Microstructure and 355 nm Laser-Induced Damage Characteristics of Al 2 O 3 Films Irradiated with Oxygen Plasma under Different Energy

Al2O3 films were prepared using electron beam evaporation at room temperature. The samples were irradiated with oxygen plasma under different energy. The variations in average surface defect density and root mean square (RMS) surface roughness were characterized using an optical microscope and an atomic force microscope. Surface average defect density increased after plasma treatment. The RMS surface roughness of the samples decreased from 1.92 nm to 1.26 nm because of surface atom restructuring after oxygen plasma conditioning. A 355 nm laser-induced damage experiment indicated that the as-grown sample with the lowest defect density exhibited a higher laser-induced damage threshold (1.12 J/cm) than the other treated samples. Laser-induced damage images revealed that defect is one of the key factors that affect laser-induced damage on Al2O3 films.


Introduction
Alumina (Al 2 O 3 ) films are among the widely used ultraviolet optical film materials because of their high thermal conductivity, good stability, and excellent optical transparency within a wide spectral range [1][2][3][4].Al 2 O 3 is used for the preparation of high-reflective mirrors, antireflective coatings, optical filters, and other laser mirrors for laser systems.In recent years, considerable attention has been given to Al 2 O 3 thin films.Many techniques can be used to prepare Al 2 O 3 films, such as reactive magnetron sputtering [5], molecular beam epitaxy [6], sol-gel [7], ion beam sputtering [8], chemical vapor deposition [9], atomic layer deposition [10], electron beam thermal evaporation [11], and so on.Among these techniques, electron beam evaporation is one of the most common methods for optical thin film deposition.
Optical coating mirrors are important but vulnerable components in laser systems [12,13].Their susceptibility to laser-induced damage is one of the obstacles in the development of high-power laser technology.Many studies reported that micro-defects produced during the coating process caused by splashing and high absorption are the two most critical factors that affect film laser damage threshold [14][15][16].To improve the film laser damage threshold, many techniques were developed, such as reactive evaporation [17], laser conditioning [18], ion beam posttreatment [19], and so on.In this paper, Al 2 O 3 films were deposited using electron beam evaporation, and the samples were irradiated with oxygen plasma under different energy densities.The microstructure and 355 nm laserinduced damage performance of the treated sample were studied.

Experiment Details
Al 2 O 3 films were prepared on BK7 substrates at room temperature using electron beam thermal evaporation with a coating material purity of 99.99%.The base pressure in the vacuum chamber was 5.3 × 10 −3 Pa.The coating material was pre-melted thoroughly before deposition until the pressure of the vacuum chamber stabilized.During deposition, the working pressure and electron beam current were maintained at 1.5 × 10 −3 Pa and 200 mA, respectively.Film thickness, which was approximately 825 nm, was controlled using an optical monitor.Oxygen gas with a purity of 99.99% was ionized by an ion source to produce oxygen plasma.The prepared film samples were irradiated with oxygen plasma under different energy densities.The distance from the ion source to the sample surface was approximately 0.5 m.The angle between the ion beam and the normal direction of the sample surface was approximately 60˚.The detailed ion source parameters are shown in Table 1.
During the treatment process, the oxygen gas flow rate was 8 sccm, the pressure of the vacuum chamber was 0.1 Pa, and the ion beam current was 10 mA.The treatment time was 10 min.The applied ion energy varied from 350 eV to 500 eV for the different samples (Table 2).
The micro-defect density of the sample surface was determined using a Leica DM4000 microscope in dark field mode.The statistical criterion was the observation of visible defect under 200× magnification.The microstructure of the sample surface before and after the treatment was examined using an atomic force microscope (AFM).The samples were scanned over a 1 μm × 1 μm area using the contacting mode.The image resolution was 256 × 256 pixels.The laser-induced damage threshold (LIDT) of the samples was tested according to the ISO11254-1-2000 standard in 1-on-1 mode [20].A Q-switched Nd: YAG single-mode laser with an 8 ns pulse width and a 355 nm wavelength was used.The laser spot area on the sample surface measured using the knife-edge scanning method [21] was 0.026 mm 2 .Laser was irradiated in the direction of normal incidence of the sample surface.The testing system could see in our previous research work [22].Laser-induced damage morphologies were characterized and analyzed under a Leica polarizing microscope.

Results and Analysis
Eight sites were randomly selected on each sample surface to measure the average defect density.The relationship between the oxygen plasma energy and average defect density is shown in Figure 1.The plot shows that defect density increased in different degrees after irradia-  tion with oxygen plasma.Considering the plasma treatment process, we can deduce that the increase in defect density may be related to the second contaminant in the pumping and plasma treatment process.Compared with that of the as-grown sample, the average defect density of the Al 2 O 3 film increased greatly after irradiation with lower plasma energy at the initial stage.This phenomenon may be attributed to the limited ability of the lowenergy oxygen plasma irradiation to remove a small amount of contaminant.As the plasma energy increased, more particle defects were partially removed, and the defect density reduced.When the plasma energy was too high, fresh defects will be produced from the berried defects, thereby increasing defect density.
The AFM images of the samples are shown in Figure 2. The relationship between the root mean square (RMS) surface roughness of the sample and the oxygen plasma energy is plotted in Figure 3. Rough structure can be clearly observed from the as-grown sample surface image.The sample RMS roughness decreased as the oxygen plasma energy increased.The RMS roughness of the samples decreased from 1.92 nm to 1.26 nm when plasma energy varied from 0 eV (as-grown sample) to 500 eV.In most cases, many surface atoms reside in metastable state.During the oxygen plasma irradiation process, these atoms are relocated to the surface and transformed from metastable to stable state after gaining sufficient energy.The surface microstructure is restructured, and the surface RMS roughness is reduced correspondingly [23].
Figure 4 shows the third harmonic LIDT fitting plots of the as-grown and treated samples at 355 nm.The largest slope of the fitting line of the as-grown sample compared with the irradiated samples can be observed.This result reveals that the laser damage characteristics of the as-grown sample determine the intrinsic properties of thin-film materials [24].However, damage on the  treated samples presents a semblance of defect-induced failure characteristics.Figure 5 shows the LIDT variations in the samples at 355 nm.The LIDT of the samples varied from 1.12 J/cm 2 to 0.82 J/cm 2 .The highest LIDT of the as-grown sample is revealed from the plots.A comparison of the defect density measurement results in Figure 1 with the lowest defect density of the as-grown sample shows that the highest LIDT observed is reasonable.Laser-damage morphologies of the films after irradiation with various energy densities are shown in Figure 6.The damage scars are in micrometer size.Given the energy density Gaussian distribution of the laser beam, a catastrophic damage in the scar center can be observed.Moreover, the damage mostly occurred and developed from the defect sites, which correspond to the LIDT measurement results in Figure 5.At low energy density, numerous pustules developed because of the increase in defects after laser energy absorption (Figure 6(a)).As the laser energy increased, the pustules cracked and damage pits were produced (Figure 6(b)).When laser energy density further increased, damage on the entire spot area occurred (Figure 6(c)).

Conclusion
Al 2 O 3 thin films were prepared using electron beam evaporation.The samples were irradiated with oxygen plasma under different energy densities.Surface average defect density increased after plasma treatment.The RMS surface roughness of the samples decreased because of surface atom restructuring after oxygen plasma conditioning.The as-grown sample with the lowest defect density exhibited a higher 355 nm LIDT (1.12 J/cm 2 ) than the other samples.Laser-induced damage images revealed that defect is one of the key factors that affect laserinduced damage on Al 2 O 3 films.Thus, defect density reduction should be monitored during Al 2 O 3 film production.

Figure 1 .
Figure 1.Average defect density of samples surface irradiated with different energy of oxygen plasma.

Figure 2 .
Figure 2. AFM images of the samples.

Figure 3 .
Figure 3. RMS roughness variations of the samples.

Figure 4 .
Figure 4. Laser induced damage threshold fitting lines of samples.

Figure 5 .
Figure 5. Laser-induced damage threshold variation of the as-grown and treated samples.

Figure 6 .
Figure 6.Microscope images of the laser-induced damage morphology of samples.