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Abstract
The laser-induced damage of ultraviolet fused silica optics is a critical factor that limits the performance enhancement of high-power laser facility. Currently, wet etching technology based on hydrofluoric acid (HF) can effectively eliminate absorbing impurities and subsurface defects, thereby significantly enhancing the damage resistance of fused silica optics. However, with an increase in the operating fluence, the redeposition defects generated during wet etching gradually become the primary bottleneck that restricts its performance improvement. The composition and morphology of redeposition defects were initially identified in this study, followed by an elucidation of their formation mechanism. A mitigation strategy was then proposed, which combines a reduction in the generation of precipitation with an acceleration of the precipitation dissolution process. Additionally, we systematically investigated the influence of various process parameters such as extrinsic impurity, etching depth, and megasonic excitation on the mitigation of deposition defects. Furthermore, a novel multiple-step dynamic etching method was developed. Through comprehensive characterization techniques, it has been confirmed that this new etching process not only effectively mitigate redeposition defects under low fluence conditions but also exhibits significant inhibition effects on high fluence precursors. Consequently, it significantly enhances the laser damage resistance performance of fused silica optics.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Due to its exceptional optical performance, fused silica optics have found extensive applications in large-scale high-power laser-driven systems, such as inertial confinement fusion and high-energy density science experimental platforms. However, the surface of fused silica is prone to laser-induced damage under intense ultraviolet laser irradiation, which significantly restricts its operational lifespan and overall performance. Despite the remarkable advancements achieved by researchers in enhancing the laser damage resistance of fused silica, the surface damage threshold of fused silica optics (at 355 nm@3 ns laser irradiation <20 J/cm2) still falls significantly below its intrinsic threshold (∼100 J/cm2). The introduction of inevitable defects during the initial processing of fused silica optics (such as grinding and polishing) is the reason behind this phenomenon. These defects would absorb sub-bandgap laser energy, resulting in localized high temperature and pressure, thereby initiating stress generation. Consequently, material melting, or explosive damage occurs, leading to the formation of a destructive absorption wavefront [1,2]. Therefore, the laser-induced damage of fused silica poses a critical bottleneck that restricts the enhancement of load capacity in laser devices. Consequently, addressing the challenge of augmenting the laser damage resistance performance of fused silica has emerged as a significant hurdle for high-power laser facility.
Reputable researchers have been continuously unearthing diverse damage precursors that contribute to the occurrence of ultraviolet damage in fused quartz components for a span of fifty years. The surface defects of high-quality fused silica optics can be broadly categorized into two groups, as illustrated in Fig. 1: one pertains to polishing residues introduced during the polishing process, such as included impurity elements like Ce and Fe present in the Beilby layer [3]; while the other group encompasses fracture-related defects including subsurface microcracks, scratches, and pits that arise during precision machining or maintain processes [4]. The presence of sub-surface defect (SSD) is considered to be the primary contributing factor to laser-induced damage in components, as they have the ability to amplify electric field intensity, reduce material mechanical strength, and incorporate absorbing impurities [5–7].
In the past two decades, a variety of surface modification and post-treatment technologies for fused silica optics have emerged with the objective of enhancing laser damage resistance by eliminating or mitigating surface defects. These techniques include wet etching [8,9], plasma/ion beam etching [10,11], magnetorheological polishing [12], magnetic field-assisted polishing [13], and CO2 laser treatment [14,15]. The wet etching method utilizing aqueous hydrofluoric acid (HF) solutions, has emerged as the most prominent and extensively employed post-treatment technique for fused silica optics among the various methods available. The reaction between HF and fused silica enables the comprehensive removal of both the Beilby layer and sub-surface defects. To eliminate polishing residues within the Beilby layer, a shallow etching depth of approximately 100 nm is sufficient. However, complete elimination of the sub-surface defects requires adequate etching time and depth to fully expose microcracks, facilitating thorough penetration of the etchant into crack tips for complete removal. The increase in etching depth gives rise to the emergence of an additional type of defect, which impedes the laser damage performance of fused silica. This specific defect originates from the excessive redeposition of reaction by-products (hexafluoro silicate salts) during the wet etching process.
Inhibiting the redeposition defects is crucial for enhancing the laser damage resistance of fused silica through wet etching. The Lawrence Livermore National Laboratory (LLNL) in the United States has introduced the concept of Advanced Mitigation Process (AMP), which utilizes a dynamic wet etching technique based on hydrofluoric acid. By implementing a series of measures, effective inhibition of reactive byproduct redeposition is achieved, leading to significant improvement in laser damage resistance [8]. The measures encompass: (1) regulating the concentration of cations (NH4+) in the etchant to optimize the solubility of reaction by-products; (2) introducing megasonic/ultrasonic excitation during etching and rinsing processes to minimize boundary layer thickness and expedite material transport rate of reaction products; (3) effectively opening microcracks and scratches on the surface/subsurface to enhance the transport rate of reaction products at crack tips; (4) ensuring meticulous control over high-purity water and environmental cleanliness to minimize introduction of extrinsic impurities.
The dynamic wet etching technique has been widely applied at the National Ignition Facility (NIF) for processing fused silica optics, owing to its remarkable efficacy [16]. As a result, extensive research on the surface damage behavior of fused silica utilizing this method has become a significant subject matter globally. However, there are still certain issues that necessitate further comprehensive research. (1) The optimal etching depth typically chosen for LLNL's AMP process ranges from 28 to 35 μm [8,17]. However, it has been observed by some researchers that excessive deep etching beyond 10 μm often results in a decline in device performance, even when employing a similar process to AMP [18–21]. How can we determine the ideal etching depth? (2) The introduction of megasonic/ultrasonic excitation during the dynamic wet etching process has been reported by LLNL to effectively mitigate redeposition defects and enhance laser damage performance. However, Ye et al. observed that introducing megasonic excitation with a power density ranging from 0.6 to 2 W/cm2 did not have a significant impact on the damage performance [9,20]. Therefore, determining the optimal settings for megasonic excitation remains an unresolved question. (3) The presence of extrinsic impurities significantly impacts the occurrence of redeposition defects, necessitating the implementation of stringent control measures. Therefore, what level of control should be achieved?
In this study, we delineate the formation mechanism of redeposition defects during the wet etching process and propose an inhibition strategy that combines minimized precipitation generation and expedited precipitation dissolution. Through systematic exploration of influential process parameters, such as extrinsic impurity content, etching depth, and ultrasonic power, regarding their effects on the behavior of redeposition defects, we have developed a novel multiple dynamic etching technique that effectively mitigates these defects and significantly enhances the laser damage resistance performance of fused silica optics.
2. Experimental methods and materials
2.1 Sample preparation
The study integrated three different sizes of fused silica samples, namely small (50 mm×50 mm×4 mm), medium (100 mm×100 mm×10 mm), and large aperture samples (400 mm×400 mm×10 mm). All the samples were acquired from Chengdu Beirui Co.td., utilizing JGS1 as the material.
Prior to dynamic wet etching treatment, fused silica samples undergo a preliminary cleaning process. Firstly, the samples are securely mounted on a specialized cleaning fixture and immersed in a 10% detergent for 30 minutes. The cleaning temperature is maintained at 50 ℃, and ultrasonic stimulation is applied at frequencies of 40 KHz, 80 KHz, and 120 KHz. Subsequently, the fixture containing the fused silica sample is rinsed in a bath tank of ultra-pure water for initial rinsing. The megasonic overflow rinsing process lasts for 40 minutes, with a flow rate of 12 L/min and an excitation frequency of 1.0 MHz in sweep mode. Upon completion of the initial rinse, the fixture is promptly removed and submerged in the final rinsing tank for 60 minutes at a flow rate of 12 L/min. Subsequently, the fixture is withdrawn at a rate of 1 mm/s. After allowing the fused silica sample to air dry naturally for 30 minutes, the surface of the sample is meticulously examined with a powerful flashlight to identify any anomalous markings prior to initiating the dynamic acid etching treatment.
The fixture containing the fused silica sample was subsequently submerged into the acid bath for the purpose of dynamic wet etching. The acid solution exhibits an etching ratio of 1:6 for HF to NH4F. After undergoing threefold dilution, the final concentrations of HF and NH4F are approximately 2.5 wt% and 12 wt%, respectively. The megasonic excitation frequency in sweep mode is set at 1.0 MHz, while the etching reaction temperature is maintained at (20 ± 3) ℃. The etching rate of fused silica sample is approximately 3 μm/h, while the circulation flow rate of the etching bath stands at ∼35 L/min. The dynamic wet etching methods include two principal types: single-step etching and multiple-step etching. The former entails a prolonged process of 120 minutes, while the latter consists of three sequential stages – shallow/deep/shallow – with corresponding etch durations of 10 minutes, 100 minutes, and 10 minutes, respectively. The cumulative depth realized through these stages reaches a value of 6 μm. Following each etching process, the fused silica samples must undergo two rinsing procedures, namely the initial and final rinses, which employ identical methods to those utilized in the preliminary cleaning process.
2.2 Laser damage tests
According to the distinct sizes of fused silica samples, specific damage testing platforms are meticulously selected for their performance appraisal. Small-aperture samples are subjected to evaluation on a dedicated small-beam damage testing platform [22], whereas medium and large-aperture samples are assessed utilizing the Multipurpose Optical Damage Science System [23]. The assessment of damage density via specific fluence testing serves as the principal method for characterizing the laser damage performance of fused silica samples. The “raster-scan” damage testing method [24] is employed on the small-beam testing platform. The test beam, with a diameter of 1.4 mm (1/e2), features a horizontal step size of 1.2 mm and a vertical step size of 1.0 mm. The test array is configured at 25 × 30, with each test point undergoing irradiation twice, denoted as a “2-on-1” protocol. The total tested area amounts to approximately 9.0 cm2, exhibiting a beam modulation ratio (Peak/Average) of 2.3. The initial occurrence of surface damage on the sample can be detected in real-time using an online optical microscope, providing a resolution of approximately 2 μm. The Multipurpose Optical Damage Science System employs a 1-on-1 damage testing protocol with a beam modulation ratio of 2.0. For the medium-aperture samples, a test array of 7 × 7 is utilized, featuring an approximately 80 mm (1/e2) beam diameter. For the large-size sample, the platform uses a test array of 20 × 18, showcasing an approximately 90 mm (1/e2) beam diameter. The cumulative testing area for each medium-aperture sample and large-aperture sample is approximately 22 cm2 and 240 cm2, respectively. All damage testing fluence in this paper have been converted based on the pulse width of 3 ns [25].
2.3 Other characterizations
An atomic force microscope (AFM) (Dimension Icon, Bruker, Germany) was used to measure the morphologies of the etched surface of the samples. Scanning electron microscopy (SEM) (FE-1050, NCSMB) is employed for the characterization of surface deposits on samples. Prior to testing, the sample surface undergoes carbon evaporation treatment to generate a carbon film with a precise thickness of 5 nm. 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 photo-thermal absorption of the samples was characterized by a photo-thermal common-path interferometer. A surface scanning of a 60 mm × 60 mm test area with 100 μm step size, for 360000 points, was executed in the photo-thermal absorption test process.
3. Results and discussion
3.1 Identification and mitigation mechanism of redeposition defects
According to LLNL researchers, the primary precursors accountable for the laser damage at low fluence (<10 J/cm2) are the Beilby layer, subsurface defects (SSD), and the redeposition of reaction by-products. [26] The Beilby layer typically exhibits a thickness within the range of several hundred nanometers, whereas SSD possess a thickness varying from a few micrometers to tens of micrometers, primarily dictated by griding and polishing process. The thorough mitigation of the Beilby layer and SSD can be effectively achieved by ensuring an adequate etching depth. A range of etching depths were applied to fused silica samples, followed by a comprehensive evaluation of their damage density.
As illustrated in Fig. 2, both the damage density and likelihood exhibit a significant increase as the etching depth increases. This result implies that the primary precursor responsible for laser-induced damage to fused silica optics following wet etching is no longer attributed to the Beilby layer and SSD, but rather to the redeposition defects. The etched samples were then analyzed using AFM characterization, as shown in Fig. 3. The surface displayed a significant number of nanoscale precipitates with an estimated density of approximately 106 particles/cm2. The SEM is also employed for the characterization of deposit morphology, as illustrated in Fig. 4. The lateral dimensions of the deposits are approximately 100∼200 nm, with its height estimated at around 10 nm, which correlates well with the AFM testing results obtained. The components of the deposits were characterized utilizing TOF-SIMS, as illustrated in Fig. 5. TOF-SIMS analysis detected the presence of components, including hexafluorosilicate ions and Na+ and ammonium ions, on the etched sample's surface. This finding implies that the by-products of the etching reactions, namely hexafluorosilicates, are dispersed across the surface of the etched sample. In conclusion, the redeposition defects formed by reaction by-products serve as the primary damage precursor for the etched samples.
Hexafluorosilicate is a by-product of the wet etching process, where SiO2 reacts with HF to generate a substantial quantity of hexafluorosilicate ions (SiF62-) that readily dissolve in the solution. These hexafluorosilicate ions subsequently react with other cations present in the solution, giving rise to the formation of various hexafluorosilicate precipitations. The precipitation reaction of hexafluorosilicate is illustrated by the Eq. (1),
The wet etching process can be categorized into three stages: etching, rinsing, and drying, with rinsing further partitioned into initial and final rinsing steps. The critical precipitation concentrations of various reaction byproducts during the etching and rinsing processes were determined by using the equilibrium constant (Ksp), as presented in Table 1. In the etching stage, the concentration of hexafluorosilicate was calculated to be approximately 37 mol/L, while the ammonium ion concentration was estimated to be around 3 mol/L. The concentrations of K + and Ce4+ in the Beilby layer were assumed to be approximately 100 ppm, whereas their content in the bulk material was around 1 ppm. The results of our calculations reveal that throughout the etching process, the concentration of cations consistently surpasses their critical precipitation threshold, resulting in the formation of hexafluorosilicate salt precipitates. Nonetheless, during the subsequent rinsing stage, a significant decline in the concentration of hexafluorosilicate anions leads to a considerable difference between the critical precipitation threshold for cations and their actual concentrations. Consequently, the reactions predominantly dissolve rather than precipitate during this rinsing stage. During the drying process, as the fused silica sample is withdrawn from the rinsing solution (high purity deionized (DI) water), a thin liquid film, approximately (1-2) μm in thickness, adheres to its surface. This film contains a certain concentration of SiF62- ions and other impurity cations. Upon evaporation of the moisture within the film, these impurity ions gradually concentrate and ultimately precipitate onto the surface of the sample.
Table 1. Critical precipitate concentration of the reaction byproduct during the wet etching process
Thus, the redeposition of by-products can occur via two distinct pathways: (1) the precipitate formed during the etching process was not completely dissolved during the rinsing stage; (2) impurities become concentrated and subsequently redeposited in residual liquid films on the surface during the drying process. Inhibiting the formation of redeposits involves minimizing precipitate generation during the etching process and ensuring complete dissolution of precipitates during the rinsing stage, thereby achieving thorough solubilization. The reduction of impurity cation concentration during the wet etching process, as depicted in Fig. 6, is advantageous for suppressing the generation of reaction by-products. Decreasing the etching depth can also mitigate precipitation formation, while still necessitating a sufficient etching depth to achieve passivation by eliminating SSD. Hence, it is worth considering adopting a multiple-etching method with shorter individual etching depth while keeping the cumulative etching depth unchanged and promptly rinsing and dissolving any potential precipitates that may arise.
To achieve complete deposit dissolution, the rinsing time can be prolonged, and the reaction product dissolution rate can be accelerated. The introduction of megasonic excitation can reduce the thickness of the viscous boundary layer, which would enhance transport rates of reaction products from solid-liquid interfaces to rinsing solutions. Additionally, sufficient etching depth can fully expose scratches/cracks, facilitating enhanced transport rates of reaction products at scratch/crack tips. Therefore, the effective mitigation of redeposition defects can be achieved by implementing measures such as controlling the extrinsic impurities, optimizing the depth of etching, introducing megasonic excitation, and implementing multiple dynamic etching.
3.2 Control of the extrinsic impurities
The introduction of extrinsic impurities significantly impacts the mitigation of redeposition defects and ultimately affects the damage performance of optics. LLNL's research has revealed that, under identical etching and rinsing process conditions, a lack of strict control over extrinsic impurities leads to a gradual decline in the damage performance of components as the etching depth increases [8]. Conversely, the implementation of stringent measures to regulate extrinsic impurities results in a proportional increase in the damage performance of optics with the etching depth. This observation suggests that the presence of extrinsic impurities contributes to an elevated incidence of redeposition defects.
The presence of foreign impurities, which can originate from diverse sources such as air, DI water, etchant, tanks/pipelines, and cleaning fixtures, has been extensively studied. Researchers have demonstrated that maintaining an ISO5 cleanroom environment significantly enhances the laser damage resistance of fused silica sample following dynamic acid etching. Uncontrolled particulate impurity concentrations in the air, if left unchecked, could potentially infiltrate both etching and rinsing solutions, ultimately precipitating onto sample surfaces as damage precursors. Therefore, ensuring optimal air cleanliness is crucial in mitigating the redeposition defects, thereby establishing ISO5 air cleanliness as an indispensable prerequisite for wet etching processes. The cleaning equipment has been optimized with an exhaust design to achieve an air cleanliness level close to ISO4, ensuring a well-controlled cleanroom environment for the entire dynamic acid etching process.
The presence of impurities in DI water also has an impact on the mitigation of redeposition defects. Common control parameters for impurities in DI water typically include resistivity, particle count, total organic carbon (TOC), individual metal impurity content, and bacterial count. Among these parameters, resistivity is widely used as it reflects the overall level of control over DI water to some extent. Research has demonstrated that the impurity content in DI water significantly influences the laser damage performance of optics after wet etching [27]. Precisely, incorporating even a minuscule 0.01% proportion of municipal water into DI water, despite its high resistivity of 17.7 MΩ·cm-1, significantly compromises the laser damage resistance of the optics. Moreover, the influence of impurities introduced at varying stages differs in terms of their impact on the damage performance. Impurities introduced during the rinsing phase exhibit a more pronounced effect on the laser damage resistance of the optics compared to those introduced during the etching stage. This suggests that effective mitigation of redeposition defects necessitates extrinsic impurities control throughout the entire dynamic acid etching process. Furthermore, stringent control measures should be exercised in subsequent stages (i.e., rinsing and drying), underscoring the indispensability of DI water as an essential component. Thus, the resistivity, particle count, and individual impurity metal content have been effectively controlled through secondary purification using DI water, elevating it to the level of electronic grade water (EW-I) as a whole.
The concentrations of impurities in the etchant constitute an essential process parameter. Currently, dynamic acid etching procedures predominantly employ a dilution of BOE (Buffered Oxide Etchant) solution as the etchant. The benefits of utilizing BOE as the etchant are twofold: (1) the buffering system ensures a more consistent etch rate, thereby enhancing process stability; (2) procuring pre-prepared BOE stock solutions from manufacturers directly mitigates the risks associated with handling and storing high concentrations of hydrofluoric acid. The etchant, besides harboring a substantial quantity of ammonium ions (NH4+), may also encompass minute amounts of other impurity metal cations, including K+, Na+, Fe2+, Al3+, and so on. In comparison to NH4+, these impurity metal cations exhibit a greater propensity to react with hexafluorosilicate anions and generate precipitates. Consequently, stringent regulation of the content of impurity metal cations in the etchant is indispensable. The research conducted by LLNL suggests that the introduction of impurity metal cations at a concentration of 100 ppb in the etchant significantly alters the damaging performance of fused silica optics [28]. We also examined the effects of etchant containing metal impurity cations at concentrations of 1 ppb, 40 ppb, and 100 ppb on the laser damage performance of the post-etching samples through experiments. The results indicate that impurity metal cation concentrations of 100 ppb have a considerable impact on the damage performance, while no significant differences were observed between 40 ppb and 1 ppb (as depicted in Fig. 7). Therefore, we have chosen to employ UP-S grade BOE reagent, which possesses a metal impurity content of approximately 1 ppb, for the dilution and formulation of the etching solution, and the BOE regent was purchased from Chengdu Kelong Co., Ltd. The selection of this grade of BOE reagent serves two purposes: firstly, as the impurity content in the etchant gradually increases with each iteration of usage, high-grade reagent would reduce the frequency at which the solution needs to be replaced. Secondly, considering that DI water also contains an impurity content around 1ppb and twice as much DI water is required compared to BOE reagent during the dilution process, using a higher-grade BOE reagent was deemed impractical.
Fig. 7. Damage performance of the fused silica as a function of impurity concentration of the etchant.
Furthermore, the selection of the tank/piping and cleaning fixtures’ material is crucial as it will be exposed to etching or rinsing solutions. Hence, materials with minimal impurity precipitation rates are preferred. Given the corrosive nature of hydrofluoric acid, it is advisable to refrain from using metallic materials for etching tanks and subsequent rinsing tanks. Consequently, it is suggested to utilize materials such as PP, PTFE, or PVDF for the applications. For components that may incorporate metallic materials like megasonic transducers, a surface coating of Teflon is recommended.
Through the series of measures mentioned above, we have successfully achieved effective control over extrinsic impurities. By comparing the variation trend of laser damage resistance of fused silica samples with etching depth before and after effective control of extrinsic impurities (as shown in Fig. 8), it can be observed that prior to effective control (Fig. 8.a), the laser damage resistance gradually decreases with increasing etching depth. However, after achieving effective control over extrinsic impurities (Fig. 8.b), on one hand, the laser damage resistance of the components significantly improves; on the other hand, the influence of etching depth on damage performance is no longer significant. This result indicates that stringent control over the introduction of extrinsic impurities helps to mitigate redeposition defects and eliminate the negative effects caused by deep etching.
Fig. 8. Damage density as function of etching depth: (a) the extrinsic impurities haven’t been under control;(b) the extrinsic impurities have been under control.
3.3 Optimization of etching depth
The depth of etching is a crucial process parameter in wet etching. Firstly, a sufficient etching depth is necessary to guarantee the complete opening of sub-surface microcracks for passivation, thereby reducing the influence of microcracks in field enhancement and enhancing the material transport rate at scratch and crack bottoms while minimizing the occurrence of redeposition defects. Secondly, excessive etching can undoubtedly impede work efficiency and surface morphology. Additionally, deep etching may also affect the control of redeposition defects. Consequently, determining the desired etching depth should adhere to the principle of minimizing etching depth while ensuring sufficient opening and passivation of sub-surface microcracks/scratches.
The etching process gradually opens and passivates scratches due to the isotropic nature of wet etching reactions, as depicted in Fig. 9. The width-to-depth ratio (w1/d1) serves as a crucial parameter for quantifying the extent of scratch opening. The influence of varying width-to-depth ratios on the transport rate of hexafluorosilicate ions at the bottom of scratches was examined by Suratwala et al. through simulation studies [8]. It was observed that an increase in the width-to-depth ratio corresponded to a parallel rise in the transport rate of hexafluorosilicate ions. When the width-to-depth ratio of a scratch reaches 1, the transport rate of reaction products at the bottom of the scratch becomes nearly indistinguishable from that on the surface. To enhance the transport efficiency of reaction products, it is imperative to achieve a width-to-depth ratio of 1 through adequate etching depth.
Our research group, led by Dr. Liu, conducted a simulation analysis to examine the effects of varying width-to-depth ratios on the field strength modulation of scratches [28]. The findings revealed that an incremental increase in the width-to-depth ratio was accompanied by a corresponding decline in the field strength modulation effect. The impact of different width-to-depth ratios on the field intensity modulation of scratches was investigated through a simulation analysis conducted by our research group, led by Dr. Liu. The results revealed a corresponding decline in the field intensity modulation effect with an incremental increase in the width-to-depth ratio. To mitigate the impact of scratches on field intensity modulation, it is crucial to ensure that the width-to-depth ratio of the scratch reaches 5. The evolution law of scratch morphology during the wet etching process has been investigated by our research group using experimental and simulation methods. Our findings suggest that (Fig. 10), typically, a depth of 2 to 3 times the scratch's width is required to ensure a passivated scratch with a width-to-depth ratio of 5.
Optimizing the etching depth necessitates determining the scratch depth or SSD depth of fused silica samples. This can be achieved through a combination of magnetic rheological gradient polishing and wet etching; however, this method remains relatively laborious [29]. Another simpler method involves utilizing a profilometer to assess the depth of scratches post-etching, subsequently employing empirical formulas for reverse calculation of the initial SSD depth. The etch rate at the bottom of the scratch is initially lower than that at the top during the initial stage of etching reaction. However, as the scratch gradually opens, the etch rate at the bottom also increases and eventually reaches parity with that at the top. The depth of scratches after etching is expected to be slightly lower than the initial depth, as evidenced by our previous research findings which indicate that the post-etching depth of scratches is approximately 0.8 times that of their initial depth.
In this study, we conducted dynamic wet etching on two medium aperture samples and six small aperture samples, achieving an etching depth of 6 μm. Subsequently, employing optical microscopy and profilometry techniques, we characterized the scratch morphology on the surfaces of these eight samples and determined their width-to-depth ratios. The morphology of the etched scratches is depicted in Fig. 11, exhibiting a width ranging from 10 to 12 μm and a depth of approximately 1 μm. The width-to-depth ratio of the etched scratches measures around 10, while the scratch density is approximately 0.1 counts/cm2. These findings suggest that SSD has depths varying between 1 and 2 μm. To achieve a desired width-to-depth ratio of 5 for the etched scratches, an additional etching of (2-6) μm is necessary. To achieve a certain degree of redundancy, we selected an etching depth of 6 μm as the dynamic wet etching process parameter for this batch of samples. The observation that the scratch width-to-depth ratio reaches 10 post-etching signifies that the scratches have been fully opened. The subsequent damage tests also revealed that the damage performance of the scratched area in the sample after 6 μm etching is comparable to other regions, further confirming the effectiveness of the current etching depth in successfully eliminating subsurface defects.
3.4 Setting of megasonic excitation
During the etching and rinsing processes, a viscous boundary layer forms between the surface of fused silica sample and the flowing liquid. Beyond this layer, reaction products rapidly diffuse with the liquid flow. However, within the viscous boundary layer, the diffusion of reaction products occurs at a slower rate, solely driven by concentration gradients in accordance with Fick's second law [30]. The transport rate of reaction products on the surface of the sample is significantly influenced by two crucial factors: the thickness of the boundary layer and the velocity of the liquid outside this layer [31]. The introduction of megasonic excitation can reduce the thickness of the viscous boundary layer and enhance the flow velocity of the liquid beyond this layer. Consequently, this technique facilitates an improved transport rate of reaction products, accelerates the depletion of hexafluorosilicate ions on sample surfaces, and effectively inhibits redeposition of reaction by-products.
The effects of static etching and dynamic etching on the damage performance of fused silica optics were compared through experiments by LLNL researchers [8]. The experimental results demonstrated that the incorporation of megasonic excitation significantly enhanced the laser damage performance of the samples, effectively mitigating the redeposition defects. However, the research conducted by Ye et al. demonstrated that the introduction of megasonic excitation, with a power density ranging from 0.6 to 2 W/cm2, during the etching process did not result in any notable influence on the damage performance of fused silica samples [9]. The contradictory phenomenon may be attributed to two factors: firstly, there exist disparities in the rinse processes employed by LLNL researchers and Ye et al., as the former introduced dynamic ultrasound excitation while the latter did not provide any relevant explanation; secondly, it could be attributed to the influence of megasonic power density.
We conducted a comparative experiment to investigate the impact of introducing megasonic excitation during etching and rinsing processes on the laser damage performance of samples. The experiment comprised four groups: Group 1 served as a control without megasonic excitation; Group 2 introduced megasonic excitation solely during the etching stage; Group 3 introduced megasonic excitation solely during the rinsing stage; and in Group 4, megasonic excitation was introduced in both stages. The results depicted in Fig. 12 demonstrate that the application of megasonic excitation at any stage effectively enhances the laser damage resistance of the component when compared to the pure static control group. Notably, introducing megasonic excitation during the rinsing stage yields significantly superior outcomes than solely incorporating it during the etching stage. Considering all factors comprehensively, optimal performance in terms of laser damage resistance is achieved by incorporating megasonic excitation in both stages.
Fig. 12. The influence of introducing megasonic excitation during etching and rinsing processes on the laser damage performance of samples.
The frequency modulation of megasonic excitation governs the thickness of the viscous boundary layer, while its power density determines the liquid flow velocity beyond the boundary layer. Consequently, the magnitude of power density significantly impacts the transport rate of reaction products on the sample surface, which is also associated with its mitigation effect on redeposition defects. We conducted experiments to investigate the correlation between the damage performance of samples in dynamic wet etching and varying megasonic power densities. According to Fig. 13, it is evident that an incremental increase in the megasonic power density during both etching and rinsing stages leads to a progressively enhanced damage performance of the fused silica samples. After conducting a thorough analysis of the results from both experiments, we have determined that incorporating megasonic excitation into wet etching process is a critical technique for enhancing laser damage performance and mitigating redeposition defects. Furthermore, it is recommended to utilize high power density megasonic excitation during both rinsing and etching stages.
3.5 Implement of multiple dynamic etching
In the wet etching process, a single-step etching strategy is typically employed to achieve the desired etch depth, followed by subsequent rinsing and drying treatments. The occurrence of redeposition defects is primarily observed during the etching and drying stages, as mentioned previously. However, in the rinsing stage, there is a continuous process of dissolution and dilution. Therefore, effective mitigation of redeposition defects can be achieved by implementing strategies to reduce precipitation formation and accelerate precipitation dissolution. This can be accomplished by minimizing the etching amount and extending the rinsing time to enhance the mitigative effect on redeposition defects. Since the total etching depth remains constant at 6 μm, we are considering implementing a multi-step etching strategy to reduce the depth of each individual etching. And rinsing process would be implemented after each etching step to dissolve reaction products promptly, thereby effectively mitigating the occurrence of redeposition defects. In this study, we propose a multi-step dynamic etching method for post-processing fused silica optics, which replaces the conventional single-step etching approach. The three-step etching strategy comprises of shallow/deep/shallow etching. The primary objective of the initial shallow etch is to selectively eliminate the Beilby layer due to its high impurity metal element content that can result in significant redeposition defects. Consequently, immediate rinsing is conducted after removing the Beilby layer through shallow etching to facilitate dissolution of reaction by-products. The subsequent two etching processes are conducted to ensure thorough removal and passivation of the SSD, achieving the desired scratch width-to-depth ratio (≥ 5).
In an environment where effective control of extrinsic impurities has not been implemented, we investigated the variations in laser damage performance of fused silica samples under the same cumulative etch depth through single-step etching, double-step etching, and triple-step etching methods. The samples subjected to triple-step etching processes, as depicted in Fig. 14, demonstrates superior laser damage performance compared to those treated with double-step etching processes and traditional single-step etching processes. After successfully achieving effective control of extrinsic impurities, we conducted a three-factor Taguchi experiment to investigate the impact of etching protocol, etching depth, and inorganic acid leaching on the damage performance of the components. The specific design of the Taguchi experiment and the results of the damage test can be found in Table 2, with an average fluence of 18 J/cm2@3 ns for the damage test. Table 3 presents the analysis of variance (ANOVA) of these etching-related factors. According to the results of ANOVA, the obtained F value for the etching protocol is 7.86, indicating a statistically significant effect (p < 0.05). Therefore, we can conclude that the etching protocol has a substantial impact on the damage performance of fused silica samples. This finding further reinforces the efficacy of multiple etching in enhancing laser damage resistance. The F-values of etching depth and inorganic acid leaching were found to be relatively low, while their corresponding P-values exceeded 0.05. This suggests that the influence of these two factors on the laser damage performance of the samples is not statistically significant.
Fig. 14. Relationship of laser damage density and damage probability with the etching protocols in the same etching depth, the average fluence is about 10J/cm2.
Table 2. Taguchi experiment array and the results obtained.
Table 3. ANOVA of Etching related factors.
The photo-thermal absorption test was using to trace the multiple dynamic etching process. As shown in Fig. 15, the photo-thermal absorption value gradually decreased with the etching process. It suggests that the damage precursors which would induced the photo-thermal absorption, have been gradually mitigated during the multi-step etching process [32,33].
Fig. 15. 2D photo-thermal absorption maps for various stage of the multiple dynamic etching process.
The speculation is that the implementation of multiple dynamic etching processes can significantly enhance the laser damage resistance of fused silica optics. This enhancement primarily stems from a reduction in redeposit accumulation, achieved by decreasing the depth of each individual etching. Additionally, redeposition defects are effectively suppressed through an extended rinsing time three times longer than that used for single-step etching, which facilitates dissolution of any generated deposits. In addition, considering the SSD depth of approximately 1 μm for this batch of fused silica samples, even with an etching depth as low as 2 μm, adequate opening and passivation of the SSD can still be achieved. Therefore, variations in etching depth within the range of (2∼10) μm have not significantly impacted the laser damage performance of the samples. Inorganic acid leaching has the potential to reduce impurity metal content in the Beilby layer, thereby theoretically minimizing precipitate formation during etching. However, Taguchi experiments indicate that this method does not significantly impact the laser damage performance. It suggests that the effectiveness of inorganic acid leaching may be overshadowed by other factors during the etching process, such as the introduction of ultrasonic waves for enhanced mitigation effects on redeposition defects during etching and rinsing process. Moreover, in multiple dynamic etching processes, the initial shallow etching exhibits a similar effect to inorganic acid leaching but with greater prominence and complete removal of the Beilby layer.
3.6 Comprehensive laser damage test
By implementing a series of optimization measures, such as strict control over extrinsic impurities, optimizing etching depth, comprehensive application of megasonic excitation, and developing multi-step dynamic etching technology, significant enhancements have been achieved in the laser damage performance of fused silica optics. With the optimization of component laser damage performance, the damage density within the low fluence range is typically numerically insignificant to such an extent that obtaining high-confidence results relying solely on small area testing becomes challenging. Consequently, large area tests are necessary to obtain more reliable conclusions. To obtain damage test results with higher confidence, we employed the Multifunctional Optical Damage Science System for conducting large-area damage density testing on fused silica samples subjected to optimized dynamic wet etching processes, encompassing five medium-aperture samples and one large-aperture sample. The medium-aperture samples were subjected to laser damage test by using an average fluence of 15 J/cm2 @3 ns as the testing condition, with each sample having an approximate test area of 22 cm2. The cumulative test area amounted to around 110 cm2, resulting in the detection of 4 damage sites. Hence, it can be inferred that the damage density of samples treated with dynamic wet etching is less than or equal to 0.1 sites/cm2 under an average fluence of 15 J/cm2, with a confidence level exceeding 90%. On the contrary, the damage performance data of the sample showed a damage density of 0.1 sites/cm2 under an average fluence of 10 J/cm2 before the optimization of the dynamic wet etching process. After implementing a series of optimization measures, the average fluence that fused silica sample can withstand at the same damage density, has been enhanced for approximately 5 J/cm2. Taking into consideration of the laser beam modulation, these optimizations further enhance the peak fluence that the sample can withstand by approximately 10J/cm2. This result indicates that the novel dynamic wet etching process significantly enhances the laser damage resistance of the samples.
Subsequently, we conducted an extensive damage assessment on a large-aperture sample, employing an average fluence of 12 J/cm2 and a cumulative test area of about 240 cm2. However, no damage points were detected. This result indicates that, under laser irradiation with an average fluence of 12 J/cm2, the performance of large-aperture samples in terms of damage density remains below 0.01 sites/cm2, with a confidence level exceeding 90% based on the test results. The optimized dynamic wet etching process has successfully achieved effective mitigation of various low-fluence precursors, including SSD and the redeposition of reaction by-products. In order to investigate the damage performance of samples in high fluence range, we conducted a damage density test on the same large-aperture sample using the fluence mapping method, which is similar to the off-line test method employed on the OSL laser system at LLNL [17,34]. The damage locations would be accurately mapped and registered onto on-sample fiducials using an automated microscope, while simultaneously registering the fluence distribution of each beam. The peak fluence in the test would reach up to ∼40 J/cm2. The results shown in Fig. 16 indicate that the performance of large-aperture sample treated with optimized dynamic wet etching is comparable to that achieved by LLNL's AMP3 laboratory in the high fluence range. This suggests that this dynamic wet etching process not only effectively mitigates low-fluence precursors, but also demonstrates excellent suppression performance on high-fluence precursors. The surface morphology of large-aperture sample after etching was characterized using AFM, revealing a significant decrease in the density of deposits on the sample surface from previous levels of ∼106 cm-2 to the level of ∼103 cm-2. These research findings further validate the substantial mitigation effect of current dynamic wet etching process on redeposition defects.
4. Conclusions
This article elucidates that the redeposition of reaction by-products poses a significant constraint on current wet etching processes, as evidenced by surface morphology characterization and composition analysis. Through in-depth analysis of the formation mechanism of redeposition defects, a technical strategy is proposed that combines inhibition of precipitation generation and promotion of precipitation dissolution. This study investigates the impact of extrinsic impurities, etching depth, megasonic excitation, and other factors on mitigating redeposition defects. Multiple dynamic wet etching techniques are successfully proposed and validated. The results indicate that strict monitoring of multiple parameters, such as air cleanliness, DI water quality, etching solution composition, and tank/pipeline material, is crucial for controlling extrinsic impurities. Achieving an ISO4 level of air cleanliness, using electric grade water (EW-I), employing UP-S grade BOE reagent, and utilizing low outgassing materials (such as PP, PVDF, PTFE, and metal-plated Teflon coating) are key measures in the dynamic acid etching process. The optimal etching depth should be 2∼3 times the depth of SSD. The depth of SSD can be determined by analyzing opened scratches after etching using a profilometer. Additionally, incorporating megasonic excitation helps to mitigate redeposition defects; therefore, maintaining a high level of megasonic power density (approximately 3 W/cm2) during both the etching and rinsing stages is recommended. Compared to the traditional single-step wet etching method, employing multiple dynamic etching techniques can significantly enhance the laser damage resistance of fused silica optics. By conducting large-area damage testing, we obtained highly reliable test results under low fluence conditions. The sample treated with dynamic wet etching exhibited a remarkably low damage density below 0.01 sites/cm2 at an average fluence of 12 J/cm2. The successful mitigation of low fluence precursors has been confirmed, while the results of fluence mapping method indicate a significant inhibitory effect of the current process on high fluence precursors.
Funding
Key Programme (22072139).
Acknowledgments
The authors wish to acknowledge the SEM measurement performed by Yuwei Zhou.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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