Laser induced damage of fused silica optics occurs primarily on optical surface or subsurface resulting from various defects produced during polishing/grinding process. Many new kinds of surface treatment processes are explored to remove or control the defects on fused silica surface. In this study, we report a new application of reaction ion etching (RIE)-based surface treatment process for manufacture of high quality fused silica optics. The influence of RIE processes on laser damage resistance as a function of etching depth and the evolution of typical defects which are associated with laser damage performance were investigated. The results show that the impurity element defects and subsurface damage on the samples surface were efficiently removed and prevented. Pure silica surface with relatively single-stable stoichiometry and low carbon atomic concentration was created during the etching. The laser damage resistance of the etched samples increased dramatically. The increase of roughness and ODC point defect with deeper etching are believed to be the main factors to limit further increase of the damage resistance of fused silica. The study is expected to contribute to the development of fused silica optics with high resistance to laser induced degradation in the future.
© 2016 Optical Society of America
Laser induced damage (LID) leading to performance degradation of optics has a huge impact on their development in the field of high energy density science including Inertial Confinement Fusion (ICF), high energy light sources, etc [1, 2]. Although the intrinsic damage threshold of bulk optical materials such as fused silica under UV irradiation can reach the value of over 100 J/cm2, the damage can easily occur on the surface when the optics are exposed to 355 nm, 3 ns laser pulses with energy density from 5 to 15 J/cm2 . Their performance degradation is closely related to the existence of both intrinsic defects and extrinsic ones which are produced during fabrication process or by environmental contamination [4, 5]. These damage precursors, which are responsible for the laser damage initiation, have been systematically studied and classified by their structures and mechanism how they interact with laser pulse. Generally, LID may occur with a high propensity caused by destructive defects, such as scratches and indentations in subsurface layer introduced by sorts of surface finishing processes [6–8]. Meanwhile, contamination defects in the form of impurity elements (e.g., Ce, Fe, etc.), which are left by the polishing process, or introduced through environmental contamination, can lead to enhanced laser absorption and thus decrease the damage threshold [9, 10]. In addition, chemical structure defects associated with non-bridging oxygen hole center (NBOHC) and oxygen deficient center (ODC) are also connected with laser damage in fused silica [11, 12].
Over the past two decades, various finishing processes and surface treatments have been developed to eliminate these damage precursors [13–16]. For instance, ion beam etching (IBE) has been used to physically polish the surface of fused silica optics [17–19]. Unfortunately, different kinds of damage precursors as we mentioned are formed in near-surface region caused by the bombardment of high energy positive ions and have an adverse effect on the life time of the optics. Buffered HF solution has been widely used to chemically remove subsurface damage (SSD) layer of fused silica optics. Great progress has been made in improving LID resistance of fused silica optics [20–22]. However, further etching often results in a decrease rather than an increase of damage threshold with enlarged and extended defects such as scratches/indentations and pits in geometrical structures. Moreover, solubility and mass transport of redeposited reaction product (SiF62-) is difficult to be controlled easily during the etching [3, 22, 23].
Reactive ion etching (RIE) process has been widely used for the fabrication of micromechanical line structures with high fidelity of pattern transfer for silicon, silicon dioxide, etc. In fact, RIE technique also possesses the potential to improve the damage performance of fused silica optics owing to its capability of providing high anisotropic etching profiles with good selectivity [24–27]. However, no study has been performed on how to improve the damage resistance of fused silica by using RIE process. The main problem is that the optical surface is contaminated by carbon residue from the dissociation of fluorocarbon reactant gas which is introduced during the fluorocarbon plasma etching process . Fortunately, we have made great efforts and succeed in controlling the carbon contamination efficiently by optimizing the parameters of the etching process.
In this paper, an overall research was undertaken on the performance of polished fused silica optics by the optimized RIE treatment. Section 2 is devoted to description of RIE mechanism of fused silica. In Section 3, we give some information on sample manufacturing. Damage performance test and multiple characterizations results are presented in Section 4. Section 5 were employed for discussion of the laser damage thresholds with respect to the analytical results. The conclusion of the work is given in Section 6.
2. RIE mechanism of fused silica
RIE is a chemical-physical etching process for microstructures with several unique advantages mentioned above. High anisotropy is attributed to the bombardment on the substrate surface by the ions being accelerated through the plasma sheath to assist the etching of the Beilby and subsurface layers where structured defects are located; High selectivity is due to the diversity of the ion sputtering energy threshold of different material types, as well as different chemical reactivities of various radicals and neutral plasma species which interact with plasma-modified subs rate surface to form volatile etch products . These products are then pumped away from the etched surface with the help of physical bombardment of energetic ions. The surface properties of fused silica after modification depends on a great number of parameters such as reactant gas type, gas flow rate, input power and working pressure, etc.
Trifluromethane (CHF3) is commonly used as reactant gas in plasma discharge for SiO2/Si etching. Significantly CHF3-contained plasma has higher selectivity of SiO2 over Si than other plasma such as CF4-contained plasma because polymer film deposition (including C: F film) can be controlled more flexibly and efficiently . As important initial neutral species, CF3 and H originate from collisions between electron and CHF3. Hydrogen atoms continually react through F abstraction reactions to form HF acid as well as unsaturated C bonds . CF2 radicals are formed as precursor of polymer film deposition, and (CF3, F) radicals participate in the etching process [32–34]. The etching of fused silica (SiO2) in CHF3 plasma mainly results in products like SiF4, CO2, COF2 and H2O. Actually, CHF3 plasma consists of a complex web of gas-phase and gas-surface reaction forming a variety of reactive neutral radicals, stable molecules, ions, volatile etch products, surface polymer film, etc [31, 35]. Ar gas is often added to make the plasma more stabling and simultaneously cooling the mixture. Furthermore, the introduction of Ar gas significantly changes the electron energy distribution and reactive species composition. Moreover, material surface can be smoothed by enhanced anisotropic etching due to strong energetic ion bombardment.
3. Samples manufacturing
Four 50 mm wide and 5 mm thick square fused silica samples (Coring 7980) were polished by the same vendor. All samples (marked as 1, 2, 3, and 4) were firstly cleaned with deionized water and allowed to air dry. Then sample 2, 3 and 4 were treated by RIE process with etching depth of 1 µm, 5 µm and 15 µm respectively in a parallel plate discharge etcher using CHF3-Ar gas mixture. The input power and the chamber pressure were 200 W and 20 mTorr, respectively. The etching rate was controlled to be roughly 2 µm/h. Sample preparation methods are given in Table 1.
4.1. Damage performance measurement
Laser induced damage threshold (LIDT) of the samples was assessed using small beam R-on-1  test in a laser damage test facility with a frequency-tripled Nd-YAG laser. The wave length, pulse width, and laser repetition rate were 355 nm, 9.3 ns (FWHM), and 1 Hz, respectively. The spatial beam distribution was flat Gaussian with diameter of 3 mm. During the test, twenty test positions in the whole sample surface were chosen randomly. With increasing laser power, damage thresholds would be obtained statistically by laser irradiating the same position of the sample surfaces with the same time span and power increment (about 0.5 J/cm2). The relationship between damage threshold and probability was fitted linearly. Then we achieved the 0% and 100% probabilities damage thresholds by extrapolating the fitting lines. The uncertainty of the energy density in the test system is 4.5%. Before the test, all the samples were rinsed using deionized water and allowed to air dry.
The laser damage probability and surface morphology of the initial damage sites are shown in Fig. 1 and Table 2. The results show that the laser damage resistance is improved dramatically with the etching process. Compared with the unetched sample (Sample 1), the damage threshold of 0% probability of the 1-µm-etched sample (Sample 2) has no obvious increasement and the laser damage threshold of 100% probability increases from 8.2 to 10.1 J/cm2. When the etching depth increases to 5 µm (Sample 3), the laser damage threshold of 0% and 100% probability both have a significant increase. As the etching depth increases to 15 µm (Sample 4), the laser damage threshold of 0% probability continually increases from 6.1 to 7.3 J/cm2. However, the laser damage threshold of 100% probability turns back to 10.2 J/cm2. The results suggest that there might be a best etching depth between 5 and 15 µm for laser damage performance.
In terms of the surface morphologies of the initial damage sites in Fig. 1 (top inset), it can be noted that the size of the damage craters is strongly influenced by etching depth. The appearances of samples with etching depth of 5 and 15 µm become larger (50-100 µm) and more typical in scale compared with the unetched sample. Moreover, some adjacent damage sites with smaller size (<10 µm) adhering to the damage crater centers are observed on the surface of the unetched and 1-µm-etched samples.
4.2. Surface contamination analysis
Contamination of the redeposition layer as a function of etching depth was investigated using an IONTOF TOFSIMS IV apparatus. The depth profiles of impurity elements in redeposition layer were detected by monitoring the entire mass spectrum. Before the detection, all the sample surfaces were first cleaned through sputtering for twenty seconds to remove the contamination induced by surrounding.
The depth profiles of impurity elements of the samples at various etching depths are shown in Fig. 2. The data have been normalized with silicon particle number (counts 10,000) as a standard. Figure 2(a) presents the depth profiles of impurity elements of the unteched sample surface, which is only pitch-polished conventionally. There are a large amount of photoactive mental impurity elements in the redeposition layer of the sample, including Ce, Fe, Al and Mg etc. The peak concentrations of these impurity elements are located on the surface and fall down with the increase of the detecting depth. Compared Figs. 2(b) and 2(c) with Fig. 2(a), all the impurity element concentrations decrease dramatically with the increase of etching depth. The peak concentrations of the impurity elements decrease by 3 orders of magnitude (from ~105 to ~102). Furthermore, in Fig. 2(d) the peak concentrations of all the impurity elements decrease down to ~101 and then keep stable at ~1 after 15 µm etching treatment. The results suggest that the impurity element defects can be effectively eliminated after the RIE treatment.
The eliminating degree of the impurity element contamination is strongly etching depth dependent. The total amounts (0-12 nm) of the impurity elements of the samples in the surface were calculated statistically in Table 3. With the increase of the etching depth, there are rarely impurity elements (especially Ce, Fe, Al and Mg) in the redeposition layer, suggesting that deeper etching can create a cleaner and purer silica surface compared with the unetched sample. Moreover, there is no additional impurity element produced during the etching.
4.3. SSD analysis
All kinds of subsurface damage formed during surface finishing process could not be thoroughly removed after final polishing process. Residual micro cracks might still embedded under the polishing layer. Photoluminescence can happen under laser irradiation when quantities of photoabsorption impurities embed in the micro cracks. Confocal fluorescence microscope was herein used to indirectly reflect the SSD since the photoabsorption impurities can be featured in fluorescence image under laser irradiation [6, 37]. The fluorescence defects aforementioned were detected in the 410-488 nm spectral band for an excitation wavelength of 405 nm and 20X objective. The detecting depth of all the samples is constantly 10 μm.
Figure 3 shows the fluorescence defects detected by confocal fluorescence microscope. The results of Figs. 3(a)-3(d) are for sample 1, 2, 3 and 4, respectively. Without etching (Sample 1), the subsurface fluorescence defect density is higher than the etched ones. With etching depth of 1 µm (Sample 2), the fluorescence defect density begins to decrease. As the etching continually increase (Sample 3 and 4), the fluorescence defect densities nearly drop to 0. The percentage of fluorescence defect area as a function of etching depth is shown in Table 4. The SSD area percentage decreases from 0.064% to 0.000% when the etching depth increases from 0 µm to 15 µm, suggesting the SSD of the fused silica samples can be removed efficiently when etching depth arrives at a certain level.
According to the fluorescence defect results detected by confocal fluorescence microscope, we also evaluated the density of SSD on the fused silica sample surface by performing a statistical analysis of fluorescence defects number with respect to different defect area ranges. As illustrated in Fig. 4, there are a great number of small scale defects with area range from 0 to 6.25 µm2 on the unetched and 1-µm-etched sample surfaces. The slight lift of the density from 9.92 to 11.36 × 10−6 µm2 is probably due to the decrease of the bigger scale defect area with etching depth of only 1 µm. The defect density in this scale range decreases sharply with the increase of etching depth. The corresponding densities of larger scale (range from 6.25 to 25 µm2) defects on the sample surfaces are greatly low. In the area range from 25 to 62.5µm2, the defect density of the unetched sample is as low as 0.16 × 10−6/µm2 and no visible defect is observed on all the etched sample surfaces.
4.4. Surface roughness analysis
White light interferometer was used to investigate the surface roughness and microscopy of the samples with different etching depths. The evolutions of the RMS roughness and the microscopies of sample surfaces were illustrated in Fig. 5 and Fig. 6. The RMS roughness of the unetched sample surface is about 0.56 nm. After the samples are exposed in the CHF3-Ar plasma at removal depth of 1 µm and 5 µm, the roughness values decrease to about 0.5 nm. It is probably due to the removal of the surface structure defects such as scratches and pits so that the optical surfaces are smoothed. However, when the etching depth increases to 15 µm, the roughness value increases to about 0.66 nm. And from the microscopes images in Fig. 6, it can be observed that the surface becomes rougher and more irregular with etching depth of 15 µm. The results indicate that there may be a best etching depth between 5 and 15 µm, which agrees well with the LIDT results.
4.5. Chemical structure analysis
X-ray photoelectron spectroscopy (XPS) was used to investigate stoichiometry and chemical bonding of the Si and C on the surfaces which are generally sensitive to local electronic environment. The X-ray source was a focus monochromatic Al Kα beam (150W) with an analysis area of 3 mm2 on the sample surfaces. The slot mode was chosen for scanning (300 × 700 μm elliptic region) with the pass energy of 20 eV and the step length of 0.05 eV. Before the XPS measurement, Ar ion beam was used to sputter-clean the top carbon contamination layer for 100 s (approximately equal to the removal of 1 nm of Ta2O5). Figure 7 illustrates the XPS Si 2p spectra of the etched and unetched sample surfaces. The XPS Si 2p peaks are fitted using Gaussian peak function to identify the chemical species. Note that the Si 2p peaks for all the samples are assigned to relatively stable-single O = Si = O bonds on the surfaces [38, 39] indicating that there is no obvious change in chemical composition of the fused silica sample surfaces during the etching process.
We also evaluated the C atomic concentrations of the sample surfaces according to the XPS core level spectra, as shown in Table 5. The C atomic concentrations slightly decrease with the increase of etching depth. The surface with the etching depth of 15 µm presents a relatively low atomic concentration of C. It indicates that optimized etching process can efficiently control the carbon contamination on the sample surface.
4.6. Fluorescence spectra analysis
Laser-induced fluorescences (PL) of the sample surfaces were examined by means of a fluorescence spectrometer furnished with a photomultiplier used as a detector and a Xe lamp used as an excitation source. The slit width for exciting and emission was 15 nm and 20 nm, respectively. The emission spectra were excited with a feak centered at 5.0 eV (~280 nm). As shown in Fig. 8, the FL emission spectrum of unetched sample surface has two peaks centered at 390 nm and 540 nm, which are attributed to the typical defects: ODC and self-trapped excition (STE), respectively. The ODC intensity decreases dramatically and the STE peak disappears after the etching treatment. However, the sample surface with etching depth of 15 µm presents a higher ODC intensity than other etched sample surfaces. The results indicate that there may be a best etching depth between 5 and 15 µm, which is in great agreement with the LIDT and roughness results.
The presence of both extrinsic and intrinsic defects in the fused silica surface can lead to absorption and subsequent damage initiation under UV illumination. As observed above, reaction fluorocarbon-based etching method is an attractive method to remove or prevent the identified surface precusors. In this section, two aspects are discussed to describe the laser damage thresholds with respect to the observed results by different analytical techniques. Firstly, what’s the reason for the increasement of damage threshold during RIE process and which kind of damage precursors are primarily responsible for that with different etching depths? Secondly, why the laser damage performance of 100% probability decreases when the etching depth increases from 5 to 15 µm?
The laser damage probability strongly depends on the etching depth, as shown in Fig. 1 and Table 2. Generally, damage threshold of 0% probability tends to reflect damage initiation induced by low threshold damage precursors with a discrete distribution (i.e. impurity element contamination or SSD); damage threshold of 100% probability tends to reflect damage initiation induced by high threshold damage precursors, which may locate on the intrinsic material surface with an intensive distribution (i.e. C deposition or ODC).
Impurity element contamination, especially Ce element which is known for its susceptibility to light, is responsible for the laser absorption and damage initiation [9, 40]. Furthermore, SSD would lead to enhanced laser absorption and thus decrease the damage threshold [7, 8]. Table 2 shows that there is no obvious increasement in 0% probability damage threshold with only 1 µm etching treatment, suggesting that the 1 µm removal amount is insufficient and there are more defects below the surface. On the other hand, the difference of damage morphologies observed in Fig. 1 (top inset) suggests that the damage craters with small scale on unetched and 1-µm-etched sample surfaces are induced by low threshold precursors such as impurity element contamination and SSD. Significant changes in damage threshold occur when the etching depth is 5 µm. And compared with the unetched sample, the 0% probability of damage threshold is increased by 29.8% and the 100% probability of damage threshold is increased by 40.2%. In this case, the peak concentration of the impurity elements in redeposition layer is decreased by 3 orders of magnitude (see Fig. 2) and the SSD area percentage is decreased by 78.2% (see Table 4). It indicates that the low threshold damage precursors are removed efficiently at this etching depth. Therefore, we believe that the removements of the impurity element contamination and SSD are the main contributions to the improvement of damage performance.
When the etching depth increases from 5 to 15 µm, the 0% probability damage threshold continually increases by 19.7% (see Table 2). In the case, although the peak concentration of the impurity elements continually decreases to 4 orders of magnitude (see Fig. 2), the corresponding SSD area percentages are almost unchanged (see Table 4). This phenomenon suggests that SSD at this depth is not the dominant factor influencing the 0% probability damage threshold. On the other hand, the 100% probability damage threshold decreases apparently with 15 µm removal amount. Accordingly, the observed damage crater with higher scale suggests that the damage initiation is induced by high threshold damage precursors. Thus, it can be concluded that there must be some kinds of high threshold damage precursors that limit the further increase of the 100% probability damage threshold and induce the damage initiation. Experimentally, a higher ODC peak is observed on the sample surface with etching depth of 15 µm (see Fig. 8). ODC, which belongs to a typical high shreshold damage precursor with an atomic scale intensive distritution, would cause rapid material heating and melting so that laser damage occurs . We therefore believe it is a dominant factor that limits the further increase of the 100% probability damage threshold. In addition, we have observed an slight increase of roughness when the etching depth is 15 µm. Previous investigators have demonstrated that there is a strong dependence of damage threshold on surface roughness . Rougher surface may induce laser scattering or coupling into longitudinal plasma oscilation. Moreover, it may increase the exposed area for a given irradiated area on which obsorbing inclusions, surface or subsurface defects can reside. Thus, the deterioration of surface roughness may be a sencondry factor that limits the further increase of the 100% probability damage threshold.
RIE is an attractive method to remove and prevent two types of key precursors: impurity element contamination in redeposition layer and SSD in polishing layer, which are associated with destructive absorption of sub band-gap light and subsequent damage initiation under UV illumination. From the above XPS results, it presents that the optimized RIE process can create a pure sample surface with relatively single-stable stoichiometry and low carbon atomic concentration. Damage test results reveal that the damage resistance of etched fused silica samples is enhanced dramatically. The increase of ODC point defect and roughness with deeper etching (~15µm) are the main factors to limit further increase of the damage resistance of fused silica. The results ultimately clarify laser induced damage mechanism and are beneficial for improving damage resistance of fused silica under UV pulse laser irradiation.
The authors wish to thank Jin Huang team for assistance in sample preparation and execution of the experiments. We are also grateful to Miao Liu and Xibin Xu for helpful discussions. The authors acknowledge financial support by the Development Foundation of China Academy of Engineering Physics under Grants 2013A0302016, the National Natural Science Foundation of China (NSFC) under Grants 61078075 and Key Laboratory of Precision Manufacturing Technology, CAEP, ZZ13021.
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