In this paper, the influence of ion beam sputtering (IBS) process on laser damage resistance as a function of sputtering depth and typical defects which associated with laser damage performance was investigated. Damage test results reveal that the damage resistance of HF etched surface can be further enhanced about 30% by appropriate IBS removal depth (less than ~1000nm). Within this removal depth, the IBS process can remove the redeposited reaction products during HF acid etching process, improve surface quality and reduce chemical structure defects concentration. However, further ion sputtering often results in a decrease rather than an increase of damage threshold with enhanced surface densification and increased chemical structure defects of ODC and NBOHC which generated from sputtering damage. Moreover, the sputtered surface will accelerate the chemical reaction of surface atoms with water molecules. Thus the newly obtained hydroxylation layer rich in highly absorptive products can result in the decrease of laser damage resistance. The study reveals the improvement mechanism by IBS process, and provides both technical guidance and theoretical basis for the optimization of the post-process of fused silica.
© 2017 Optical Society of America
Fused silica is widely used in the field of high energy density science such as the Inertial Confinement Fusion (ICF) and high energy light sources because of its good optical properties [1,2]. However, laser-induced damage on the exit surface of fused silica remains today a key limitation for the stable operation and load capacity improvement of high power laser systems. Moreover, the laser-induced damage threshold (LIDT) of conventionally polished optical surface is much lower than the dielectric breakdown threshold of its bulk . Therefore, it has become a hot and difficult issue in recent decades to reveal the improvement mechanism and find ways to improve LIDT of fused silica.
Over the past two decades, various finishing processes and surface treatments such as magnetorheological finishing (MRF), HF acid etching, ion beam sputtering (IBS) and so on, have been developed to eliminate these damage precursors produced during fabrication process or by environmental contamination [4,5]. MRF can effectively remove the defect layers by shear stress, but Fe, Ce contamination is introduced in the subsurface because of the carbonyl iron powder contained in the MRF fluid polishing, which suppresses the improvement of LIDT . HF acid etching is an effective post-processing treatment for increasing the optic damage threshold, since it can chemically remove the subsurface damage (SSD) layer [7,8]. However, further etching often results in a decrease rather than an increase of damage threshold with enlarged and extended defects such as scratches or indentations and pits in geometrical structures. Moreover, solubility and mass transport of redeposited reaction product (SiF62-) is difficult to be controlled during the etching process, thereby it redeposited on the surface and became laser damage precursors [9,10]. Compared with the MRF and HF acid etching methods, IBS has the characteristics of no pollution, non-contact press and has advantages in the control of destructive defects such as residual stress and contamination [11–13]. So IBS has been applied to physically polish fused silica surface and achieve good effects in improving the LIDT.
In our current researches, IBS is more applied to polish the HF etched surface to remove the redeposited reaction product and improve surface quality [11,12]. However, further researches have shown that with the increase of IBS removal depth, the damage threshold of fused silica declined greatly, even reduced to the threshold after the optimized HF acid etching treatment. In addition, the LIDT of the ion sputtered surface is significantly reduced after immersed in deionized water. These phenomena cannot be explained by the evolution laws of damage precursors we now have studied. In other words, new damage precursors appeared with the increase of IBS removal depth, and thus result in the decrease of laser damage resistance of the sputtered surface. So, what are the new damage precursors appearing in IBS process? How does it evolve? And what is the mechanism that leads to the decrease of laser damage resistance after the ion sputtered surface immersed in deionized water? These problems are of great significance for understanding the influence law of IBS on the damage performance and optimizing the surface treatment process. In this paper, an overall research was conducted on the performance of HF etched fused silica optics by the IBS treatment. Multiple characterizations results of ion sputtered surface are presented. The results can systematically reveal the influence law of IBS and also provide technical guidance for the optimization of post-processing for fused silica.
2. Damage model of IBS
During IBS process, the incident ions transfer energy and momentum to the target atoms in the form of elastic collisions, then the target atoms consume these energies to overcome the lattice binding energy and the surface binding energy . Thereby chemical structure defects such as lattice dislocation, vacancy and displacement are produced during collisions. The accumulation of these defects leads to the lattice distortion, which is the subsurface sputtering damage due to internal force. According to Sigmund sputtering theory, the sputtering damage depth is defined as the thickness of incident ion energy scattering in the target bulk. As shown in Fig. 1, the relation between the sputtering damage depth and incidence angle can be expressed as :
Figure 2 is the simulation result of sputtering damage depth with different incidence angles and ion energies. The result shows that sputtering damage depth increases with the increase of ion energy, while increasing incidence angle can decrease the sputtering damage depth. For IBS process with low energy (<2keV), the sputtering damage depth is general less than 10nm. The vacancy and displacement induced by ion collisions will lead to the structure change of lattice, which is the main cause of chemical structure defects. Figure 3 shows the depth distribution of vacancy number produced by the vertical incidence of Ar + ions with different energies in fused silica. Obviously, the vacancy number increases with the increase of ion energy, and the peak value appears at the end of the ion range. Simulation results indicate that the chemical structure defects induced by low energy IBS cannot be neglected.
3. Sample preparation
The fused silica samples (Heraeus 312) with a size of 50 × 50 × 10 mm3 were polished by the same vendor. Three samples (marked as #1, #2 and #3) were firstly ultrasonically etched for 120min with 5% HF acid solution, which has been optimized by our research group . Next, the samples were washed in deionized water and soaked in water under ultrasound for 30min. Then the samples were dried by absolute alcohol. All IBS experiments were performed in our self-developed IBS system under the bombardment of Ar + ions at normal incidence. Within the experiments, the sputtering conditions are fixed at ion energy Eion = 900eV, beam current Jion = 5mA. Both the two surfaces of sample #1 were sputtered with the same removal depth of 300nm, 900nm, 1500nm and 2000nm before damage performance test. An R-on-1 procedure was used in the LIDT test in which a ramping fluence focused on a single area until damage was registered on the imaging CCD. Ten positions were chosen randomly during each LIDT test and taken the average value as the final damage threshold. A table-top 3ω Nd: AG laser with pulse width of 7ns operated at a repetition rate of 1Hz was employed in the test. The spatial beam distribution was almost flat Gaussian with diameter of ~1.2 mm. The laser beam was focused on the backside of the sample. Sample #2 was sputtered with different removal depths for the measurement of surface roughness and weak absorption. Sample #3 was sputtered with different removal depths for the spectral analysis.
4. Experimental results and analysis
4.1 Damage performance test
The LIDT test results with different IBS removal depths are shown in Fig. 4. The LIDT increases gradually with the increase of IBS removal depth. The maximum LIDT (10.5 ± 0.3 J/cm2) is obtained when the IBS removal depth reaches 900nm, which is 30% higher than that of HF etched surface (8.1 ± 0.3 J/cm2). However, with the further increase of IBS removal depth, the damage threshold decreases to the level of the optimized HF acid etching treatment. In addition, it is found that the damage threshold is much lower than that of IBS surface after the sample is immersed in deionized water for 30min. In the three contrast experiments, the damage threshold reduces by 21.2%, 22.9% and 14.5%, respectively. As a result, there are 3 problems worthy of further study in the LIDT experiment:
- (1) What is the improvement mechanism of LIDT by IBS? Previous studies on the LIDT improvement by IBS were focused on the removal of SSD layer and the improvement of surface quality. In this experiment, we use various detection methods to systematically study and characterise the improvement mechanism of LIDT by IBS.
- (2) What is the reason leading to the decrease of LIDT with the further increase of IBS removal depth? Previous researches on the LIDT improvement by IBS were mainly focused on the shallow surface, in which the IBS removal depth was generally no more than 1000nm. The lack of further researches about the influence law of IBS on fused silica surface, especially the mechanism of laser damage resistance weakened by IBS, has not yet been clearly understood.
- (3) What is the reason leading to the LIDT reduces by about 20% when the IBS surface is immersed in deionized water? It is the first time found in experiments that the LIDT of IBS surface reduces significantly after immersed in water. This means that the sputtered surface has some changes after contacting with water, which leads to the decrease of laser damage resistance. To reveal the mechanism of this change is very important for the optimization of IBS process.
4.2 Surface roughness analysis
The ScanAsyst mode of Bruker atomic force microscope (AFM) was used to measure the surface roughness in different removal depths and 3 positions were selected randomly to calculate the average roughness for each region. The scanning size is 10µm × 10µm. The typical surface microtopography evolution of fused silica during IBS process is shown in Fig. 5. As can be seen from Fig. 5(a), the original polished surface is relatively smooth with a roughness of 0.36nm RMS. There are no other obvious manufacturing defects in the field of vision except for subtle polishing traces. However, after the optimized HF acid etching treatment for 120min, the surface quality deteriorates seriously because of the enlarged and extended defects such as scratches and pits. Accordingly, the surface roughness increases dramatically from 0.36nm RMS to 1.24nm RMS, as shown in Fig. 5(b). The surface quality improves significantly with the increase of IBS removal depth, and the roughness value decreases sharply to a level [0.54 nm RMS, Fig. 6(e)] close to the original surface. Figure 5(f) displays the corresponding change of surface roughness. Previous researches have confirmed that the optimized HF acid etching can effectively remove the SSD layer of fused silica optics, while the IBS can further remove the redeposited reaction product and improve the surface quality .
4.3 Weak absorption analysis
The absorption loss of high power laser optics causes the deposition of laser energy on surface, which is one of the key factors leading to laser damage. As a nondestructive testing method, the weak absorption detection is directly related to the damage precursors, and reflects the damage resistance performance of the high power laser optics . The detection system used in experiment is PTS-2000-RT-C made in China, and the detection sensitivity is better than 0.1ppm. The pump laser wavelength is 355nm, with a pump power of 3.91W, and the test area is 3mm × 3mm. Three regions are selected randomly to calculate the average weak absorption intensity before the LIDT test.
The weak absorption intensity can be well matched with the damage precursor. If there is a damage precursor on the surface, a absorption peak will appear correspondingly. Figure 6 shows the weak absorption evolution during different treatment processes. Since there are many highly absorptive photoactive impurities in the polishing redeposition layer (e.g., Ce, etc) and SSD layer, the weak absorption intensity of the polishing surface is the strongest, with an average absorption intensity of 5.2ppm, and a peak value of 360ppm, as shown in Fig. 6(a). After HF acid etching for 120min, with the complete removal of the polishing redeposition layer and the partial removal of SSD layer, the highly absorptive photoactive impurities are significantly reduced. As a result, the weak absorption intensity decreases obviously. The average absorption intensity of the chemically etched surface is 3.0ppm, with a peak value decreases rapidly to 30ppm, as shown in Fig. 6(b). Figures 6(c)-6(e) show the weak absorption distribution of the IBS surface with a removal depth of 400nm, 700nm and 1000nm, respectively. The figure shows that the highly absorptive photoactive impurities on the etched surface are further removed by IBS. Accordingly, the weak absorption intensity is becoming weaker and weaker. At last, the average value drops to 0.2ppm, and the peak value decreases rapidly to 2.5ppm. Figure 6(f) shows the change of weak absorption intensity during different treatment processes, which fully demonstrates the advantage of IBS as a post-processing treatment in removing the absorptive defects.
4.4 Raman spectra analysis
In this paper, the structure changes of fused silica during different processes are studied by Raman spectra analysis. A Bruker Senterra confocal Raman spectrometer with a excitation laser of 532nm was used for Raman spectra measurement. Figure 7 shows the typical Raman spectra of fused silica after HF acid etching for 120min, IBS removal of 600nm and 1500nm, respectively. These Raman spectra consist of a series of broad bands reflecting the coupled vibrational modes of the silica random network . In addition, Fig. 7 shows two relatively sharp bands (D1 and D2) centered on ~490 (D1) and ~606 (D2) cm−1, which have previously been assigned to in-phase breathing motions of oxygen atoms in puckered 4- and planar 3-membered ring structures, respectively . It is obvious that the relative intensities of the D1 and D2 lines firstly decrease when the HF etched surface is sputtered with IBS removal depth of 600nm, and then significantly increase when the IBS removal depth increases to 1500nm. Such an increase in the number of small-membered rings indicates sputtering-induced material densification . This result is also consistent with previous reports on the densification of a-SiO2 by x-ray, γ-ray, electron, ion, neutron, and low-intensity laser beams . Compared with the HF etched surface, the peak intensity of D1 and D2 lines decrease after the IBS removal of 600nm, indicating that the densification degree of fused silica is reduecd. This maybe because the subsurface deformation layer formed by the polishing pressure is gradually removed by the chemical etching and physical ion sputtering, and the newly obtained fused silica surface is closer to its intrinsic surface. Therefore, the densification degree is reduecd after the ion sputtering. However, when the IBS removal depth increased to 1500nm, the peak intensity of D1 and D2 lines are significantly enhanced, even stronger than that of the HF etched surface. This indicates that the densification degree is enhanced with the increase of IBS removal depth. This maybe because the ion bombardment effect for the newly obtained intrinsic surface, so that the peak intensity of D1 and D2 lines is enhanced significantly.
4.5 Fluorescence spectra analysis
The French JY TAU-3 fluorescence spectrometer with a excitation laser of 248nm was used to analyze the chemical structure defects evolution under different treatment processes. We used a 320nm long-pass filter to collect fluorescence spectra for wavelengths above ~400nm in order to avoid the influence of the second diffraction order of the 4.4eV PL band .
Figure 8 shows the characteristic changes of fluorescence spectra of fused silica after HF acid etching for 120min, IBS removal depth of 600nm and 1500nm, respectively. It can be seen that the fluorescence spectra has two characteristic peaks at the wavelengths of 300nm to 800 nm, representing the oxygen vacancy defects ODC (~400 nm) and the non-bridge oxygen defect NBOHC (~650 nm),respectively [22,23]. When the HF etched surface is removed 600nm by IBS, the peak fluorescence intensity of ODC and NBOHC decreases obviously, which indicates that the chemical structure defects in SSD layer is reduced by IBS. The mechanism of HF acid etching is to break the three-dimensional silicate network structure formed by the combination of ≡Si-O-Si≡ (siloxane) bonds using HF, HF2- and H+ in HF acid solution. Although HF acid etching removes the SSD layer, but a new surface with chemical structure defects has been formed during the etching process. And the following IBS just removes these chemical structure defects formed during HF acid etching. Therefore, the fluorescence intensity of ODC and NBOHC decreases. However, with the IBS removal depth increases to 1500 nm, the fluorescence intensity appears to be enhanced. This means that new chemical structure defects have been produced due to sputtering damage of fused silica. When the polished surface is etched by HF for 120min, the material removal depth is about 5μm, and then the IBS removal depth is about 1μm. Thereby the SSD layer has been removed obviously, and the newly obtained surface is closer to the intrinsic surface. The simulation analysis of sputtering damage in Section 2 indicates that a sputtering damage layer with a depth of several nanometers is formed on the surface of fused silica. The chemical structure defects such as lattice dislocation, vacancy and displacement are produced, resulting in the increase of fluorescence intensity. Reference  pointed out that ODC and NBOHC defects can form in SiO2 under a variety of ionizing radiation conditions such as various particle and electromagnetic radiation.
4.6 Influence analysis of OH- groups
Infrared spectroscopy is the main means to study the structure characteristics of hydroxyl group (OH- group) in fused silica, which has an absorption peak near 3660cm−1 in infrared spectra. A Bruker Vertex70 high resolution fourier transform infrared spectroscopy (FTIR) was used to detect the surface hydroxylation process during different treatment processes.
As shown in Fig. 9, the hydroxyl absorption peak intensity of fused silica sample after IBS removal depth of 600nm and 1000nm decrease to some extent compared with that of the HF etched surface. After IBS removal depth of 1000nm, the sample was soaked in deionized water for 30min, then dried by high pressure nitrogen gas. After that, the infrared spectra was measured immediately. It can be seen from Fig. 9 that the hydroxyl absorption peak intensity is obviously enhanced after the immersed in water, even stronger than that of HF etched surface. The HF etched surface shows a strong hydroxyl absorption peak is mainly due to the etching process occurring in the water environment. The fresh surface is easy to react with the surrounding water molecules, and thus forming lots of OH- groups on its surface. The chemical reaction process can be expressed by Eq. (2) . The hydroxylation layer is removed after IBS removal depth of 1000nm, leading to the decrease of the hydroxyl absorption peak intensity. However, the chemical activity of Si and O atoms is enhanced by ion sputtering, and thus the probability of reaction between surface unsaturated structure and water molecule increases significantly. When the ion-sputter-treated sample is completely immersed in deionized water, the hydroxylation process is accelerated and the concentration of OH- groups increase further. According to the infrared spectral transmittance curve shown in Fig. 9, the intensity of OH- groups increases by 18% after immersed in deionized water for 30min as compared with the ion-sputter-treated surface .
The OH- group can be chemisorbed on the glass surface and destroy the original space network structure of fused silica, leading to the decrease of surface mechanical strength and laser damage resistance . Therefore, the ion-sputter-treated surface should be strictly prevented from contacting with water environment in order to avoid surface hydroxylation.
In order to further reveal the damage mechanism of the hydroxylation layer to the laser damage resistance, the weak absorption distribution of fused silica surface before and after immersed were measured. Figure 10 displays the weak absorption distribution of three different regions during different processes. The corresponding treatment processes were HF acid etching for 120min, IBS removal depth of 1200nm and the following immersed in deionized water for 30min.
Figure 11 shows the change of weak absorption intensity during different processes. The weak absorption intensity of the HF etched surface reduces obviously after IBS removal depth of 1200nm, this is because the removal of residual redeposited reaction contaminants and hydroxylation layer on the surface. This also verifies the cleaning effect of IBS on the surface. However, after the ion sputtered surface is immersed in deionized water, the weak absorption intensity is significantly enhanced, even stronger than the HF etching surface. The reason is because of the highly absorptive products introduced in the hydroxylation process. Comparing the weak absorption distribution of the same area after different treatments in Fig. 10, it is obvious that the weak absorption distribution of HF etched surface is relatively uniform, and there are almost no absorption peaks, while there are many absorption peaks on the ion sputtered surface. Although ion sputtering can reduce the overall weak absorption intensity of HF etched surface, the chemical structure defects caused by ion sputtering accelerate the hydroxylation process, thus further enhance the weak absorption intensity of corresponding regions. Comparing the absorption peaks in Figs. 10(b) and 10(c), it can be found that the weak absorption regions after water immersed have a one-to-one correspondence with those of the sputtered surface. This is also consistent with the conclusion that the hydroxylation reaction is more likely to occur with the unsaturated structures formed during ion sputtering.
Based on the above analysis of the experimental results, we try to answer the 3 questions raised in section 4.1.
- (1) Improvement mechanism of damage threshold by IBS process. Firstly, the residual contaminants produced during HF acid etching can be removed by IBS, resulting in a significant reduction in weak absorption intensity [Fig. 6]. Secondly, due to the ion-induced smoothing effect, the surface micromorphology deterioration caused by HF acid etching is suppressed, and the surface quality is improved by removing the surface micro-nano-scale defects, and the passivation of scratches and corrosion pits. [Fig. 5]. Thus, the effect of laser scattering or coupling into longitudinal plasma oscilation caused by the rougher surface is weakened. Thirdly, IBS can further weaken the surface densification [Fig. 7] and reduce the chemical structure defects concentration such as the ODC and NBOHC [Fig. 8] by removing the subsurface deformation layer. In addition, IBS can further reduce the absorption intensity by the removal of the hydroxylation layer formed during HF acid etching process [Fig. 10]. All these factors contribute to the continuous improvement of LIDT of the HF etched surface.
- (2) Decrease mechanism of damage threshold due to the increase of IBS removal depth. Although the surface roughness and weak absorption intensity are further reduced with the continuous increase of IBS removal depth, the LIDT appears to decrease or even lower than the HF etched surface. Based on the spectral analysis related to SiO2 structure, we believe that it is these chemical structure defects induced by the ion sputtering damage that lead to the decrease of damage threshold. This is similar to the influence law of reaction ion etching (RIE) process on laser damage resistance of fused silica . Chemical structure defects, which belong to typical high threshold damage precursors with an atomic scale intensive distribution, would cause rapid material heating and melting so that laser damage occurs . Although IBS gradually removes the subsurface damage, making it closer to the intrinsic surface, continuous ion sputtering will lead to the amorphization of surface structure, the enhancement of surface densification [Fig. 7], and the concentration increase of lattice dislocation, vacancy and displacement, and thus resulting in the decrease of damage threshold.
- (3) Mechanism of damage threshold decrease after the ion sputtered surface immersed in deionized water. Ion sputtering leads to the separation of lattice atoms from original positions, forms a large number of unsaturated structures on the surface layer, and also improves the chemical activity of Si and O atoms, thereby accelerating the chemical reaction of surface atoms with water molecules. This leads to the hydroxylation layer as well as highly absorptive products [Fig. 10]. The presence of OH- groups reduce the bonding strength of Si-O and destroy the space network structure of substrate, which reduces the mechanical strength of fused silica and leads to the decrease of laser damage resistance.
In this paper, surface characteristics evolutions during IBS process and their influence law on the LIDT of fused silica have been experimentally and theoretically investigated. Damage test results reveal that the damage resistance of HF etched surface can be enhanced about 30% by appropriate IBS removal depth (less than ~1000nm). The above measurement results present that the IBS process can create a pure sample surface by removing the redeposited reaction products during HF acid etching process, improving surface quality and reducing chemical structure defects concentration. However, further sputtering often results in a decrease rather than an increase of damage threshold with enhanced surface densification and increased chemical structure defects of ODC and NBOHC which generated from sputtering damage. Moreover, the sputtered surface will accelerate the chemical reaction of surface atoms with water molecules. The newly obtained hydroxylation layer rich in highly absorptive products can result in the decrease of laser damage resistance. The studies further clarify the improvement mechanism of laser damage performance by IBS process, and also provide important technical guidance for optimizing the post-process to improve the LIDT of fused silica.
National Natural Science Foundation of China (NSFC) (No.91323302, No.91523101, No. 51605476).
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