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Surface damage precursor evolution has great influence on laser-induced damage threshold improvement of fused silica surface during Ion beam etching. In this work, a series of ion sputtering experiment are carried out to obtain the evolutions of damage precursors (dot-form microstructures, Polishing-Induced Contamination, Hertz scratches, and roughness). Based on ion sputtering theory, surface damage precursor evolutions are analyzed. The results show that the dot-form microstructures will appear during ion beam etching. But as the ion beam etching depth goes up, the dot-form microstructures can be mitigated. And ion-beam etching can broaden and passivate the Hertz scratches without increasing roughness value. A super-smooth surface (0.238nm RMS) can be obtained finally. The relative content of Fe and Ce impurities both significantly reduce after ion beam etching. The laser-induced damage threshold of fused silica is improved by 34% after ion beam etching for 800nm. Research results can be a reference on using ion beam etching process technology to improve laser-induced damage threshold of fused silica optics.
© 2016 Optical Society of America
1. Introduction
Fused silica optic is of critical importance in the development of high-power laser systems. To improve the laser induced damage threshold (LIDT) of fused silica optics is one of the basic pursuing objectives for high-power laser systems [1–3]. In the fabrication process of fused silica optical components, a variety of ionic species at size scales below that which can be reliably observed by optical or even electron microscopy, when precipitated on optical surfaces, can seriously lead to laser damage. Those high fluence precursors represent a significant barrier to the fabrication of UV optics for high fluence applications [4].
After the end of polishing, the optical components should be post processed to increase the LIDT. Currently, the post processing technology consists of magnetorheological finishing (MRF), HF acid etching, ion beam etching (IBE) and so on. MRF has the features of high removal efficiency, strong controllability, a high degree of conformability, and good polished surface quality. MRF can remove defect layers, when it removes material by shear stress [5]. Lawrence Livermore National Laboratory (LLNL) found that the MRF can effectively remove scratches on the surface of the component in the productive process. But Fe, Ce contamination is introduced on the component surface because of the magnetorheological fluid containing carbonyl iron powder been used in the MRF polishing, which suppresses the improvement of LIDT [6]. HF acid etching can react with fused silica, to remove the residual impurities from the subsurface of fused silica, which has a mitigative effect on the subsurface defects [7]. At present, LLNL uses “advanced Mitigation Process” (AMP) to treat fused silica optics in high-peak-power laser optics. AMP uses an optimized hydrofluoric(HF) acid or a buffered oxide etch(BOE) etch to remove both the polishing layer and fracture-induced electronic defect layer [8].The aqueous fluoride-based etching processes can significantly improve the LIDT of scratched fused silica surfaces. But the HF acid etching will worsen the surface quality and the redeposition of the reaction products in etching processes will become the new damage precursor to lead to laser damage initiation [9–11]. Thus, MRF and HF acid etching should be improved on removing high fluence precursors.
Ion beam etching uses ion sputtering effect to remove material at atomic scale and get super-smooth surface. Compared with MRF and HF acid etching, IBE is controllable, highly stable and non-contact with advantages over conventional etching technology [12]. That is, metal contamination can be avoided to introduce on the component surface in the IBE. The re-deposition of the reaction products in HF etching processes is also not a consideration. As a potential post processing technology, IBE has a possibility to be used in surface cleaning of fused silica optics and wipe out high fluence precursors at nanoscales. Laboratory for Laser Energetics in Rochester University of the United States first uses IBE to increase the LIDT of fused silica. The LIDT was increased by 2 times when they used the ion beam etching with a voltage of 200-400eV for one hour. Kamimura found improvement of the surface LIDT of fused silica after surface IBE as compared with that of the as-polished surface. They thought Ion beam etching could removal contamination impurity such as CeO2 polishing powder from fused silica subsurface, and would not damage the surface quality of substrate [13–16]. In these studies, the IBE techniques for improving laser damage resistance were focused on the final effect.
Recent studies have shown that there are three kinds of precursors: (1) impurities (e.g., Ce, Fe, etc.) in the 50~100nm Beilby layer; (2) a shallow intrinsic defect layer found on fracture surface (such as scratches); and (3) re-deposited compounds. All these precursors will cause a localized optical absorption to lead to laser damage when exposed to sufficient laser fluence [4, 17]. But it is not yet clear what are the nature and characteristics of the precursors that lead to laser damage initiation in IBE. And in the previous study [13–16], researchers usually focus on the results on getting super-smooth surface or improving LIDT in the IBE. Details in the ion beam polished surface morphology evolution is rarely shown. The phenomenon on improving LIDT by IBE still need to be further studied from the source.
With the identification of these precursors, this investigation of IBE on fused silica is conducted focus on the surface morphology evolution and the development of precursors. A series of etching processes are carried out to measure the changing profiles of the dot-form microstructures and scratches by Atomic Force Microscopy (AFM), which is of significance to make sense the nature and characteristics of the precursors that lead to laser damage initiation in IBE. In addition, ion sputtering theory is used to analyze the surface morphology evolutions in IBE. Then, polishing-induced contamination measurement is analyzed as well. Finally, the LIDT test experiment is carried out. The results reveal that IBE can improve the LIDT greatly without destroy the surface quality. Above all, results can be a reference on using ion beam etching process technology to improve laser-induced damage threshold of fused silica optics.
2. Experimental
2.1 Sample preparation
Three commercial fused silica samples (Heraeus 312-#1, #2, and #3) with size of 50 × 50 × 5mm3 are prepared. Sample #1 and #2 are treated by a conventional polishing process. CeO2 with diameter 0.2µm is used as the abrasive particle. They need to be treated until surfaces are bright. Sample #3 is polished by MRF. It is processed for 200min, and the removal depth is 350nm. Its parameters are shown in the Table 1.
Table 1. Parameters of MRF
The surface of #1 fused silica sample is divided into eight areas. Those areas all are processed in the IBE machine but their ion beam etched depths are 0nm, 50nm, 100nm, 150nm, 200nm, 250nm, 300nm, 800nm, respectively. The sample is used to research the relationships among initial LIDTs, surface roughness and ion beam etched depths. Therefore, surface roughness measurements and LIDT measurements will follow after IBE.
The #2 fused silica’s surface is used to investigate the evolution of a typical Hertz scratch in the IBE. And the surface of #3 fused silica sample is processed to study the metal impurities (Fe, Ce) before and after IBE. The removal depth in Sample #3 is 100nm.The two samples mainly focus on the evolution of defect profiles and change on relative content of polish-induced impurities to make sense the effect on surface quality in IBE.
The parameters in the ion beam etching (Sample #1, #2 and #3) are the same. Three samples are all processed in the KDIBF650L-VT Ion beam etching machine developed by ourselves. Figure 1 shows the experiments in the KDIBF650L-VT. The value of IBE parameters are shown in Table 2. The removal rate of material in the IBE is a determined value. Therefore, when the processing time and processing area are determined, the etched depth is determined as well.
Table 2. Parameters of IBE
2.2 Measurements on surface morphologies
Bruker NanoScope V Atomic Force Microscopy (AFM) is used to investigate the surface morphology after IBE in the #1 fused silica’s surface. The measuring range of AFM is 10μm × 10μm. The roughness values and the profile of surface microstructure of different etched depths are obtained by AFM.
In the #2 fused silica’s surface, a typical Hertz scratch is investigated. The size and morphology of the Hertz scratch on surface are recorded through the whole IBE process. Every time the scratch is etched by IBE for certain depth (0nm, 200nm, 500nm, 700nm, 1000nm), AFM microscope is used to investigate the Morphologies of scratch. The profiles are obtained by slicing the defects lengthways. The measuring range of AFM is 30μm × 30μm.
2.3 Polishing-induced contamination measurement
Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) is a surface-sensitive analytical method that can collect elemental, chemical state and molecular information from surfaces of solid materials. TOF-SIMS is accomplished by exciting a samples surface with a finely focused ion beam which causes secondary ions and ion clusters to be emitted from the samples surface. A time-of-flight analyzer is used to measure the exact mass of the emitted ions and clusters. From the exact mass and intensity of the SIMS peak, the identity of an element or molecular fragments can be determined.
In this paper the apparatus for contamination measurement is Model 2100 Trift II TOF-SIMS made by Electronics Physical Company. The ion source for analysis and the initial ion source are both the gallium source. The ion beam energy is 25 keV. The analysis range is 10x 10µm2. Polished induced contaminants before and after IBE on Sample #3 are analyzed. We investigate the elements likely to be introduced by the MRF process: Fe, Ce [7].
3. Result
3.1 Surface roughness change
After Sample #1 is ion beam polished for different depths, the surface morphologies of the eight areas observed by atomic force microscopy are obtained. Its roughness values and the surface morphologies of different etched depths are shown in Fig. 2. There is little visible defect on the initial surface of sample except for subtle polishing trace. The initial roughness value is 0.262nm RMS. Because of the existence of residual polishing particles and the subsurface damage in the polishing re-deposition layer, as the depth goes up to 50nm, the surface quality becomes worse. It direct-viewing represents the number of the “bright spots” in the morphology images increases significantly. Therefore, the roughness value increases from 0.262nm RMS to 0.339nm RMS. The Fig. 2(b) image for 50nm etched depth indicates a much rougher surface than the surface for no etched at all. The roughness value of 50nm is 0.339nm RMS, which is the worst. Later, as the etched depth increases, the surface roughness decreases generally to a level (0.238nm RMS). The surface quality is improved during IBE, due to the smoothing effect induced by ion sputtering at near-normal incidence.
3.2 Morphology evolution of microstructure
As the depth goes up to 50nm, the number of “bright spots” increases rapidly to maximum visibly. It could be think that the polishing re-deposition layer has been removed and the subsurface damage is exposed totally. Therefore, the “bright spots” shall be represented for subsurface dot-form microstructures, including in the subsurface damage. When the etched deep increases, the number of dot-form microstructures decreases. And at the 800nm etched depth, there are little dot-form microstructures in the measurement. To further study and characterize the sizes of dot-form microstructures in the surface morphologies pictures, the widths and heights of five dot-form microstructures in the six Figs. [Fig. 2(b)-2(g)] are measured along the center line by AFM, respectively. These five dot-form microstructures are selected randomly from the surface. The sizes of dot-form microstructures are shown in Fig. 3. The morphology evolution of dot-form microstructures showed in Fig. 4. The initial width of dot-form microstructures is about 200nm, and the height is less than 3.5nm. It is obvious that the sizes of dot-form microstructures are becoming shorter as the IBE depth increases to200nm. But the width changes little. When the etched depth reaches 800nm, the height of dot-form microstructures remain decreases (less than 1nm. The width becomes increases from 200nm to 500nm. The result indicates that IBE can mitigate the dot-form microstructures in the subsurface.
3.3 Morphology evolution of Hertz scratches
According to previous research [4, 17], brittle scratches play an important role in decrease the laser damage performance. In order to investigate the morphology evolution of brittle scratches during IBE, a typical Hertz scratch on Sample #2 will be ion beam polished. Since defect pits have similar evolution trends, only pits 2, 3 are investigated. To quantify the change in the morphologies, the profiles of defect pits 2, 3 in the Hertz scratch on Sample #2 are measured by AFM along the straight direction, shown in Fig. 5. The AFM images are 30μm × 30μm. Focus on defect pit 2, there are two stages during IBE. The initial depth is about 750nm and the initial width is 2µm. When the ion beam etched depth reaches 200nm in Stage 1, the depth of pit 2 increases to 2µm rapidly, while the width changes little. There are lots of polishing compounds, structural densification of the glass, plastic deformation and fracture in the defect pits, whose sputtering yield is greater than that of the bulk material. In the IBE process, compared with the defect-free surface, the removal rate inside the pit is faster than outside and the change in depth is more obvious. In fact, the re-deposition layer is removed out at this time. Later in the Stage 2, when the etched depth exceeds about 200nm, with the increasing etched depth of ion beam etching, the width of defect pit becomes larger, and the depth increases little. Eventually, the shape of profile is becoming smooth.
3.4 Polish-induced contaminants analysis
Polish-induced contaminants (Fe, Ce) before and after IBE on Sample #3 are analyzed. The results of the impurities (Fe, Ce) before and after IBE on the Sample #3 are given in Fig. 6(a) and 6(b). It is obvious that the relative content of Fe and Ce impurities both significantly reduce after IBE. The surface relative number of Fe falls from 3.5% to less than 0.5%, and the relative number of Ce drops from 0.08% to less than 0.003%.That is, the surface impurity concentrations of Fe and Ce reduce to less than 1/5 of the initial level without IBE. As a reference, Fe, Ce contamination is introduced on the surface in MRF, which suppresses the improvement of LIDT. And the decrease of polish-induced contamination can enhance the LIDT. Therefore, the result from sample #3 can be a suggestion that IBE can wipe out the polish-induced contaminants from sample surface to raise the LIDT.
Fig. 6 Comparison of Fe, Ce before and after IBE (Sample #3). (a) Comparison of Fe before and after IBE (Sample #3) (b) Comparison of Ce before and after IBE (Sample #3).
4. Discussion
All Ion beam processing method uses the ion sputtering effect to remove the material and obtain the super-smooth surface (0.238nm RMS). Because of non-contact processing characteristics, IBE can be used to remove or mitigate the laser damage precursors from sample surface. This etching passivation phenomenon has been investigated in previous research [18]. In this paper, two major kinds of visible subsurface damage: dot-form microstructures and a shallow intrinsic defect layer found on surface (such as scratches), which may play an important role in laser damage resistance as damage precursors, are investigated.
Alternative effects lead to the observed dot-form microstructures, it is possible that dot-form microstructures may be the residual polishing powder in nanoscale and the internal defect clusters of fused silica. However, there is no direct method to study the composition of dot-form microstructures here. Therefore, a hypothesis for what the dot-form microstructures are offered below. Dot-form microstructures may represent the hardening of subsurface layer in nanoscale, whose material density, hardness and elastic modulus are higher than the fused silica matrix. Dot-form microstructures may result from the surface densification of fused silica. In the crystalline structure of fused silica, there is considerable space in Si-O, O-O and tetrahedral units. Under certain pressure, a fused silica surface will always be densified, shown in [18]. According to [19–21], the fused silica surface will be densified permanently under pressure 2GPa. As the pressure goes up to 12GPa, the densification saturation phenomena will occur. Thus, the conventional polishing process can cause the densification in fused silica surface and obtained an anisotropic surface with various material properties. The visible feature is the existence of scratches in surface and subsurface.
During the IBE in Sample #1, the local erosion rate would be influenced by the initially existing local densification. The residual polishing powder cause local densification in fused silica surface and result in the difference of erosion rate at nanoscales. As a result, dot-form microstructures with a height of 3.5nm appear when IBE for 50nm. And their width is close to the width of polishing powders. In response, the morphology evolution in IBE at nanoscales does not totally match with the Bradley and Harper (BH) model based on Sigmund’s sputtering theory. And the erosion rate cannot be ignored. According to [22, 23], is redefined as a variable and related to the local material properties at coordinate point (x, y)
Where is the molar weight of the sputtering target; and represent the local material density and local sputtering yield of the corresponding point; NA is Avogadro number; is ion current density. For a given material, Eq. (1) indicates that the local erosion rate is mainly determined by material density and sputtering yield at the same ion sputtering conditions. And is determined by local binding energy and.So the local erosion rate is determined by material density and local binding energy and [18]. According to the research by Keller [24], the local erosion rate is mainly determined by material density and the effect of can be ignored. So during the IBE, the local erosion rate will be limited by the initially existing local densification and lower than the erosion rate of surface without densification. When the polishing re-deposition layer is removed at IBE 50nm, large numbers of the dot-form microstructures appear. Then, as the IBE depth goes up, the dot-form microstructures can be mitigated during IBE. And the number of dot-form microstructures reduces significantly as well. This may be because the smoothing effect induced by ion sputtering at near-normal incidence. Mayer points out that ion-induced viscous flow have an obvious effect on smoothing fused silica surfaces [25].
The roughness results from sample #1 indicate that IBE has a smooth effect on the optic surface as well. To prove that the evolution of the roughness is due to the dot-form microstructures, subregions of the AFM image [Fig. 2(b)] with the dot-form microstructures censored are evaluated in Zygo, shown in Fig. 7. When the bulges (height>2nm) are censored in Fig. 7(a), the roughness value has a dramatic decrease (0.236nm RMS) in the absence of the dot form microstructures. It is close to the initial roughness value of unetched surface (0.262nm RMS). The results indicate that dot-form microstructures have an adverse impact on roughness. When the dot-form microstructures become mitigated in size and amount, the roughness value decreases. The roughness results accord with the evolution of dot-form microstructures.
Fig. 7 Influence on the roughness of dot-form microstructures (Sample #1). (a) The image [Fig. 2(b)] in Zygo (b) The image [Fig. 2(b)] with the dot-form microstructures censored in Zygo.
In addition, IBE can broaden and mitigate brittle scratches. However, Fig. 5 shows that IBE is different from other post processing technology in mitigate scratches. With the increasing etched depth, HF etching is isotropic and the aspect ratio decreases. And in MRF, with the increasing etched depth, the width of defect pit increases little, and the depth decreases remarkably, shown in [26]. They both select a typical Hertz scratch and study the evolution trends of defect pits in the post processing technology. In the IBE, when the removal depth is 200nm, the depth of defect pit increase to 2µm from 750nm, and the width does almost not change. This is because of the broken structure and impurities contained in the Hertz scratches, whose sputtering yield is greater than that of the bulk material. As a result, the material removal rate in the depth direction is significantly faster than that of the width direction [27]. After the broken structure and impurities contained in the Hertz scratches are totally removed, with the increasing etched depth of IBE, the width of defect pit becomes larger, and the depth increases little. Previous researches show that the erosion rate is dominated by incidence angle . Liao uses TRIM ion sputtering simulation and points out that the erosion rate will increase when the incidence angle is between 0° and 80° [18]. When the incidence angle reaches 60°, the erosion rate is maximum. In conclusion, there is almost the same material removal rate at the defect pit bottom and the optic surface due to the same ion beam incident angle (≈0°). But the actual incident angles along the pit 2 edge are about 45°, shown in Fig. 8. So the erosion rate in width direction is faster than the in depth direction. And there is little change in height and the width is enlarged. The shape of profile is becoming smooth. After IBE 800nm, it takes about 2.1 hours to enlarge the width/depth from 2 of 200nm to 2.75. The mitigating efficiency is close to MRF, but lower than HF acid etching [26]. But in effect, all three post processing technologies make the width/depth increase.
In order to investigate the influence on LIDT of Hertz scratches during IBE, LIDT tests for the third-harmonic (355nm) at a repetition rate of 1 Hz are performed on the same Hertz scratch in the Sample #2 as a reference. An R-on-1 procedure is used in LIDT tests in which a ramping fluence focuses on a single area until damage, whose diameter is beyond 10 µm, is appeared on the imaging CCD. The pulse length τ of the table-top Q-switched Nd:YAG laser is 7ns. The laser beam profile is near-Gaussian with a focused area of ~4.5mm2. The laser beam is focused on the Hertz scratch on the backside of the sample. It should be noted that only three groups of tests are conducted (0nm, 500nm, 1000nm) limited by the length of Hertz scratch. Figure 9 shows optical microscope images of laser damage on the Hertz scratch. The LIDTs of Hertz scratch during IBE are in Table 3. The LIDT goes up from 3.4 J/cm2 of unetched to 4.4J/cm2 of 1000nm etched depth. So IBE is beneficial to improve the damage performance of Hertz scratch on fused silica.
Fig. 9 Microscopic images of damage morphology with IBE depths (Sample #2). (a) Damage morphology before IBE (b) Damage morphology after IBE 500nm (c) Damage morphology after IBE 1000nm.
Table 3. Hertz scratch LIDTs with various IBE depths
Finally, the surface LIDTs of various IBE depths are investigated on sample #1. The involved parameters in LIDT test measurement system are described before. The laser beam is focused on the backside of the sample and the test sites are chosen randomly in the different eight areas on sample #1.
According to the results in Table 4, IBE can raise the LIDT. An overall consideration is necessary to investigate the impact of various precursors on laser induced damage of fused silica optics. Relationships between LIDT and various damage precursors are shown in Fig. 10. The change of LIDT during IBE contains two stages. The critical point is the situation of IBE 50nm. It should be noted that when the IBE removal depth increases to 50nm, the LIDT gets a decrease and reaches the minimum value. Interestingly, at the same time the dot-form microstructures appear. Its number reaches to the maximum, and the roughness value is worst (0.339nm RMS). It is indicated that the polishing deposit is removed entirely and all the subsurface defects in Hertz scratches are exposed. It seems that a large quantity of defects appears in the early IBE and restrains the improvement of LIDT. So the LIDT decreases in the Stage 1. When the IBE depth increases further in Stage 2, the IBE begins to dominate to raise the LIDT of fused silica. Meanwhile, the dot-form microstructures are mitigated both in size and amount during IBE. The surface quality comes to be improved as well (0.238nm RMS). Hertz scratches are broadened and passivated without increasing in the depth direction. Dot-form microstructures, Hertz scratches and roughness value have a good correlation with LIDT. The change of LIDT implies that the effect on laser-resistant capacity is dominated by comprehensive factors. Although alternative effects lead to the observed dot-form microstructures, the elimination of dot-form microstructures has no negative effects on improving LIDT. When the IBE depth reaches 300nm, although the roughness value has a slight increase, the LIDT still get increase. Finally, the LIDT reaches 9.9J/cm2, which is 1.34 times than the initial value of unetched surface. Laser damage test reveals that the dot-form microstructures, Hertz scratches and roughness value play an important role in causing a localized optical absorption to lead to laser damage when exposed to sufficient laser fluence. And IBE process can mitigate dot-form microstructures, Hertz scratches and roughness value to enhance the laser-resistant capacity of fused silica.
Table 4. Surface LIDTs with various IBE depths (Sample #1)
5. Conclusions
The experiments have something meaningful to clear what are the nature and characteristics of the precursors that lead to laser damage initiation in IBE. And the evolution of fused silica surface in IBE can affect sample laser damage resistance in some way.
The mechanism of LIDT improvement for the IBE processes has been explained by mitigating the dot-form microstructures and passivating scratches. The dot-form microstructures are mitigated both in size and amount and the concentration of polishing powder in the polishing re-deposition layer can be wiped away from fused silica surface. With the increasing etched depth of IBE, this technique enlarges the width of defect pit but changes the depth little. The shape of scratch profile becomes smooth. What is more, the surface quality of substrate will not be damaged and a super-smooth surface (0.238nm RMS) is obtained. Therefore, the laser damage test results argue that IBF can improve the LIDT greatly. After ion beam etching for 800nm, the mitigated sample has a 134% higher LIDT than the initial sample. But the composition of dot-form microstructures here is still uncertain. Dot-form microstructures will most likely represent the hardening of subsurface layer in nanoscale. This work has significance in optimizing post processing technologies to improve LIDT of fused silica.
Funding
National Natural Science Foundation of China (NSFC) (51675526, 91323302, 51275521); China Postdoctoral Science Foundation (CPSF) (42469).
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