Influence of Thermal Annealing on Mechanical and Optical Property of SiO2 Film Produced by ALD

The application range of fused silica optical components can be expanded and the cost of fused silica components can be reduced by depositing the same material film on fused silica substrate. However, due to the different manufacturing process, the performance of ALD SiO2 film is lower than that of fused silica substrate, which also limits the use of this process. In this paper, ALD SiO2 film with different thicknesses were deposited, and then the structure and properties were tested. Finally, the ALD SiO2 film was treated via the annealing process. Transmission electron microscopy (TEM) showed that the ALD SiO2 film had good compactness and substrate adhesion. The Raman spectra showed that the ALD SiO2 film and substrate had the same structure, with only slight differences. The XRD pattern showed that ALD-fused silica did not crystallize before or after annealing. The infrared spectra showed that there was an obvious Si-OH defect in the ALD SiO2 film. The laser damage showed that the ALD SiO2 film had a much lower damage threshold than the fused silica substrate. The nanoindentation showed that the mechanical properties of the ALD SiO2 film were much lower than those of the fused silica substrate. After a low-temperature annealing treatment, the ALD SiO2 film Si-OH defect was reduced, the ALD SiO2 film four-member ring content was increased, the elastic modulus of the ALD SiO2 film was increased from 45.025 GPa to 68.025 GPa, the hardness was increased from 5.240 GPa to 9.528 GPa, and the ALD SiO2 film damage threshold was decreased from 5.5 J/cm2 to 1.3 J/cm2.


Introduction
Current global concerns about energy resources are seeing a shift towards sustainable energy generation technologies [1].Inertial fusion energy (IFE) is a new type of energy that obtains clean deuterium tritium (DT) fusion energy based on inertial confinement fusion (ICF) and has commercial application value.The drive device of laser inertial confinement nuclear fusion is a large and complex optical system that requires a large number of highquality optical components.Fused silica is the amorphous state of silicon dioxide; has excellent heat resistance, with a melting point temperature as high as 1730 • C; can work at a high temperature of 1450 • C for a short time; has excellent transmittance in far ultraviolet light, visible light, and near infrared light; has higher mechanical properties than ordinary glass; and has a manufacturing process.Because of these excellent properties of fused silica, it is often used as a basic material for optical components [2].Taking the National Ignition Facility (NIF) [3,4] built in the United States in 2009 as an example, there are 1728 fused silica windows and lenses in the diameter range of 0.5 to 1.0 m and 192 fused silica gratings and shields [5,6].The Laser Megajoule (LMJ) device in France and the Shen Guang III series device constructed in China also use a large number of fused silica components.The most common method for producing fused silica workpieces is grinding [7].Due to the tional electron beam evaporation deposition.The effects of the protective SiO 2 layers and annealing on the laser-induced damage threshold (LIDT) of the films were investigated.Annealing was effective at decreasing the microdefect density and the absorption of the films.Moreover, the combination of the protective SiO 2 layer and annealing maximized the LIDT of the Ta 2 O 5 film [21].At the same time, annealing of the film improved the damage threshold of HfO 2 under near-infrared laser irradiation [22,23].
Previous studies have shown that SiO 2 films prepared by ALD process have good light transmission and substrate-binding force.However, the difference in the ring structure and vibration form of the ALD SiO 2 from the substrate, the hardness and elastic modulus from the substrate, and the effect of heat treatment on the molecular structure and mechanical properties of ALD SiO 2 on fused silica substrate have not been extensively studied.
Annealing coatings is an effective method to improve their properties [24].In this study, ALD SiO 2 films of different thicknesses were deposited on fused silica substrate, and the same process was used to deposit SiO 2 on silicon wafer substrate.We first studied the relationship between ALD SiO 2 thickness and molecular structure, laser damage threshold, and mechanical properties.Secondly, we treated ALD SiO 2 film on fused silica substrate with the annealing process.Thirdly, we studied the effect of temperature on the structure and properties of ALD SiO 2 film.

Experimental Process 2.1. Sample Preparation
Fused silica substrate with a surface roughness of less than 2 nm (RMS) was polished before deposition.The substrate thickness was 2 mm.The silicon wafer substrate was polished before deposition and had a surface roughness of less than 2 nm (RMS).Prior to deposition, all substrates were ultrasonically cleaned with a mixture of deionized water and ethanol.Then, the fused silica substrate was chemically etched in a 5% hydrofluoric acid solution for 80 min to remove absorbed impurities (Ce, Fe, etc.) and passivation [25].The substrate was then rinsed with deionized water to clean the surface.By measuring the height of the corroded part from the original surface, the corrosion depth was calculated to be about 4 µm.In beneq TFS 500, using BTBAS as a Si precursor and O 3 as an oxygen source, a single SiO 2 film was deposited on the previously prepared fused silica substrate and silica wafer substrates via the thermal ALD method at substrate temperatures of 300 • C. Per the typical ALD cycle, the first step was filling it with BTBAS for 0.5 s, the second step was filling it with nitrogen for 3 s, the third step was filling it with O 3 for 2 s, and the last step was filling it with nitrogen for 3 s.The film thickness and growth rate of the deposited SiO 2 were measured via spectroscopic ellipsometry.In our experiment, the growth rate of ALD SiO 2 was 0.1 nm/cycle.In a super-clean room, samples were placed in Petri dishes and wrapped in tinfoil and then annealed in a box-type resistance furnace (Model: KXL-1200X, Hefei, China).The samples were heated at rate of 10 • C/min, then held for 1 h, and last, cooled to normal temperature in the furnace.

Characterization
The microstructure of the ALD SiO 2 film sample on the silicon wafer substrate was observed with a spherical transmission electron microscope (model: Titan G2 60-300, FEI Eindhoven, The Netherlands).The microstructure of the ALD SiO 2 film on the fused silica substrate was observed via X-ray diffraction (model: Bruker-AXS D8 Advance, Bruker, Karlsruhe, Germany).A laser damage test platform (355 nm@10 ns) was used to test the ALD SiO 2 film on the fused silica substrate via the R-on-1 method, and the laser damage threshold of the ALD SiO 2 film on the fused silica substrate before and after heat treatment was studied.A Fourier transform infrared (FTIR) spectrometer (model: NICOLETIS10, Thermo America, Medley, FL, USA) was used to measure the infrared spectra of the ALD SiO 2 film and the fused silica substrate in order to study the structure and defects of the ALD SiO 2 film before and after heat treatment.Raman spectrometers (model: Aramis, HORIBA H.J.Y Company, Loos, France) were used to measure the Raman spectra of the ALD SiO 2 film and the fused silica in order to study the structure of the ALD SiO 2 film before and after heat treatment.A nano-indentation instrument (model: Keysight G200, Keysight Technologies, Inc. Santa Rosa, CA, USA) was used to measure the mechanical property in order to study the hardness and the elastic modulus of the ALD SiO 2 film before and after heat treatment.Transmission electron microscopy [26] can be used to observe the crystal structure of a sample and is commonly used to study the structure of thin film [27].ALD SiO 2 film on silicon wafer substrate was studied via transmission electron microscopy, and the images obtained are shown in Figure 1.Periodic arrangement of atoms in crystals can yield regular dark field images.It can be seen in Figure 1a that the ALD SiO 2 film had a clear interface with the substrate, and the interface width was about 1.5 nm, which was close to the roughness of the silicon wafer.There were no obvious micro-defects such as holes or cracks at the interface, indicating that the adhesion of the atomic layer deposition was excellent.The accelerated and concentrated electron beam was projected onto a very thin sample, and the electrons collided with the atoms in the sample and changed direction, thus obtaining an image of the atomic positions and defects.The upper left corner depicts obvious crystal silicon lattice characteristics, whereas the lower right corner shows that the ALD SiO 2 had no crystal lattice characteristics.After electrons passed through the sample, electrons from the same direction were focused on the same point behind the objective lens, resulting in an electron diffraction pattern.Because the atoms of a crystal have periodic laws, in a crystal material, the diffraction image is composed of a series of regular light spots.Because the atoms of amorphous materials lack the periodic rule of long-range order, the diffraction image turns out to be dispersed concentric circles.The electron diffraction pattern at the interface was obtained via diffraction operation on the ALD SiO 2 , as shown in Figure 1b.Because the electron beam diameter was much larger than the interface width, the diffraction ring formed by the ALD SiO 2 and the reciprocal lattice formed by the crystal Si could be seen in the electron diffraction pattern.

Experimental Results and Analysis
SiO2 film and the fused silica substrate in order to study the structure and defects of the ALD SiO2 film before and after heat treatment.Raman spectrometers (model: Aramis, HORIBA H.J.Y Company, Loos, France) were used to measure the Raman spectra of the ALD SiO2 film and the fused silica in order to study the structure of the ALD SiO2 film before and after heat treatment.A nano-indentation instrument (model: Keysight G200, Keysight Technologies, Inc. Santa Rosa, CA, USA) was used to measure the mechanical property in order to study the hardness and the elastic modulus of the ALD SiO2 film before and after heat treatment.Transmission electron microscopy [26] can be used to observe the crystal structure of a sample and is commonly used to study the structure of thin film [27].ALD SiO2 film on silicon wafer substrate was studied via transmission electron microscopy, and the images obtained are shown in Figure 1.Periodic arrangement of atoms in crystals can yield regular dark field images.It can be seen in Figure 1a that the ALD SiO2 film had a clear interface with the substrate, and the interface width was about 1.5 nm, which was close to the roughness of the silicon wafer.There were no obvious micro-defects such as holes or cracks at the interface, indicating that the adhesion of the atomic layer deposition was excellent.The accelerated and concentrated electron beam was projected onto a very thin sample, and the electrons collided with the atoms in the sample and changed direction, thus obtaining an image of the atomic positions and defects.The upper left corner depicts obvious crystal silicon lattice characteristics, whereas the lower right corner shows that the ALD SiO2 had no crystal lattice characteristics.After electrons passed through the sample, electrons from the same direction were focused on the same point behind the objective lens, resulting in an electron diffraction pattern.Because the atoms of a crystal have periodic laws, in a crystal material, the diffraction image is composed of a series of regular light spots.Because the atoms of amorphous materials lack the periodic rule of long-range order, the diffraction image turns out to be dispersed concentric circles.The electron diffraction pattern at the interface was obtained via diffraction operation on the ALD SiO2, as shown in Figure 1b.Because the electron beam diameter was much larger than the interface width, the diffraction ring formed by the ALD SiO2 and the reciprocal lattice formed by the crystal Si could be seen in the electron diffraction pattern.The atoms that make up chemical bonds or functional groups are in a state of constant vibration, and their vibration frequency is comparable to that of infrared light.When organic molecules are irradiated with infrared light, the chemical bonds or functional groups in the molecules can be vibrationally absorbed, and different chemical bonds or functional groups have different absorption frequencies, specifically in different absorption peak positions in the infrared spectrum.In this way, the structure of the samples could be obtained by detecting the position of the infrared absorption peak [28,29].Infrared spectrum tests were carried out on different thicknesses of ALD SiO 2 film on fused silica substrate, and the results are shown in Figure 2.Each sample was measured at three different locations.The structure of the ALD SiO 2 film was the same as that of the fused silica substrate, both of which contained a stretching vibration mode and a stretching vibration mode of Si-O.In Figure 2a, the measurement of 980 cm −1 is attributed to Si-O asymmetric stretching vibration (Vs), 784 cm −1 to Si-O bending vibration (Vb) [30,31], and 950 cm −1 and 3300 cm −1 to Si-OH vibration [32,33].It can be seen in Figure 2b that the asymmetric stretching vibration peak position was related to the film thickness and that the vibration peak position gradually increased with the increase in film thickness.It can be seen in Figure 2c that the asymmetric stretching vibration peak intensity was related to the film thickness and that the peak intensity decreased with the increase in film thickness.It can be seen in Figure 2d that the peak intensity of Si-OH increased with the increase in film thickness.It can be judged that the structure of the ALD SiO 2 film was almost the same as that of the fused silica substrate.The ALD SiO 2 film contained Si-OH defect and the peak strength of the asymmetric tensile vibration was weaker than that of the fused silica substrate.

Experimental Results and Analysis
The atoms that make up chemical bonds or functional groups are in a state of con vibration, and their vibration frequency is comparable to that of infrared light.Whe ganic molecules are irradiated with infrared light, the chemical bonds or funct groups in the molecules can be vibrationally absorbed, and different chemical bon functional groups have different absorption frequencies, specifically in different ab tion peak positions in the infrared spectrum.In this way, the structure of the sam could be obtained by detecting the position of the infrared absorption peak [28,29].I red spectrum tests were carried out on different thicknesses of ALD SiO2 film on f silica substrate, and the results are shown in Figure 2.Each sample was measured at different locations.The structure of the ALD SiO2 film was the same as that of the f silica substrate, both of which contained a stretching vibration mode and a stretchin bration mode of Si-O.In Figure 2a, the measurement of 980 cm −1 is attributed to Si-O a metric stretching vibration (Vs), 784 cm −1 to Si-O bending vibration (Vb) [30,31], and cm −1 and 3300 cm −1 to Si-OH vibration [32,33].It can be seen in Figure 2b that the a metric stretching vibration peak position was related to the film thickness and tha vibration peak position gradually increased with the increase in film thickness.It ca seen in Figure 2c that the asymmetric stretching vibration peak intensity was relat the film thickness and that the peak intensity decreased with the increase in film thick It can be seen in Figure 2d that the peak intensity of Si-OH increased with the increa film thickness.It can be judged that the structure of the ALD SiO2 film was almos same as that of the fused silica substrate.The ALD SiO2 film contained Si-OH defec the peak strength of the asymmetric tensile vibration was weaker than that of the f silica substrate.Raman spectroscopy is an analytical method based on the Raman scattering effect discovered by Indian scientist C.V. Raman.It analyzes the scattering spectra with different frequencies of incident light to obtain information about the molecular vibration and rotation, and it is applied to the study of molecular structure [34].It is generally believed that fused silica is a spatial network topology composed of a silicon atom and an oxygen atom tetrahedron as the basic unit.In the middle range, silicon and oxygen atoms are connected to each other to form ring structures, such as three-membered rings, fourmembered rings, and higher-membered rings.In this experiment, fused silica substrate and ALD SiO 2 film with different thickness were tested by Raman spectrum, and the test results are shown in Figure 3.Each sample was measured at five different locations.In the Raman spectrum, 440 cm −1 (ω 1 ) is attributed to oxygen atoms in the ring structure with more than five silicon atom rings, 490 cm −1 (D 1 ) is attributed to the vibration of oxygen atoms in the four-membered ring, 606 cm −1 (D 2 ) is attributed to the vibration of oxygen atoms in the three-membered ring, and 800 cm −1 is attributed to the bending vibration of Si-O-Si [35,36].It can be seen in Figure 3 that the peak shape and the peak position of the ALD SiO 2 film were consistent with those of the fused silica substrate, indicating that the ALD SiO 2 film and the fused silica substrate had the same structure.
vibration peak position of Si-O-Si, (c) asymmetric stretching vibration peak intensity of Si-O-Si, and (d) Si-OH peak intensity.
Raman spectroscopy is an analytical method based on the Raman scattering effect discovered by Indian scientist C.V. Raman.It analyzes the scattering spectra with different frequencies of incident light to obtain information about the molecular vibration and rotation, and it is applied to the study of molecular structure [34].It is generally believed that fused silica is a spatial network topology composed of a silicon atom and an oxygen atom tetrahedron as the basic unit.In the middle range, silicon and oxygen atoms are connected to each other to form ring structures, such as three-membered rings, four-membered rings, and higher-membered rings.In this experiment, fused silica substrate and ALD SiO2 film with different thickness were tested by Raman spectrum, and the test results are shown in Figure 3.Each sample was measured at five different locations.In the Raman spectrum, 440 cm −1 (ω1) is attributed to oxygen atoms in the ring structure with more than five silicon atom rings, 490 cm −1 (D1) is attributed to the vibration of oxygen atoms in the four-membered ring, 606 cm −1 (D2) is attributed to the vibration of oxygen atoms in the three-membered ring, and 800 cm −1 is attributed to the bending vibration of Si-O-Si [35,36].It can be seen in Figure 3 that the peak shape and the peak position of the ALD SiO2 film were consistent with those of the fused silica substrate, indicating that the ALD SiO2 film and the fused silica substrate had the same structure.

ALD SiO2 Film Optical Properties
The laser damage threshold is an important parameter for characterizing the ability of a laser irradiated medium to resist laser damage [37].The damage threshold of the sample in this experiment was obtained with a damage test platform of 355 nm@10 ns.The damage test results for the ALD SiO2 film with different thicknesses on the fused silica substrate of the same batch are shown in Figure 4.As can be seen in Figure 4, the 355 nm laser damage thresholds of the fused silica substrate and 198 nm, 388 nm, and 692 nm ALD SiO2 were 28.7 J/cm 2 , 15.8 J/cm 2 , 10.3 J/cm 2 , and 5.5 J/cm 2 , respectively.The damage threshold of the ALD SiO2 film was much lower than that of the fused silica substrate, and the damage threshold decreased with the increase in film thickness.It can be seen that the film had a greater impact on laser damage than the interface.

ALD SiO 2 Film Optical Properties
The laser damage threshold is an important parameter for characterizing the ability of a laser irradiated medium to resist laser damage [37].The damage threshold of the sample in this experiment was obtained with a damage test platform of 355 nm@10 ns.The damage test results for the ALD SiO 2 film with different thicknesses on the fused silica substrate of the same batch are shown in Figure 4.As can be seen in Figure 4, the 355 nm laser damage thresholds of the fused silica substrate and 198 nm, 388 nm, and 692 nm ALD SiO 2 were 28.7 J/cm 2 , 15.8 J/cm 2 , 10.3 J/cm 2 , and 5.5 J/cm 2 , respectively.The damage threshold of the ALD SiO 2 film was much lower than that of the fused silica substrate, and the damage threshold decreased with the increase in film thickness.It can be seen that the film had a greater impact on laser damage than the interface.

ALD SiO2 Film Mechanical Properties
The mechanical properties of a material [38,39], especially the hardness and elastic modulus, directly determine the wear resistance of the material and affects the service life of the material.The hardness and the elastic modulus of the sample in this experiment were obtained with the nano hardness tester.The ALD SiO2 film with different thicknesses

ALD SiO 2 Film Mechanical Properties
The mechanical properties of a material [38,39], especially the hardness and elastic modulus, directly determine the wear resistance of the material and affects the service life of the material.The hardness and the elastic modulus of the sample in this experiment were obtained with the nano hardness tester.The ALD SiO 2 film with different thicknesses on the fused silica substrate of the same batch were pressed into a depth of 100 nm for testing, and the results obtained are shown in Figure 5.The same sample was measured in five different regions.It can be seen that the hardness and the elastic modulus of the film were much lower than those of the substrate, and with the increase in film thickness, the hardness and the elastic modulus decreased gradually.This is because as the thickness of the film increased, the measured data were less and less affected by the substrate.

ALD SiO2 Film Mechanical Properties
The mechanical properties of a material [38,39], especially the hardness and ela modulus, directly determine the wear resistance of the material and affects the service of the material.The hardness and the elastic modulus of the sample in this experim were obtained with the nano hardness tester.The ALD SiO2 film with different thickne on the fused silica substrate of the same batch were pressed into a depth of 100 nm testing, and the results obtained are shown in Figure 5.The same sample was measu in five different regions.It can be seen that the hardness and the elastic modulus of film were much lower than those of the substrate, and with the increase in film thickn the hardness and the elastic modulus decreased gradually.This is because as the thick of the film increased, the measured data were less and less affected by the substrate.

Effect of Annealing Temperature on ALD SiO2 Film Structure
In this experiment, the ALD SiO2 film on the fused silica substrate of the same b was used, and then the spectral changes to the ALD SiO2 film were measured via infra reflection.The results are shown in Figure 6.It can be seen in Figure 6a that the wavef changed before and after annealing treatment, indicating that the structure of the A SiO2 film underwent some changes.It can be seen in Figure 6b that after low-tempera annealing, the peak position of the asymmetric stretching vibration peak of Si-O-Si creased from the initial 1005.848cm −1 to 985.993 cm −1 .It can be seen in Figure 6c that a annealing ALD SiO2, the peak strength of the asymmetric stretching vibration of Si-  In this experiment, the ALD SiO 2 film on the fused silica substrate of the same batch was used, and then the spectral changes to the ALD SiO 2 film were measured via infrared reflection.The results are shown in Figure 6.It can be seen in Figure 6a that the waveform changed before and after annealing treatment, indicating that the structure of the ALD SiO 2 film underwent some changes.It can be seen in Figure 6b that after low-temperature annealing, the peak position of the asymmetric stretching vibration peak of Si-O-Si decreased from the initial 1005.848cm −1 to 985.993 cm −1 .It can be seen in Figure 6c that after annealing ALD SiO 2 , the peak strength of the asymmetric stretching vibration of Si-O-Si increased from 1.143 to 1.502.It can be seen in Figure 6d that the peak intensity of the Si-OH peak decreased from 0.039 to 0. According to Lamberbier's law, the content of the asymmetric stretching vibration of Si-O-Si increased after annealing and the content of Si-OH decreased after annealing.
After the ALD SiO 2 (692 nm) film on the fused silica substrate of the same batch was annealed, the detection was carried out under the Raman spectrometer.The wavelength of the detection laser was 532 nm.The laser was focused onto the surface of the film.In order to reduce the experimental error, five random regions were used for detection.The Raman spectrum was obtained, as shown in Figure 7.It can be seen in Figure 7a that there was no significant change in Raman peak shape and position before or after annealing.In order to reduce the measurement error, the ratio of the peak strength of the four-member ring (D 1 ) to the peak strength of main ring (ω 1 ) was used in this paper to study the change in the structure of the ALD SiO 2 , and Figure 7b was obtained.It can be seen in Figure 7b that the four-member ring content of the ALD SiO 2 film increased after annealing.
increased from 1.143 to 1.502.It can be seen in Figure 6d that the peak intensity of the Si-OH peak decreased from 0.039 to 0. According to Lamberbier's law, the content of the asymmetric stretching vibration of Si-O-Si increased after annealing and the content of Si-OH decreased after annealing.After the ALD SiO2 (692 nm) film on the fused silica substrate of the same batch was annealed, the detection was carried out under the Raman spectrometer.The wavelength of the detection laser was 532 nm.The laser was focused onto the surface of the film.In order to reduce the experimental error, five random regions were used for detection.The Raman spectrum was obtained, as shown in Figure 7.It can be seen in Figure 7a that there was no significant change in Raman peak shape and position before or after annealing.In order to reduce the measurement error, the ratio of the peak strength of the four-member ring (D1) to the peak strength of main ring (ω1) was used in this paper to study the change in the structure of the ALD SiO2, and Figure 7b was obtained.It can be seen in Figure 7b that the four-member ring content of the ALD SiO2 film increased after annealing.X-ray diffraction is especially suitable for phase analysis of crystalline substance order to test whether the ALD SiO2 crystallized, X-ray diffraction detection of ALD before and after heat treatment was carried out in this paper, and the detection result shown in Figure 8.For amorphous materials, because there is no long-range order of arrangement in the crystal structure, there is only short-range order in a few at ranges, so the XRD pattern of amorphous materials is bulged.It can be seen in Fig that in the whole range of the scanning angle, the scattered X-ray intensity changed ge during which there was only a maximum value.At the beginning, because the inte was close to the direct beam, the intensity decreased rapidly with the increase in a and the intensity of the high angle slowly tended to the background value of the in ment.The ALD SiO2 was amorphous before and after heat treatment, and no crysta tion was found.This is because the annealing temperature was much lower than the perature required for the recrystallization of fused silica.X-ray diffraction is especially suitable for phase analysis of crystalline substances.In order to test whether the ALD SiO 2 crystallized, X-ray diffraction detection of ALD SiO 2 before and after heat treatment was carried out in this paper, and the detection results are shown in Figure 8.For amorphous materials, because there is no long-range order of atom arrangement in the crystal structure, there is only short-range order in a few atomic ranges, so the XRD pattern of amorphous materials is bulged.It can be seen in Figure 8 that in the whole range of the scanning angle, the scattered X-ray intensity changed gently, during which there was only a maximum value.At the beginning, because the intensity was close to the direct beam, the intensity decreased rapidly with the increase in angle, and the intensity of the high angle slowly tended to the background value of the instrument.The ALD SiO 2 was amorphous before and after heat treatment, and no crystallization was found.This is because the annealing temperature was much lower than the temperature required for the recrystallization of fused silica.
before and after heat treatment was carried out in this paper, and the detection results are shown in Figure 8.For amorphous materials, because there is no long-range order of atom arrangement in the crystal structure, there is only short-range order in a few atomic ranges, so the XRD pattern of amorphous materials is bulged.It can be seen in Figure 8 that in the whole range of the scanning angle, the scattered X-ray intensity changed gently, during which there was only a maximum value.At the beginning, because the intensity was close to the direct beam, the intensity decreased rapidly with the increase in angle, and the intensity of the high angle slowly tended to the background value of the instrument.The ALD SiO2 was amorphous before and after heat treatment, and no crystallization was found.This is because the annealing temperature was much lower than the temperature required for the recrystallization of fused silica.

Effect of Annealing Temperature on Optical Properties of ALD SiO2 Film
Fused silica is an important component in the laser system, so it is very important to measure its laser damage.When a new component is put into use, the evaluation or determination of its laser damage capability is an essential procedure.The ALD SiO2 film of 692 nm thickness on the fused silica substrate of the same batch was irradiated on the damage test platform to obtain the laser damage threshold, and the results are shown in Figure 9.It can be seen in Figure 9 that with the increase in annealing temperature, the damage threshold decreased gradually from the initial 5.5 J/cm 2 to the lowest point of 1.3 J/cm 2 .In the rapid annealing process, the expansion rate of the film and the substrate was inconsistent, stress between the film and the substrate formed, and the stress led to

Effect of Annealing Temperature on Optical Properties of ALD SiO 2 Film
Fused silica is an important component in the laser system, so it is very important to measure its laser damage.When a new component is put into use, the evaluation or determination of its laser damage capability is an essential procedure.The ALD SiO 2 film of 692 nm thickness on the fused silica substrate of the same batch was irradiated on the damage test platform to obtain the laser damage threshold, and the results are shown in Figure 9.It can be seen in Figure 9 that with the increase in annealing temperature, the damage threshold decreased gradually from the initial 5.5 J/cm 2 to the lowest point of 1.3 J/cm 2 .In the rapid annealing process, the expansion rate of the film and the substrate was inconsistent, stress between the film and the substrate formed, and the stress led to electronic defects in the film or the expansion of the original electronic defects, thereby reducing the laser damage threshold of the film.

Effect of Annealing Temperature on Mechanical Properties of ALD SiO2 Film
In order to reduce the influence of the equipment and the experimentalists on the experimental results, all samples in this experiment were tested at the same time.A diamond indenter was pressed into the ALD SiO2 film (692 nm) at a depth of 100 nm to measure the hardness and the elastic modulus.Each specimen was measured at five different locations and the results were obtained, as shown in Figure 10.It can be seen in Figure 10a,b that after the annealing treatment, the elastic modulus of the ALD SiO2 increased from 45.025 GPa to 68.025 GPa and the hardness increased from 5.240 GPa to 9.528 GPa, which is attributed to the increase in four-member ring content causing densification [40], the decrease in Si-OH content, and the increase in the Si-O-Si peak intensity.In order to reduce the influence of the equipment and the experimentalists on the experimental results, all samples in this experiment were tested at the same time.A diamond indenter was pressed into the ALD SiO 2 film (692 nm) at a depth of 100 nm to measure the hardness and the elastic modulus.Each specimen was measured at five different locations and the results were obtained, as shown in Figure 10.It can be seen in Figure 10a,b that after the annealing treatment, the elastic modulus of the ALD SiO 2 increased from 45.025 GPa to 68.025 GPa and the hardness increased from 5.240 GPa to 9.528 GPa, which is attributed to the increase in four-member ring content causing densification [40], the decrease in Si-OH content, and the increase in the Si-O-Si peak intensity.
In order to reduce the influence of the equipment and the experimentalists o experimental results, all samples in this experiment were tested at the same time.A mond indenter was pressed into the ALD SiO2 film (692 nm) at a depth of 100 nm to m ure the hardness and the elastic modulus.Each specimen was measured at five diff locations and the results were obtained, as shown in Figure 10.It can be seen in F 10a,b that after the annealing treatment, the elastic modulus of the ALD SiO2 incre from 45.025 GPa to 68.025 GPa and the hardness increased from 5.240 GPa to 9.528 which is attributed to the increase in four-member ring content causing densification the decrease in Si-OH content, and the increase in the Si-O-Si peak intensity.

Conclusions
By using BTBAS as an Si precursor and O3 as an oxygen precursor, a single lay ALD SiO2 film was deposited on the silicon wafer and on the fused silica substrate, re tively.High-resolution electron microscopy was used to study the ALD SiO2 cross-se The results show that the deposited SiO2 had good densification, there were no cavit other microscopic defects in the internal or interface of the film, and the ALD SiO2 cided with a typical amorphous state.The XRD spectra show that the ALD SiO2 di crystallize after low-temperature annealing.The Raman spectra show that the stru

Conclusions
By using BTBAS as an Si precursor and O 3 as an oxygen precursor, a single layer of ALD SiO 2 film was deposited on the silicon wafer and on the fused silica substrate, respectively.High-resolution electron microscopy was used to study the ALD SiO 2 crosssection.The results show that the deposited SiO 2 had good densification, there were no cavities or other microscopic defects in the internal or interface of the film, and the ALD SiO 2 coincided with a typical amorphous state.The XRD spectra show that the ALD SiO 2 did not crystallize after low-temperature annealing.The Raman spectra show that the structure of the ALD SiO 2 was consistent with that of the fused silica substrate.The infrared spectrum detection shows that the ALD SiO 2 asymmetric stretching vibration peak position was greater than that of the fused silica substrate, the ALD SiO 2 asymmetric stretching vibration peak intensity was smaller than that of the fused silica substrate, and at the same time, the ALD SiO 2 Si-OH peak was greater than that of the fused silica substrate and was proportional to the film thickness.The damage test of the ALD SiO 2 film on the fused silica substrate shows that the laser damage threshold decreased with the increase in film thickness.The nanoindentation test shows that the hardness and the elastic modulus of the ALD SiO 2 were much lower than those of the fused silica substrate.After annealing at a low temperature with a rapid warming rate, the ALD SiO 2 four-member ring content increased, the peak position of the asymmetric stretching vibration of Si-O-Si decreased, the peak Intensity of the asymmetric stretching vibration of Si-O-Si increased, the peak intensity of Si-OH decreased, the laser damage threshold of the ALD SiO 2 was reduced, the elastic modulus of the ALD SiO 2 increased from 45.025 GPa to 68.025 GPa, and the hardness increased from 5.240 GPa to 9.528 GPa.

Figure 1 .
Figure 1.Transmission electron microscopy (TEM) photo of ALD SiO2 on silicon wafer substrate.(a) Lattice image of film interface and (b) electron diffraction pattern at the interface.

Figure 1 .
Figure 1.Transmission electron microscopy (TEM) photo of ALD SiO 2 on silicon wafer substrate.(a) Lattice image of film interface and (b) electron diffraction pattern at the interface.

Figure 2 .
Figure 2. Infrared absorption spectra of ALD SiO2 film and fused silica substrate and related tion peak position and peak intensity.(a) Infrared absorption spectra, (b) asymmetric stret

Figure 2 .
Figure 2. Infrared absorption spectra of ALD SiO 2 film and fused silica substrate and related vibration peak position and peak intensity.(a) Infrared absorption spectra, (b) asymmetric stretching vibration peak position of Si-O-Si, (c) asymmetric stretching vibration peak intensity of Si-O-Si, and (d) Si-OH peak intensity.

Figure 3 .
Figure 3. Raman spectra of ALD SiO2 film and fused silica substrate.

Figure 3 .
Figure 3. Raman spectra of ALD SiO 2 film and fused silica substrate.

Figure 4 .
Figure 4. Laser damage threshold of ALD SiO2 film and fused silica substrate.

Figure 4 .
Figure 4. Laser damage threshold of ALD SiO 2 film and fused silica substrate.

Figure 4 .
Figure 4. Laser damage threshold of ALD SiO2 film and fused silica substrate.

Figure 5 .
Figure 5. Mechanical of ALD SiO2 film and fused silica substrate.(a) Hardness an elastic modulus.

Figure 5 .
Figure 5. Mechanical properties of ALD SiO 2 film and fused silica substrate.(a) Hardness and (b) elastic modulus.

4 .
Effect of Annealing on the Structure and Properties of ALD SiO 2 Film 4.1.Effect of Annealing Temperature on ALD SiO 2 Film Structure

Figure 6 .
Figure 6.Infrared spectrum of ALD SiO2 film before and after thermal annealing.(a) Infrared spectrum of ALD SiO2 film, (b) stretching vibration peak position of Si-O-Si, (c) stretching vibration peak intensity of Si-O-Si, and (d) Si-OH peak intensity.

Figure 6 .Figure 7 .
Figure 6.Infrared spectrum of ALD SiO 2 film before and after thermal annealing.(a) Infrared spectrum of ALD SiO 2 film, (b) stretching vibration peak position of Si-O-Si, (c) stretching vibration peak intensity of Si-O-Si, and (d) Si-OH peak intensity.Materials 2024, 17, x FOR PEER REVIEW 9

Figure 7 .
Figure 7. Raman spectra of ALD SiO 2 film treated at different temperatures.(a) Raman spectra of ALD SiO 2 film and (b) the relationship between D 1 /ω 1 and temperature.

Figure 8 .
Figure 8. X-ray diffraction spectra of ALD SiO2 before and after heat treatment.

Figure 8 .
Figure 8. X-ray diffraction spectra of ALD SiO 2 before and after heat treatment.

Materials 2024 ,
17,  x FOR PEER REVIEW 10 of 13 electronic defects in the film or the expansion of the original electronic defects, thereby reducing the laser damage threshold of the film.

Figure 9 .
Figure 9. Relationship between ALD SiO2 film damage threshold and temperature.

Figure 9 .
Figure 9. Relationship between ALD SiO 2 film damage threshold and temperature.

4. 3 .
Effect of Annealing Temperature on Mechanical Properties of ALD SiO 2 Film