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Abstract
We investigate the nanoscale damage precursors that will cause laser damage initiation on fused silica surface during KOH-based wet etching. Some nanoscale damage precursors, like impurity contamination and chemical structure defects on different etched surface with a KOH solution, are explored through a variety of testing methods at nanoscale spatial resolution. The etched surface roughness and photothermal absorption level are also studied. The results show that KOH-based etching can keep a good surface roughness, reduce impurity contamination significantly, and thus decrease surface photothermal absorption level. However, it can mitigate little chemical structure defect and has a risk of secondary pollution induced by residual deposition such as K2SiO3. The work can be a reference on using KOH-based wet etching technology to mitigate nanoscale damage precursors of fused silica ultraviolet optics.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Inertial confinement fusion (ICF) research that has become a key and frontal subject is a vital technical way to study high energy density science (HEDS) in the laboratory [1]. In ICF research, fused silica is widely used as focusing lens, grating or other optics at the wavelength of 351 nm. They must have high-precision surface shape as well as excellent laser damage resistance to withstand high fluence laser at 351 nm [2].
Numerous studies have demonstrated that subsurface crack, impurity contamination and chemical structure defects from the manufacturing process are main laser damage precursors [3–7]. Under ultraviolet (UV) laser irradiation of sufficient intensity, they will act as precursors, induce laser damage and make optical component operating hours decrease. To improve laser induced damage threshold (LIDT) of fused silica optics, tremendous efforts have been devoted to promote technological advance on damage precursor suppression and control, and many achievements have been made [8–11]. Magnetorheological finishing (MRF) can effectively remove subsurface cracks and obtain high-precision surface shape. However, hydrofluoric acid wet etching must be introduced to clean iron impurity contamination after MRF [8]. Hydrofluoric acid wet etching is the most widely used post-processing technology to mitigate damage precursors and improve laser damage resistance of fused silica. Researchers in Lawrence Livermore National Laboratory (LLNL) get a series of great achievements on hydrofluoric acid wet etching and summarize research results as “advanced mitigation process (AMP)” [9]. However, hydrofluoric acid (HF) etching will deteriorate surface shape precision of optics and some residual production of $\textrm{SiF}_\textrm{6}^{\textrm{2 - }}$ will be adsorbed on the optic surfaces, causing secondary pollution [10]. The use of hydrofluoric acid also has great threat to human body and is not conducive to environmental protection. In view of the above considerations, it is necessary to develop new post-processing technologies to machine low-damage fused silica optics economically.
In microfabrication field, the potassium hydroxide (KOH) solution as a fused silica etchant is studied [12]. However, the literature on KOH-based etching for fused silica UV optics that has high laser damage resistance is rarely reported. M. Pfiffer et al. have utilized KOH solution to fabricate high damage threshold UV optics [13]. They have made a preliminary study on the damage precursor mitigation with deep wet etching in KOH solutions and achieved good results. They used KOH solution to etch 12 µm depth and effectively mitigated fracture scratches, thereby improving the LIDT and maintaining the surface roughness of fused silica. L. Sun et al. employed KOH-based shallow etching to improve laser damage resistance of fused silica and considered residual K-containing salts may be the potential obstacle that limit the LIDT of fused silica UV optics [14]. As a potential post processing technology, KOH-based etching can be utilized to mitigate cracks and improve laser damage resistance of fused silica based on findings to date. However, many important aspects of KOH-based wet etching for mitigating damage precursors of fused silica remain inadequate, especially impurity contamination and chemical structure defects hidden or beneath the polished redeposition layer. It is also absent of characterization results of potential factors limiting surface damage threshold improvement after etching. Thus, more characterization results of KOH-based etched surface should be presented, and considerable efforts should be exerted to achieve accurate analysis through mutual corroboration in the experiments.
This paper aims to determine the characteristics and evolution of nanoscale damage precursors, such as contamination and chemical structure defects during KOH-based wet etching. Section 2 introduces the sample preparation and experimental design. Section 3 presents the measurement results and analysis. Section 4 discusses the influences of measurement results on laser damage resistance of fused silica optics. Section 5 provides the conclusions.
2. Sample preprocessing and experimental design
Commercial fused silica samples prior to wet etching in KOH solutions are treated by abrasive-based polishing with small-tool. Their sizes are all 10 mm × 10 mm × 3 mm. Cerium oxide slurries are used in polishing, the polishing force is 0.02 MPa, the rotating speed is 20 rpm, the swinging frequency is 12 min−1, and the polishing time is 150 min. There are few visible defects on sample surface except for the polished redeposition layer. Before KOH-based wet etching, samples are cleaned in deionized water under 7 different ultrasonic frequencies. The ultrasonic condition is listed in Table 1. The total cleaning time is 35 min and the cleaning temperature is 25 °C.
Table 1. Cleaning conditions of samples.
After the cleaning, the samples are allowed to be etched in KOH solution. Table 2 lists the etching conditions and tests used of each sample. Six samples are statically etched at 0, 200, 400, 600, 800, and 1000 nm depths using KOH solution, respectively. #1-#5 are putted in 30% KOH solution for thermostatic water bath. The etching temperature is 80 °C and the etching rate of fused silica is approximately 400 nm/h. After KOH-based wet etching, #1-#5 are rinsed in deionized water again under ultrasonic condition in Table 1. Then, etched samples should be dried with nitrogen. Thanks to the strict environmental controls in term of cleaning, etching, rinsing and drying of the samples in a Class 100 clean room, the sample surfaces should not be affected by uncontrollable contaminants.
Table 2. Etching conditions and tests used for single-factor tests.
The surface morphology and roughness at different etched depths are directly measured via atomic force microscope (AFM, Bruker Dimension Icon). What is more, AFM-based infrared spectroscopy (AFM-IR) are conducted to investigate nanoscale surface property in-situ [15]. AFM-IR can locally detect the thermal expansion induced by the absorption of infrared radiation on sample surface with the aid of the tip of an AFM probe, thereby providing chemical analysis and composition spectra as well as the spatial resolution of AFM [16]. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is conducted on samples #0 and #2 to characterize impurity contamination evolution on the KOH-based etched surface. The spectral information on #0, #2, and #4 surface for characterizing chemical structure defects are obtained via JY TAU-3 fluorescence (FL) spectrometer. A photothermal (PT) absorption detection system is used on #0, #2, and #4 surface to evaluate weak absorptance, thereby estimating the laser damage resistance of different KOH-based etched surfaces.
3. Results and discuss
3.1 Surface morphology evolution
AFM analysis in ScanAsyst mode is performed to investigate each sample surface morphology. Each analysis area is 10 µm×10 µm. Figure 1 illustrates the surface morphologies and roughness Ra of six samples in AFM. The original sample surface is covered with some polishing marks, whose roughness Ra is 0.32 nm. After 200 nm is etched in #1 surface, a few bright spots are exposed, suggesting polished redeposition layer is removed. At this point, the roughness Ra increases to 0.40 nm slightly. Later, with the depth increases, the surface roughness changes little, which indicates that the surface quality in term of surface roughness obtained after polishing can be maintained during KOH-based wet etching.
3.2 AFM-IR analysis
AFM-IR analysis is performed to investigate the surface chemical composition at nanoscale spatial resolution of #0, #3, and #5 sample. Each sample has two testing point in order to eliminate the interference of accidental factors. The AFM-IR analysis uses a pulsed, tunable IR source to excite molecular vibrations in a sample that is mounted on an IR-transparent prism of zinc selenide.
The infrared spectra within a range of wavenumber (1800cm−1∼900 cm−1) of three sample surfaces are shown in Fig. 2–Fig. 4, respectively. In Fig. 2, there are two absorption peaks in the spectra. The one within 1400 cm−1∼1200 cm−1 and the unnoticeable peak within 1200 cm−1∼1000 cm−1 are both arise from the functional group: -SiO2 [17,18]. However, the intensity of unnoticeable peak within 1200 cm−1∼1000 cm−1 significantly enhanced illustrated in Fig. 3–Fig. 4, indicating changes in the local chemical signatures. After KOH-based wet etching, KOH solution will react with fused silica and result in surface residual products such as K2SiO3. The functional group $\textrm{SiO}_\textrm{3}^{\textrm{2 - }}$ will cause a wide and strong absorption peak in infrared spectrum within 1100 cm−1∼900 cm−1 [17]. So, a varying relative peak intensity within 1200 cm−1∼1000 cm−1 appears. What needs to be explained in Fig. 4 is that the peak intensity within 1200 cm−1∼1000 cm−1 is high whether on the substrate or on the bright spot in AFM, suggesting reaction products do not exist locally but spread across the whole etched surface.
Fig. 2. Surface morphology and chemical composition at nanoscale spatial resolution of sample #0 in AFM-IR. (a) Surface morphology in AFM with two testing points on substrate (b) infrared spectra of two testing points
Fig. 3. Surface morphology and chemical composition at nanoscale spatial resolution of sample #3 in AFM-IR. (a) Surface morphology in AFM with two testing points on substrate (b) infrared spectra of two testing points
Fig. 4. Surface morphology and chemical composition at nanoscale spatial resolution of sample #5 in AFM-IR. (a) Surface morphology in AFM with two testing points, Point 1 is on substrate while Point 2 is on a bright spot (b) infrared spectra of two testing points
3.3 TOF-SIMS analysis
TOF-SIMS surface analysis is performed on the #0, #2 samples for investing these following ions: Al+, K+, Ca+, Ce+, Fe+, H−, F−, OH−, and NH−. In TOF-SIMS analysis, a 500 µm × 500 µm area to be tested in the center of each sample is randomly selected.
Figure 5 illustrates the relative impurity contamination concentration change before and after KOH-based wet etching. The value of relative counts is ×106 and normalized. For positive elements, lower intensities of Al+, Ca+ and Ce+ ions are detected on #2 as compared to the origin. The original Ce+ content affecting laser damage is introduced in the prior abrasive-based polishing. KOH-based wet etching can remove the polished redeposition layer and clean the subsurface cracks at once, which makes Ce+ concentration immensely decrease by 98.2%. in the meanwhile, Al+ concentration decreases significantly by 95.8% and the Ca+ concentration decreases by 90.1%. But Fe+ concentration changes little, indicating that KOH-based etching cannot remove Fe contamination very well. K+ concentration of #2 surface increases 13.4 times after etching, suggesting KOH solution will react with the substrate and result in residual potassium salt like K2SiO3. For negative elements, higher intensities of H− is detected on #2 as compared to the origin, while lower intensities of F−, OH−, and NH− ions is detected. H− concentration increases by 53.6%. F− concentration decreases by 62.0%. OH− and NH− concentration decreases by 26.7% and 43.5%, respectively. Above all, the TOF-SIMS results demonstrate that KOH-based wet etching has a positive effect on impurity contamination mitigation except for K, Fe and H.
Fig. 5. The relative concentration of impurities detected by time-of-flight secondary ion mass spectrometry (TOF-SIMS).
3.4 Fluorescence spectra analysis
FL spectra analysis is performed on the #0, #2, and #4 sample for observing characteristic peak of chemical structure defects such as oxygen-deficient center (ODC) and nonbridging oxygen hole centers (NBOHC). The fluorescence emission spectra in FL analysis are excited using a 248 nm laser. By comparing the observed FL peaks intensities during etching, the evolutions of ODC and NBOHC are obtained.
Figure 6 illustrate the FL spectra evolution before and after KOH-based wet etching. As shown in Fig. 6, there are two sensible peaks on FL spectra. The peak centered at approximately 440 nm originates from the ODC defects. The peak centered at approximately 650 nm originates from the NBOHC defects [19,20]. As the KOH-based etching depth increases, the intensity of ODC drops slightly and the intensity of NBOHC change little. KOH-based wet etching has little effect on mitigating ODC and NBOHC in concentration.
3.5 Photothermal absorption analysis
In photothermal absorption analysis, the photothermal absorption signal is excited using a 355 nm laser. The measurement system is configured to be in reflectance mode. Four areas on a sample are randomly selected for photothermal absorption analysis, and the average value of the four test results is taken as the photothermal absorption level of the surface. The measurement region is 1.0 mm × 1.0 mm and the accuracy in measurement is 0.1 ppm. Under UV laser irradiation, the photothermal absorption has a strong correlation with LIDT of UV optics [21]. By comparing the surface photothermal absorption value of UV optics, the increase or decrease of surface laser damage resistance of UV optics can be obtained easily. Therefore, photothermal absorption analysis as a non-destructive testing method can be used to predict the surface laser damage resistance of fused silica [22,23].
Figure 7 illustrates the 2-dimensional (2D) photothermal absorption distributions on samples (#0, #2, and #4). Figure 8 shows the average photothermal absorption evolution during KOH-based wet etching. On the original surface, some visible absorption peaks appear effected by the polished redeposition layer. Therefore, the initial average absorption value is relatively high and reaches 1.33 ppm. Later, when 400 nm depth is removed in KOH solution, the number of visible absorption peaks and average absorption value both decrease significantly. Only an absorption peak is on #2-4 and the average absorption value decreases to 0.92 ppm, which indicates that the surface laser damage resistance is improved by KOH-based etching. While the etched depth is 800 nm, two absorption peaks emerge again at #4-3 and the average absorption value goes up to 1.24 ppm. The laser damage resistance had a slight fall but is still better than the origin. The increased photothermal absorption on #4 suggests that some other damage precursors after the etching have turned up and begun to hold dominant position, thereby affecting the photothermal absorption level.
4. Discuss
The photothermal absorption results in Fig. 8 indicates that the laser damage resistance of fused silica rises first at etched 400 nm and then drop at etched 800 nm. Here, this section discusses how KOH-based wet etching can mitigate nanoscale damage precursors and change laser damage resistance. The major obstacles in laser damage resistance improvement during KOH-based wet etching are discussed as well.
Surface roughness is very important for fused silica UV optics. Pfiffer et al. [13] in 2017 and Sun et al. [14] in 2020 both have evidenced that neither deep etching nor shallow etching with a KOH solution will destroy surface roughness modified surface morphology. In our case, there is little deterioration in surface quality after KOH-based wet etching based on the results in section 3.1. Invariable surface roughness has little effect on the change of laser damage resistance with different etching depth. The surface roughness after KOH-based etching is very likely to be secondary factors influencing the laser damage resistance change in Fig. 8. Besides, two typical chemical structure defects such as ODC and NBOHC are strongly correlated with the laser damage resistance of fused silica. In 2017, M. Xu et al. [24] enhanced the laser damage resistance of fused silica by reducing ODC and NBOHC during ion beam sputtering. However, FL spectra analysis in section 3.4 indicates that KOH-based etching has little effect on mitigating ODC. The mechanism of KOH-based etching is to break the three-dimensional silicate network structure formed by the combination of ≡Si-O-Si≡ (siloxane) bonds using OH− in KOH solution. Although KOH-based etching removes the Beilby layer, but a new surface with chemical structure defects has been formed during the etching process. Therefore, as the KOH-based etching depth increases, the intensities of ODC and NBOHC in fluorescence spectra change little. Chemical structure defects still limit the laser damage resistance improvement after etching.
For impurity contamination, a lot of impurities induced by polishing process will absorb sub-band gap light and evolve into damage precursors when exposed to sufficient laser fluence, thereby causing a significant reduction in LIDT [25–27]. In section 3.3, the results demonstrate that KOH-based wet etching has a cleaning effect on impurity contamination except for K, Fe and H. The mitigation phenomenon benefit from the removal of polished redeposition layer. An indirect evidence is the surface appearance in Fig. 1(c), suggesting that KOH-based etching have removed the polished redeposition layer thereby exposing subsurface scratches. In 2015, T. Suratwala investigated the chemical characteristics of the polished redeposition layer and found impurities like Ce are active participants in abrasive-based polishing process on fused silica [28]. Thus, with the removal of polished redeposition layer, the concentration of Al, Ca and Ce decreased. As a result, the laser damage resistance is improved when 400 nm are etched in a KOH solution. While the etched depth is 800 nm, the laser damage resistance decreases. This decrease is the result of many factors. During KOH-based wet etching, little chemical structure defects are mitigated. The intensities of ODC and NBOHC in fluorescence spectra remain high. Moreover, KOH-based etching cannot remove Fe contamination very well. Though the Fe concentration is small, the residual Fe can influence laser damage resistance as well. Most important of all, the TOF-SIMS analysis in section 3.3 and the infrared spectra in section 3.2 demonstrate that etched fused silica surface is covered with reaction product K2SiO3. With the depth increase during KOH-based wet etching, the accumulation of chemical structure defects, residual Fe contamination and reaction product will lead to the increase of average photothermal absorption value, and a few photothermal absorption peak appears, which obstructs the enhancement of laser damage resistance. Similar to HF-based etching, this technology limited by material removal mechanism of wet etching can easily lead to deposition of reaction products and cause surface secondary pollution. It is a good choice to utilize KOH-based wet etching combined with HF-based wet etching and ion beam etching to mitigate all types of damage precursors. The biggest advantage of KOH-based wet etching is that it can remove most of impurity contamination and maintain the surface roughness of fused silica optics. When a polished fused silica is processed during KOH-based wet etching, most of impurity contamination and subsurface damage are removed. Therefore, shallow HF-based wet etching is enough to remove Fe contamination and a good surface roughness of fused silica can be maintained. To equip shallow wet etching technology with multi-frequency ultrasonic or megasonic agitation is important to prevent residual deposition as much as possible. Finally, noncontact ion beam etching can be used to mitigate chemical structure defects and residual product deposition after wet etching to get a super smooth surface with high laser damage resistance. Efforts on optimizing combined processes for mitigating nanoscale damage precursors and improving the surface laser damage resistance of fused silica will be made in future studies.
5. Conclusion
We investigate KOH-based wet etching as a post-process technology for fused silica UV optics. It has been proven that KOH-based wet etching can increase laser damage resistance and maintain surface roughness at once. KOH-based etching has no obvious mitigation effect on chemical structure defects. After etching, the ions generated from the prior polishing had a dramatic decrease in concentration except K, Fe and H. Reaction product K2SiO3 on the etched surface is also an important obstacle for further improving laser damage resistance via AFM-IR and TOF-SIMS. They do not exist locally but spread across the whole etched surface. Above all, KOH-based etching is a promising technique for post-processing fused silica UV optics and provides a new idea for process selection. In order to mitigate all types of damage precursors comprehensively, it is the best choice to utilize KOH-based wet etching combined with other post-processing technologies like HF-based etching or ion beam processing. The research results have something meaningful to clear what features of the damage precursors and to develop a more in-depth understanding in KOH-based wet etching.
Funding
National Natural Science Foundation of China (No. 51835013, No. 51991374, No. U1801259); National Key Research and Development Program of China (No. SQ2020YFB200368-04); Strategic Priority Research Program of the Chinese Academy of Sciences (No. XD25020317).
Disclosures
The authors declare no conflicts of interest.
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