it can receive more deposited atoms. Generally, the inclined cylindrical structure forms a structure containing a certain amount of holes along the incident direction of the evaporation beam. Due to the existence of holes, the density of the film is less than that of the film with vertical incidence. Moreover, due to the inclined deposition, the film thickness changes along the beam evaporation direction, and the film thickness is uneven in the inclined direction. Uneven film thickness will lead to uneven optical transmittance and reflectivity, which will affect the beam quality of the optical system. Such a film, regardless of its local performance, can not be applied in practice.
3. Experiment
3.1 Sample preparation
We deposited three types of thin films on a fused silica substrate by electron beam evaporation deposition.
The first type of films are SiO2 films with different deposition angles and different substrate rotation rates. The deposition angles were 0°, 30°, 60°, and 85°with a rotation rate of 0 r/min, 24 r/min, 80 r/min, 160 r/min, and 320 r/min, respectively. The second type of films are variable refractive index film systems with discontinuous changes in the refractive index and thickness. This film system was designed using the TFCalc software. During the deposition process, two different processes were carried out: setting the substrate rotation rate to 0 r/min or variable speed combinations. The vacuum degree of the film chamber was maintained at 1×10−2 Pa. The deposition was performed at room temperature, and the average deposition rate was approximately 0.3 nm/s. The schematic of the deposition method is illustrated in Fig. 2.
The third type of films are broadband anti-reflective films prepared using traditional multilayer dielectric films. The initial membrane system of 0.45(0.5H L 0.5H)10 is used to design broadband anti-reflective film by Macleod software, where H represents HfO2 material and L represents SiO2 material.
3.2 Uniformity, spectra and appearance test
The uniformity of the film thickness was tested using a Bruker Dektak XT step meter (probe surface profiler). The surface morphology and surface roughness of the films were observed using a Dimension-3100 atomic force microscope (AFM). The transmission and reflection spectra were measured using a LAMBDA-1050 spectrophotometer, and the measurement accuracy of the spectrophotometer was ±0.08%.
3.3 Laser-induced damage test
A schematic of the laser-induced damage test (LIDT) apparatus is shown in Fig. 3. Laser damage experiments were carried out using an 8.8-ns pulse from a Nd:YAG laser with a wavelength of 1064 nm and a near Littrow angle of 23° incidence. A half-wave plate and a polarizer were used to constitute the energy attenuation system, and another half-wave plate was used to vary the polarization state of the incident pulse from TM polarization to TE polarization. The effective area of the spot on the sample was 0.74 mm2. A He-Ne laser was used to adjust and calibrate the test beam path. Online plasma flash detection was used to determine whether the radiation sites were damaged. The LIDT was defined as the maximum fluence at which no damage occurred in the test area. In our study, the LIDTs were given in the beam normal.
4. Results and discussion
4.1 Refractive index analysis
SiO2 films were prepared with deposition angles of 0°, 30°, 60°, and 85°. The rotation rate of the substrate was set at 24 r/min, 80 r/min, 160 r/min, and 320 r/min. The transmittance curves of the SiO2 films are shown in Fig. 4. The refractive index of the film at different deposition angles and rotational speeds was fitted using Macleod software. After reviewing the previous research [11] and comparing with the refractive index (590 nm) of SiO2 films at different deposition angles and different rotational speeds, the results were recorded and are shown in Fig. 5. The refractive index of the film gradually decreases with an increase in the deposition angle under the same rotational speed and increases significantly with the increase in substrate rotation speed when the deposition angle is the same. The change in the refractive index is related to the variable density of the film, which is caused by the different deposition angles and rotation rates.
Fig. 4. Film transmittance curves, (a) the deposition angles were 0°; (b) the deposition angles were 30°; (c) the deposition angles were 60°; (d) the deposition angles were 85°, with a rotation rate of 24 r/min, 80 r/min, 160 r/min, and 320 r/min, respectively.
Fig. 5. Refractive index (590 nm) of SiO2 films at different deposition angles and rotational speeds
The film microstructure at 85° was examined using scanning electron microscopy (SEM). As shown in Fig. 6, the inclined columnar structure of the film disappeared and a vertical columnar structure was obtained when the substrate rotated. When the substrate rotates slowly, the columnar structure had an obvious spiral structure. When the rotational speed was accelerated, the spiral structure disappeared, and the diameter of the column increased.
Fig. 6. Visible cross-sectional morphology of SiO2 films at 85°deposition angle, (a) with 24r/min rotational speeds; (b) with 80r/min rotational speeds; (c) with 160r/min rotational speeds; (d) with 320r/min rotational speeds.
The film surface roughness information can be obtained from the measurement results of the atomic force microscope, as shown in Fig. 7. There is a positive correlation between the deposition angle and root-mean-square roughness RMS. With an increase in the rotational speed, the roughness of the film surface decreased. This result can be explained by the shadow effect of the inclined deposition reduced by the increase in rotational speed. In the process of film preparation, the shadow area of the initially deposited film particles becomes narrower owing to the rotating substrate, and the later deposited film particles can be deposited in more positions, resulting in a reduction in the deposition roughness.
Since the GLAD-grown AR coatings are highly porous, in principle, scattering will adversely affect the antireflective performance of the interference-based AR coatings: scattering induced by imperfections in AR coatings will cause additional optical loss and should, so the scattering should be kept as small as possible. The scattering of the films is generally caused by the roughness of the surface or the interface of the film. The volume scattering value of the films is measured with the self-developed integrated scattering test system, as shown in Fig. 8. It is found that the scattering loss induced by the AR coating itself is negligible: scattering loss is less than 0.05% above 632 nm. Low scattering loss will help maintain good collimation for the incident light propagating inside the AR coating, and thus constructive interference between transmitted beams can be achieved efficiently.
Fig. 8. Volume scattering of films under the different inclined deposition angles and rotational speeds.
4.2 Optical performance
The SiO2 broadband AR film was designed using TFCalc software, with discontinuous changes in the refractive index and thickness and was fabricated on a fused silicon substrate. The variable refractive index anti-reflection film without rotational speed on the substrate are prepared according to our previous research [13]. The structures and parameters of variable refractive index anti-reflection film with rotational speed on the substrate are listed in Table 1. As shown in Fig. 9, in the wavelength range from 400 nm to 1800nm the maximum residual reflectance is less than 0.91%, and the average residual reflectance is approximately 0.59% at an incident angle of 0°.
Table 1. The structure and parameters of the graded refractive index broadband anti-reflection films
The traditional broadband anti-reflection film was prepared using HfO2 and SiO2 materials and the traditional design method of high- and low refractive index film stacks. The initial stack structure was designed as follows: G/0.45(0.5HL0.5H)10/A. where G is the quartz substrate, A is the incident medium air, and H and L are HfO2 and SiO2, respectively. The global optimization and local optimization methods and MacLeod software were applied to optimize the initial film system design by simplex optimization. The traditional broadband anti-reflection film was deposited on a fused silica substrate by electron beam evaporation deposition. The measured residual reflectance curve of the anti-reflection film is shown in Fig. 10. It is obvious that the traditional broadband anti-reflection film has a limitation on the transmission bandwidth. As shown in Fig. 11, the residual reflectance of the traditional anti-reflection film, variable refractive index anti-reflection film with rotational speed on the substrate, and variable refractive index anti-reflection film without rotational speed on the substrate are compared.
Fig. 11. The measured residual reflectance curve of the broadband anti-reflective film under three conditions.
The film thickness changes at the diameters in the relative positions of the three broadband anti-reflective films were measured using a step meter. The test positions and results are presented in Fig. 12. It can be concluded that because the film is prepared with zero rotational speed by the inclined deposition method, the thickness of the film is uneven in the inclined direction, which is negatively related to the distance from the evaporation source. The film thickness decreases from 781 nm to 615 nm in the position range of 36 cm in diameter, and the variation rule is linear. By increasing the substrate speed during deposition, the change in film thickness on the whole substrate is very small, and the changes are close to a constant, and the purpose of uniform film thickness is achieved. Based on the definition of film thickness uniformity, it is calculated that the film thickness uniformity is changed from 25.88% to 0.906%, and the film thickness uniformity was significantly improved. The uniformity of the traditional anti-reflection film was 0.366% owing to the process technology.
Fig. 12. The measured film thickness curve and the test positions of the three anti-reflective films at different positions.
The adhesion of the three anti-reflective films was tested according to severity level 1 of test method 2 in “ISO 9211-5:2018 Optics and photonics—Optical coatings—Part 5: Minimum requirements for antireflecting coatings”. The test positions are shown in Fig. 13 with 3M tape. During the adhesion test, it was found that both SiO2 films were peeled off. The thickness of the peeling position was tested using a step meter to detect the degree film peeling. The test position was from the film surface to the peeling area. The test results are presented in Table 2. The peeling thickness of the anti-reflective film prepared with rotational speed is half that of those prepared without rotational speed, indicating that the rotational speed has a significant improvement on the adhesion of the anti-reflective film. The traditional anti-reflective film has no peeling in the adhesion test, which shows that the adhesion of the traditional anti-reflective film has huge advantages over the film prepared by inclined deposition.
Table 2. Comparison of peel thickness at different positions of three anti-reflective films
4.3 Laser-induced damage threshold
Three types of anti-reflective films were selected for the laser-induced damage threshold test. The results of the LIDT are shown in Fig. 14 according to the 0% probability fitting method. Subsequently, the typical damage morphologies of these broadband anti-reflective films were obtained using scanning electron microscopy (SEM) and focused ion beam (FIB), as shown in Fig. 14. The damage morphologies of the three types of samples exhibit different characteristics as shown in Fig. 15.
Fig. 15. Typical damage morphology, (a) variable refractive index AR film with rotating speed; (b) (c)variable refractive index AR film without rotating speed; (d) (e) traditional AR film.
Typical damage morphologies of the anti-reflection film without rotational speed are shown in Fig. 15(b) and Fig. 15(c), which demonstrate the ablation of the surface materials and the peeling of the film, with obvious signs of melting. We can clearly see that the damage is caused by the thermal effect and is characterized by melting and boiling. A possible damage mechanism is that free carriers are heated by interaction with the laser beam and subsequently transfer energy to the crystal lattice. Therefore, damage occurs via conventional heat deposition, resulting in the melting and boiling of the graded index dielectric material.
When there was no substrate rotation during the preparation of the anti-reflective film, the film peeling was weaker than that with rotational speed, and the peeling process was incomplete. Moreover, the threshold of the anti-reflective film without rotational speed was higher than that with rotational speed. The film structure of the anti-reflective film without rotational speed was looser, which is conducive to releasing the structural stress of the film. Therefore, part of the damage morphology of the anti-reflective film without rotational speed was transferred from film peeling to surface ablation. When the rotational speed of the inclined deposition film substrate increased, the film structure became denser. The structural stress release ability of the film decreased, which reduced the laser damage resistance of the SiO2 variable refractive index anti-reflection film with rotational speed. Therefore, these factors promote the near threshold damage morphology to be mainly film peeling, as shown in Fig. 15(a). Conventional anti-reflective film damage types mainly include surface ablation and nodule damage because of the better stress matching, as shown in Fig. 15(d) and (e), respectively.
5. Conclusion
In our work, the substrate was rotated during the preparation of the broadband anti-reflective film. Compared to the previous preparation method, the broadband anti-reflection film with substrate rotation has excellent broadband effect and stability in the range of 400–1800 nm. In the laser damage threshold, the broadband anti-reflection film with substrate speed is lower than that without substrate speed, but it is much higher than the traditional anti-reflection film. The damage to the three types of anti-reflection films were analyzed. The broadband anti-reflection film with substrate speed has higher stability compared with the broadband anti-reflection film without substrate speed, and it has a higher damage threshold than the traditional anti-reflection film, which improves the application prospects of inclined deposited films in high-power laser systems.
Funding
National Key Research and Development Program of China (2018YFE0115900); The Joint Research Fund in Astronomy under cooperative agreement between the National Natural Science Foundation of China (NSFC) and the Chinese Academy of Sciences (CAS) (U183120047); National Natural Science Foundation of China (11874369, 52002271); CAS special research assistant project; China Postdoctoral Science Foundation (2021M703326); Key foreign cooperation projects of Bureau of International Cooperation of Chinese Academy of Sciences (181231KYSB20210001).
Acknowledgments
This research was funded by the National Key R&D Program of China (Grant No. 2018YFE0115900), the Joint Research Fund in Astronomy under cooperative agreement between the National Natural Science Foundation of China (NSFC) and the Chinese Academy of Sciences (CAS) (Grant No. U183120047), National Natural Science Foundation of China (Grant No. 11874369 and 52002271), CAS special research assistant project, China Postdoctoral Science Foundation (Grant No.2021M703326) and Key foreign cooperation projects of Bureau of International Cooperation of Chinese Academy of Sciences (Grant No. 181231KYSB20210001).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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