Buffered HF-based etching can effectively improve the laser damage resistance of the fused silica, but deep etching would cause the deteriorations in surface roughness and hardness, and decrease the laser-induced damage threshold. Capping a glass thin layer on the etched surface via plasma chemical vapor deposition in one step could overcome those deteriorations. We found that the deposition of the glass thin layer can further reduce the impurity element contamination and the PL intensity while retaining the low subsurface defect density as well as for the deeply etched sample. The surface quality, surface hardness and the laser damage resistance of the fused silica can be significantly improved by the glass thin layer, which reveals the potential application in high power laser facility.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Laser-induced damage of fused silica considerably affects the operational durability and lifetime of optic components in high power laser facility. The damage would easily occur on the fused silica surface while under UV illumination with an energy density from 5 to 15 J/cm2 . Literature shows that subsurface defect (SSD) sites, a cracked layer introduced by sorts of grinding and polishing processes, are one of the primary factors for the laser damage initiation [2–5]. The other one primary factor is the Beilby polishing layer, which has many embedded many impurities . The influence of SSD on the laser damage includes three aspects, enhancing the electric field intensity, weakening the mechanical strength of materials and embedding the impurities which absorb laser light [5–7]. Hence, eliminating SSD would be an ultimate goal to obtain the perfect surface with high laser damage resistance.
During the past decades, various surface modified processes and finishing processes have been developed to eliminate or suppress SSD, for example, buffered HF-based etching [1,8,9], reaction ion etching [10,11] and CO2 laser conditioning [12–15]. Buffered HF-based etching is one of the most attractive post-process treatments for enhancing the laser damage resistance of optics. By revealing and blunting the cracks, buffered HF-based etching can suppress the electric field intensity and enhance the laser damage resistance of the surface. In the meanwhile, the contaminant concentration and laser damage density can also be reduced effectively. However, deep etching (more than 20μm) would result in a decrease rather than an increase of LIDT due to the deterioration of surface shape and roughness caused by enlarged scratches and pits [8,16]. The laser beam quality would be restricted heavily while the deep etched fused silica components are working at the high power laser facilities. Besides, deep etching would reduce the surface hardness of fused silica and the decrease of mechanical strength would decrease the laser damage threshold .
Capping a glass thin layer on the etched surface of fused silica would be a potential method to improve the laser damage performance of the fused silica by overcoming the deteriorations of surface shape and surface hardness. Shen et al. used the plasma-enhanced chemical vapor deposition (CVD) method to coat a SiO2 thin film on the HF-etched surface in a SiH4/N2O precursor gas with a substrate temperature of ~300°C, and then a scanned CO2 laser beam at 10.6μm wavelength was used to anneal the as-deposited CVD film with various heat treatment temperatures . They found that the CO2 laser heat treatment can effectively improve the laser damage threshold of the as-deposited CVD film by decreasing the concentration of the intrinsic defects (i.e. oxygen-deficiency centers and non-bridging oxygen hole centers). Chen et al. used a CVD process to deposit amorphous SiO2 particles on the surface of fused quartz ceramic, and then sintered those particles into a glass thin layer via hydrogen-oxygen torch . The results revealed that the glass thin layer can improve the strength of fused quartz ceramic by ~15%. Note that the LIDT of the as-deposited SiO2 film is less than half of the etched surface’s LIDT, a subsequent heat treatment is necessary for enhancing the laser damage resistance of the CVD film. Moreover, little work has been done to understand the correlation of the glass thin layer with those extrinsic and intrinsic defects of the fused silica substrate.
Plasma chemical vapor deposition (PCVD) is one of the methods to synthesize the type IV fused silica with ultrahigh purity and ultralow content of hydroxyl groups (less than 1 ppm) [20,21]. Comparing to the conventional plasma-enhanced CVD, PCVD process is carried on at a high temperature of 800~1500°C, and it can form the glass thin layer in one step by depositing and sintering the amorphous SiO2 particles in the same time. In this work, a facile method was proposed to enhance the laser damage performance of fused silica optics by depositing a glass thin layer on the deeply etched surface via PCVD. The fused silica surface was firstly treated by buffered HF-based deep etching to completely remove SSD. And the scratches and pits would be exposed and blunted during the deep etching process, which is convenient for the following PCVD depositing. Due to the fluidity of fused state glass, the glass thin layer can fill-in and heal these scratches and pits. After the deposition of the glass thin layer, the surface quality, photo-thermal intensity, surface contamination concentration, surface hardness, photoluminescence, and laser damage resistance were characterized. We found that the deposition of the glass thin layer can further reduce the impurity element contamination and the PL intensity while retaining the low SSD density as well as for the deeply etched sample. The laser damage performance, the surface quality, and the surface hardness can all be significantly improved, as shown in this work.
2. Experimental setup
2.1 Sample preparation
In this experiment, 4 square fused silica samples (Corning 7980, 50 mm wide and 5 mm thick) were selected, ground and polished by the same vendor. Sample A1 is an original sample without any post-treatment. Sample B1 was etched to a depth of 20 μm by the dynamic chemical etching (DCE) process. The detailed etching process has been described in . The whole DCE process was carried out in a multi-frequency ultrasonic transducer (Blackstone multiSONIKTM 40, 80, 120, 140, 170, 220 and 270kHz). Firstly, the sample was submerged in a 10 vol. % solution of Micro-90 (a commercial cleaning agent) for 30 min. Subsequently, the sample was removed and rinsed with deionized water. And then the sample was submerged in an HF-buffered solution (consisting of ~10 wt. % HF and ~25 wt. % NH4F) for 120 min. After the same rinsing process, the sample was sprayed with deionized water and air dried for the following process. Sample C1 and C2 were firstly treated by DCE process as well as Sample B1. And then the PCVD method was used to deposit a glass thin layer on the etched surface. As shown in Fig. 1, clean dry air and a 200kW high-frequency plasma heat source were used to generate the high-frequency plasma torch with a diameter of 40mm and a central electric temperature of ~104K at the depositing site. To improve the affinity of SiO2 particles, the temperature of the fused silica surface was pre-heated to ~1400°C before the deposition. Subsequently, a SiCl4/O2 precursor gas was added into the plasma torch, and the SiO2 particles were generated, aggregated and carried to the surface of the fused silica by the flow. Finally, the SiO2 particles were deposited and sintered to form the glass thin layers with the thickness of 2μm and 20μm, respectively. Due to its fluidity during the sintering process, the glass thin layer can fill in and heal the scratches and pits which were exposed and blunted by deep etching and result in a smooth surface. All the samples with different post-process parameters were obtained, as shown in Table 1.
All the samples were rinsed using deionized water and allowed to air dry before the measurement of characteristics. The surface roughness and microscopy of the samples were measured by a white light interferometer, and 10 sites were randomly tested on the surface of each sample. The photo-thermal absorption of the samples was characterized by a photo-thermal common-path interferometer. A surface scanning of a 1mm × 1mm test area with a 20μm step size, for 2500 points, was executed in the laser weak absorption test process, and five regions with 1mm × 1mm were randomly chosen from each sample surface. The surface impurities of the samples were investigated by an IONTOF secondary ion mass spectrometry (TOF-SIMS) IV apparatus. All the sample surfaces were first cleaned through sputtering for 20 seconds to remove the surrounding contamination before the investigation. The SSD of the samples was investigated by confocal fluorescence microscope. Fluorescence images were detected by using a 410nm high-pass filter with 20 × objective. And the detecting depth of all the samples is constantly 10μm. Photoluminescence (PL) spectra of the samples were measured by means of the fluorescence spectrometer. A photomultiplier was used as the detector and a Xe lamp used as the excitation source. The slit widths of the exciting and emission light were 15nm and 20nm, respectively. The surface hardness of the samples was measured by a nanoindenter system (MTS Nano Indenter) with a radius of curvature about 50nm, and the hardness can be calculated from the load-displacement curves.
The LIDT test was carried out on a laser damage test platform using a frequency-tripled Nd: YAG laser, which provided a near-Gauss-type pulse beam at 355nm wavelength. The fluence fluctuations of the laser facility have a standard deviation of ~4.5% at 355nm. The pulse width was ~7ns, while the spot diameter was 1.2mm, and the effective area was about 1.13mm2. The 1-on-1 testing procedure was carried out at 20 locations for each energy density. The LIDT of the samples were measured in air according to the international standard ISO21254. The distance between any two close irradiated spots was about 3 mm, which could shun the overlapping of the damaged regions. The samples were shot by the increasing energy with an increment of 0.5mJ. And the energy was recorded when breakdown damage occurred. The morphologies of the damage sites were observed by an optical microscope.
3. Results and discussion
3.1 Surface roughness evolution
The white light interferometer was utilized to characterize the surface morphologies and surface roughness of samples with various treatment processes. The surface morphologies and surface RMS roughness values are displayed in Fig. 2. The roughness value of the original sample surface without any treatment is about 1.02 ± 0.08nm. The surface morphology in Fig. 2(a) indicates that the original sample has a very smooth surface. After being deeply etched for 20μm, the Beilby layer would be removed and the subsurface defects such as cracks and pits are completely revealed, blunted and enlarged, resulting in a deterioration of surface morphology and surface roughness . The RMS roughness of the B1 sample has been increased to 11.36 ± 0.32nm during the deep etching process and the surface morphology shows many deep scratches and pits. However, the RMS roughnesses of sample C1 and C2 have reduced to 1.05 ± 0.09nm and 1.01 ± 0.02nm, respectively. In the meanwhile, we note that the wave front properties of the samples, as displayed in Fig. 2(e), show the same tendency as well as that of the surface roughness. The surface morphologies reveal that the glass thin film deposited by PCVD could heal the deterioration of surface morphology and roughness caused by deep etching.
3.2 Photo-thermal absorption analysis
The laser weak absorptions of fused silica samples were measured by photo-thermal deflection technique. Figure 3 presents the 355nm laser absorption distribution for fused silica samples with various post-treatments. The average absorption values reveal the absorption of high-density intrinsic surface defects, such as the uniform distribution of surface impurities, while the peak absorption values reveal the strong absorption by low-density defects such as SSD . The statistic results of average absorption and peak absorption are shown in Fig. 4 for all the samples. The average absorption value of the original sample A1 is about 3.6ppm. Since the deep etching process can thoroughly eliminate the surface impurities and SSD, the average absorption value of the sample B1 has been reduced to a very low level as 0.5ppm. We note that the UV-absorption levels of the samples C1 and C2 are similar to that of sample B1, and the peak absorption results show the same tendency. It suggests that the glass thin layer has a high purity and the PCVD process doesn’t introduce any new surface defects.
3.3 TOF-SIMS analysis
Figure 5 displays the depth profiles of the impurities in the surface of the samples with respect to various post treatments, which detected by TOF-SIMS. All the data has been normalized with silicon particle number (counts 10,000) as a standard. As shown in Fig. 5(a), there are a large number of metal impurity elements in the subsurface layer of the original sample A1, including Na, Al, K, Fe, and Ce etc. After a deep etching process, most of the impurity elements have been removed and the concentrations of the impurity elements decreased a lot in Fig. 5(b). Table 2 presents the total amount of four elements (Al, K, Fe, and Ce) in the subsurface of the samples. We can find that the Ce element introduced by the conventional polishing process has been thoroughly eliminated by the deep etching process. In the meanwhile, the concentration of the Fe element increased appreciably, which suggests that it would be introduced from the etching solution. However, the concentrations of the Ce and Fe elements could be reduced further in sample C1 and C2, and the Ce element can even totally disappear in the glass thin layer surface, which reveals the high purity of the glass thin layer surface. Since the Ce and Fe elements have a strong influence on the laser-induced damage threshold , the deep etching process can improve the laser-induced damage threshold, and the Ce-free glass thin layer deposited by PCVD would have a potential to obtain a higher damage threshold.
3.4 SSD analysis
Confocal fluorescence microscope was used to indirectly detect the SSD since laser-induced fluorescence imaging is an effective method to feature the photo absorption impurities which are embedded in the microcracks. The scratches and pits displayed only in the fluorescence image are considered as the SSD . Figure 6 displays the fluorescence images detected on fused silica surface by confocal fluorescence microscope with respect to various post-treatments. There are numerous fluorescence points in the image of sample A1, which indicates that a large number of SSD are distributed in the original fused silica sample. After a deep etching process, the magnitude of fluorescence points has reduced to a very low level, suggesting that SSD of fused silica can be removed thoroughly by the deep etching process. We note that the fluorescence point magnitudes of sample C1 and C2 are similar with that of sample B1, which reveals that the deposition of glass thin layer could retain the low SSD density of the deep etched fused silica.
3.5 Photoluminescence (PL) spectra analysis
The comparison of normalized fluorescence emission spectra of the samples is shown in Fig. 7. There are two prominent emission peaks centered at ~390nm and ~650nm, corresponding to oxygen-deficiency centers (ODC) and non-bridging oxygen hole center (NBOHC), respectively [10,26,27]. Deep etching process could efficiently eliminate ODC defects and NBOHC defects. In the meanwhile, the glass thin layer deposited on the etched surface could further reduce PL intensities by mitigating ODC and NBOHC defects. It would be caused by two reasons. One is the ultralow intrinsic defects density of the new-born glass thin layer. Since the deposition process is similar to the manufacturing process of type IV fused silica, it would endow the resulting glass thin layer a high surface quality and low intrinsic defects density. The other one is the annealing effect. During the PCVD process, a high-temperature sintering process (more than 1200°C) would be applied on the etched surface, which could be considered as an annealing process. As reported by Raman  and Shen , annealing process could be an effective way to eliminate those ODC and NBOHC defects. Note that sample C2 with a thicker glass thin layer presents a lower PL intensity than sample C1. It is probably due to that the thicker glass layer needs a longer time to deposit, which means a longer time to anneal the surface.
3.6 Surface hardness analysis
The surface hardness of the fused silica samples with respect to various post-treatments are shown in Fig. 8. As reported by Zheng  and Li , the hardness increases with the increasing indentation depth until the depth reaches~200nm. Thus, the hardness value at the depth of 200nm is considered as the hardness of the fused silica surface. We find that the hardness of the original sample A1 is ~15GPa. After a deep etching process, it has decreased to ~11.5GPa, which can be considered as the hardness of the bulk. Li et al. indicated that the conventional chemical mechanical polishing (CMP) process would form a compressive stress layer over the fused silica surface, and result in a higher hardness than the bulk . Since the thickness of the harder layer wouldn’t be more than 1μm, the deep etching process can thoroughly remove the harder layer and reveal the intrinsic hardness of the bulk. Note that the hardness of the sample C1 and C2 have recovered to ~17GPa and ~18GPa, respectively. It suggests that capping the glass thin layer on the etched surface can significantly enhance the surface hardness of the fused silica. We deduce that the capping process would form a new compressive stress layer as well as the conventional strengthen glass. Comparing to the sample A1, the glass thin layer can improve the surface hardness for 15~20%, which is in good agreement with .
3.7 Laser damage performance testing
Figure 9 displays the laser-induced damage probability results of the samples with respect to various post-treatments. Comparing with the original sample A1, the LIDT of 0% probability and 100% probability of the deep etched sample B1 both have a large improvement. The LIDT of 0% probability of the sample B1 increases from 10.1 to 15.8J/cm2, while the LIDT of 100% probability of the sample B1 increases from 14.0 to 24.5J/cm2. It suggests that the deep etching process can effectively improve the LIDT of the fused silica. Note that the LIDT of 0% probability ranges from 15.8 (sample B1) to 18.1 (sample C1) to 21.3J/cm2 (sample C2), while the LIDT of 100% probability ranges from 24.5 to 33.4 to 36.7J/cm2. The results reveal that the deposition of the glass thin layer can further improve the LIDT of the etched fused silica surface.
Generally, the LIDT of 0% probability always represents the damage initiated by the low threshold damage precursors (i.e. SSD or impurity element contamination), while the LIDT of 100% probability represents the damage initiated by the high threshold damage precursors (i.e. ODC or NBOHC) . The photo-thermal absorption analysis (Fig. 3) and the fluorescence images (Fig. 6) show that the deep etching process can thoroughly remove the SSD of fused silica. The TOF-SIMS analysis results (Fig. 5) show that most of the impurity elements, especially Ce element which plays the dominant role in impairing the laser damage resistance , can be removed by the deep etching process. Furthermore, the PL spectra (Fig. 7) displays that the deep etching process can efficiently eliminate ODC defects and NBOHC defects. It would explain the significant improvement of the LIDT for the deeply etched sample.
Comparing with the deep etched sample B1, the deposition of the glass thin layer in the sample C1 and C2 could further reduce the concentration of the impurity element contamination, and the Ce element even totally disappears in the glass thin layer surface. Since PCVD can be used to manufacture the type IV fused silica, it would endow the high purity to the glass thin layer. In the meanwhile, the SSD analysis results indicate that the deposition of glass thin layer would retain the low SSD density as well as the deeply etched surface, and PCVD process wouldn’t introduce any new defects. Moreover, the high-temperature sintering process could further eliminate the ODC and NBOHC defects by the annealing effect. Due to the longer annealing time, the sample C2 with thicker glass thin layer would have a lower PL intensity and higher LIDT than the sample C1.
The surface morphologies of the initial damage sites of the samples are shown in Fig. 10. Figure 10(a) presents a discrete smooth crater appearance, the typical damage morphology of fused silica with high LIDT . Usually, there would be a delamination morphology surrounding the damage crater when the as-deposited SiO2 film was damaged by laser irradiation [18,31]. However, the damage morphologies of sample C1 and C2 show the similar appearance with that of sample B1, which reveals that the glass thin layer deposited by PCVD has a good affinity with the fused silica substrate.
In this work, a facile method was proposed to enhance the laser damage performance of fused silica optics by depositing a glass thin layer on the deeply etched surface via PCVD. PCVD can deposit a glass thin layer with high purity in one step. We found that the deposition of glass thin layer can further reduce the content of impurity elements, and a Ce-free surface would be obtained. Due to the annealing effect, capping the glass thin layer can effectively remove the intrinsic defects as ODC and NBOHC. In the meanwhile, the SSD analysis results revealed that capping the glass thin layer would retain the low SSD density as the deeply etched sample, and it didn’t introduce any new defects. Finally, the surface roughness, surface hardness, and the laser damage resistance of the fused silica can all be significantly improved by the deposition of glass thin layer, and the glass thin layer had a good affinity to the substrate surface.
National Natural Science Foundation of China (61705204, 61705206, 51606158, 11174258, and 51602296); Development Foundation of China Academy of Engineering Physics (2015B0403095); Frontier Exploration Foundation of China Building Materials Academy (2016-YT-119); Laser Fusion Research Center Funds for Young Talents (LFRC-PD011).
We would like to gratefully acknowledge the help of Xuefu Song and his colleagues from China Building Materials Academy for the sample preparation. We also would like to acknowledge the help of Xin Ye’s team from CAEP for the sample characterization.
The authors declare that there are no conflicts of interest related to this article.
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