The laser damage resistance of an optical element in high power laser systems depends significantly on the surface quality of the optical substrate. In this experiment, commercially polished fused silica substrates were etched in argon plasma generated by a RF source and their surface roughness, flatness and optical properties were investigated. This method can be applied in a vacuum chamber prior to deposition of the multilayer coatings without breaking the vacuum. It was shown that by etching the resistance to 355 nm, laser radiation could be improved more than 8 times. However, it strongly related with primary substrate quality. The etching depth from 100 nm suggests the optimum performance of surface quality in terms of surface low roughness, high flatness, and high laser damage threshold. These results are of significant importance for the manufacture of high quality laser optics on fused silica substrates. As an example of an application of our technology, anti-reflective and polarizing optical interference coatings were deposited on etched substrates and the increase of their resistance to laser radiation was measured.
© 2017 Optical Society of America
It is well known that the conventional CeO2 abrasive polishing technique, which relies on a normal load applied to an abrasive particle, causes material removal and generates scratches and subsurface damage (SSD) of fused silica substrate [1–3]. Subsurface damages are defined as residual digs and scratches, some of which are filled with polishing slurry and covered with so-called Bielby layer (polished layer) . These hidden and un-hidden scratches are responsible for the combination of two phenomena: light absorption due to residual polishing slurry, which is especially significant in UV wavelength of laser radiation [5–7] and enhancement of electric field distribution of laser pulses due to surface pits and micro-cracks [8, 9]. Previously, it was shown that the polishing residuals have the values from 30 ppm to 5 ppm in 50 nm depth  or even up to 80 nm  of polished surface. Multilayer coatings deposited on fused silica (FS) substrates without and with pits, precisely fabricated on the surface using femto-second laser processing, were analyzed in recent Chai and associates publication . They have showed that the laser damage probability was 20% - 60% higher for multilayer coating on substrate with pits than without. Authors finally conclude that the pits on substrate surface could contribute to the occurrence of damages.
The strong effect on reduction of scratches and SSD was reached by using magnetorheological finishing process [12, 13]. However, this very complex finishing procedure essentially did not eliminate the problem of SSD. For this reason, various post-polishing techniques were suggested by many authors during the past decade: 1) wet etching in hydrofluoric acid/ammonium fluoride based solutions (HF/NH4F) [6, 14, 15], 2) leaching in concentrated nitric acid and hydrogen peroxide (HNO3/H2O2) solution , 3) ion etching [17, 18], 4) UV laser conditioning  and 5) polishing in dielectric barrier discharge inert gas plasma at atmospheric pressure .
Recently, it was reported about increase in laser induced damage threshold (LIDT) by a factor of 1.5 after direct dielectric barrier discharge plasma treatment at atmospheric pressure . Ion beam etching for 800 nm, improved LIDT of fused silica for 1.34 times . In our previous research , the LIDT (at 355 nm laser radiation) of commercial polished FS substrates was successfully increased up to 4 times by performing HF/HNO3 acid etching. Developed etching procedure has removed the polished layer and has cleaned out the slurry containing subsurface damages, however, surface quality (roughness and flatness) were substantially impaired. The same conclusion was reported by the other authors  .
Therefore, an alternative method for removal of Bielby layer containing SSD by plasma argon etching/polishing is presented in this work. The purpose of this work is to create a simple and rapid method to enhance the surfaces of optical substrates with no surface roughness increment and no substantial surface flatness degradation. Moreover, this method should be applied under vacuum in physical vapor deposition (PVD) chamber, therefore, multilayer coatings can be deposited immediately after etching on improved FS substrates. However, we have to note, that in this research plasma etching and coating process were done in separate chambers. The explanation is in section 2C.
Hafnium oxide (HfO2) and silicon oxide (SiO2) were chosen as high and low refractive index materials for coating deposition. It was reported that in visible spectra HfO2 has refractive index more than 2 [23, 24] and SiO2 around 1.5 .
2. Experimental procedure
A. Substrate preparation
To guarantee clean etching, FS optical substrates (25.4 mm diameter and 5 mm thickness), were washed in four-stage ultrasonic cleaning system “UCS40” (Optimal Technologies) before etching and coating deposition. First stage included ultrasonic cleaning in alkali solution with heating and filtration; second- ultrasonic rinsing with flowing water while heating; third- rinsing with de-ionized water while heating and slowly draining; last stage was drying in 60°C environment. The cleaned samples then were placed in a special holder and moved to vacuum chamber for etching and coating process.
B. Plasma etching
FS substrates prepared in ultrasonic bath were etched by “RADIANCE” sputter cluster platform (Evatec AG), equipped with three magnetron sputter sources and radio frequency (RF) plasma source. Schematic diagram of plasma etching procedure and RADIANCE sputter platform are shown in Fig. 1. Etching procedure was performed at 0.23 Pa argon pressure and 1kW RF power. Substrate holder was rotating at the rate of 53 rpm, turn table does not rotate in this experiment. Firstly, calibration procedure was done to estimate etching rate. For this purpose, three etching procedures on polished FS substrate for the same duration were performed. Then procedures were repeated 4 times in 3 days in order to confirm plasma stability and get statistics. For measuring the etching thickness before procedure, part of substrate surface was covered by special mask. FS surface under mask was not influenced by plasma, therefore edge on the surface was created. This edge was measured by profilometer Dektak150 (Veeco). For every thickness determination, 20 scans were done. The maximum measurement error of this technique was 7%. The etching rate was determined as the ratio of the measured thickness to the etching time. Controlled by etching time the layers with thicknesses of 20 ± 1.4 nm, 50 ± 3.5 nm, 100 ± 7 nm, 200 ± 14 nm and 400 ± 28 nm were removed from the FS substrates surface.
C. Coatings deposition
Anti-reflective (AR<0.2%@355 AOI = 0 deg) and polarizing (Pol@355 AOI = 56 deg, Tp>95%, Rs>99.5%) coatings for 355 nm wavelength were deposited on etched and non-etched substrates by the e-beam process. Reflectance and transmittance spectra of deposited optical interference coatings (OIC‘s) are shown in Fig. 2. The motivation to use e-beam evaporation rather than sputtering process was to avoid unexpected absorption loss in coatings that may appear due to incomplete metal oxidation during sputtering [26, 27] of metal targets. Therefore, evaporation from stoichiometric SiO2 granules and HfO2 tablets (both from Umicore AG) in O2 environment was chosen to investigate FS etching performance and resistance of deposited coatings to UV laser radiation. Details of e-beam deposition setup could be found in Ref .
The HfO2 and SiO2 were chosen as high (H) and low (L) refractive index materials respectively; both H and L had a quarter-wave optical thickness at the reference wavelength of 355 nm (H: 43.40 nm, L: 60.30 nm).
AR coating with a layer structure Substrate/0.40H1.29L/Air and polarizing coating with a structure Substrate/1.23H0.74L0.93H0.92L0.95H1L(HL)^12_0.77H1.05L0.85H0.68L/Air were deposited. The working pressure and temperature of substrates were 2 × 10−2 Pa and 300°C respectively.
Transmittance spectra were measured in the 190<λ<1000 nm wavelength range using a Lambda 950 (Perkin Elmer) spectrophotometer.
Surface roughness was measured using a Dimension Edge atomic force microscope (AFM) (Bruker) in tapping mode, over a 20 × 20 μm2 scan area. In order to achieve better statistics of roughness values, four areas (20 × 20 μm2) were analyzed before and after etching of each FS sample and root mean square average of the profile height deviations (Rrms) result is reported.
FS surface flatness was analyzed by interferometric system “MarSurf FI 1040Z” (Mahr ESDI) based on peak-to-valley measurements and comparison before and after etching of each sample flatness (λ/x, λ = 633 nm) is presented.
LIDT (1-on-1 test) measurements of all FS substrates (coated and uncoated) were performed to investigate possible correlation with etching depth. 1-on-1 tests, according to slightly modified method described in ISO 21254-1 procedure  with micro-focusing approach. Laser setup is shown in Fig. 3.
The third harmonic of pulsed (frequency 15 Hz) nanosecond (pulse duration of 3 ns) Nd:YAG laser (Ekspla) was used for the LIDT measurements. The space filter (SF) was applied and Gaussian beam shape was formed. Attenuator, consisting from λ/2 phase plate and polarizer (P) allowed to change pulse energy without influence to pulse duration, beam shape, etc. Short distance focusing lens (f = 20 cm, focal waist diameter w0~65µm) was chosen for sample LIDT testing in order to avoid damage of backward surface of samples due to laser beam self-focusing. Scattered light was collected by scattering detection module (SDM) using lens L2 and separated by ~0.5 mm aperture. Aperture position enabled us to choose the exact projection of laser exposed sample surface. The shielding of main beam was used by displacing ∅10 mm metal circle near lens position in order to get more precise LIDT results. In this setup, the beam transmitted through the sample without any damage or defect was fully blocked by this shield and no scattered signal was registered. When sample was positioned on damaged area, exposed light scattered widely and photodiode PD2 registered scatter signal. Dependent on sample, SDM can be used in transmission configuration at 0° angle of incidence (AOI) or reflection at any angle. In this work, SDM was in transmission configuration at 0° AOI for anti-reflective coating measurement, and reflectance configuration at 0° AOI for bare substrates, and 56° AOI for polarizing coating. All LIDT measuring system was controlled by personal computer. Automated program has controlled X-Y-Z direction of motorized sample holder, operated shutter (SH) and collected incident and scattered energy data of each pulse. Sample holder itself was adjusted at 0° for bare substrates and antireflective coating measurements and 56° for polarizing coating. Manual rotating of λ/2 phase plate allowed control of energy fluency in focused area in wide interval. For a better precision of measured data after laser exposition in a few hundred places, additional scanning of the same area was done using small exposing laser pulse energy. This method allowed us to achieve automated LIDT registration (damaged or not damaged). The results of automated evaluation of damage areas and those tested with microscope fully coincided. This method speeded up a few times the measurement procedure and enabled us to avoid mistakes.
To estimate laser beam energy in damage area, calibrated energy meter was used. The spot diameter was measured by “moving knife-edge” method using the same X-Y motorized sample holder with submicron step scanning [30, 31]. LIDT for anti-reflective coating was measured at normal incidence, polarizing coating for p polarization at 56° of incidence.
3. Results and discussion
For loss evaluation of FS substrates before and after etching, optical transmittance measurements were analyzed. Many authors have noticed significant decrease of transmittance in UV range after exposure to ion flux due to contamination of the surface from filament/grid erosion or other contaminants . Transmittance spectra of FS substrates before and after plasma etching are compared in Fig. 4.
Spectrophotometric transmittance measurements have revealed no significant increase of loss in etched FS substrates. If there are vanishingly small quantities of photo-absorbing contaminants, for example tungsten (W), which could originate from plasma source grid, this method is not enough sensitive to detect it.
B. Surface roughness and flatness
The most common deliverable of etching smooth and flat surface is degradation of its quality, i.e. increasing roughness  and decreasing flatness. Despite those negative aspects etching is still applied in many fields. However, etching of optical substrates, combined with those negative deliverables is unsuitable for laser optics. Therefore, it is necessary to develop soft and homogenous etching procedure to prepare high surface quality optical substrates for high LIDT OIC deposition.
Comparison of FS surface roughness and flatness values before and after etching of different depths is presented in Fig. 5. Measurements have revealed that the roughness of etched samples did not increase comparing to the initial values, within the error margins. Moreover, surface roughness values of laser grade optical substrates varied from 0.4 nm to 0.7 nm (RMS), which is acceptable for laser optics.
Flatness analysis revealed, that no significant change in surface flatness after plasma etching procedure was observed.
C. Laser damage resistance
All etched FS samples have demonstrated significantly higher laser damage threshold (from ~4 to ~8 times) than the ones without etching. The LIDT (1-on-1) measurements at 355 nm wavelength, showed more than 8 times higher threshold values for (50.0 ± 3.5) nm and (100 ± 7) nm etched FS substrates respectively, compared with non-etched ones (Fig. 6). It suggests that the etching depth from around 50 nm to 200 nm, demonstrates the best performance of surface quality (in terms of RMS, flatness, and LIDT). It was previously reported, that absolute value of non-etched commercial FS substrates varies between 5 J/cm2  to 22 J/cm2  dependent on polishing method performed by different vendor. Therefore, we investigated LIDT of 5 different commercially available FS substrates from 4 different vendors. The measurements showed, that LIDT values (at λ = 355 nm) varied from 5.8 ± 0.4 J/cm2 to 12.6 ± 1.0 J/cm2. Substrates, used in this work, was taken from the one vendor, where LIDT varied from 5.8 ± 0.4 J/cm2 to 8.1 ± 0.7 J/cm2 with the average value 7.0 J/cm2. LIDT values difference between non-etched and etched substrates depend significantly on substrate vendor. It should be also noted, that LIDT is strongly depend on parameters of laser, such as pulse duration, repetition rate, beam size and shape, laser type, laser stability and LIDT evaluation from damage data [32, 33]. Those result suggest, that dependent on primary substrate quality, LIDT values after etching can increase from ~4 to 8 times.
The results also showed, that after etching of more than 100 nm, resistance of etched FS substrates to laser radiation started to decrease. This could be related with some tungsten contamination from ion source during exposure, because etching of 200 nm and 400 nm took much longer time, ~29 min and ~58 min respectively compared to cases of etching the thickness of 100 nm or below. It requires additional investigation.
C. OIC on non-etched and etched FS substrates
As an example of possible substrate plasma etching benefits, anti-reflective (AR<0.2%@355 AOI = 0 deg) and polarizing (Pol@ 355 AOI = 56 deg, Tp>95%, Rs>99.5%) OIC‘s were deposited on etched (100 nm depth) and non-etched FS substrates. This type of coatings has one common feature - almost all laser energy after irradiation is transferred to the substrate-coating interface, therefore positive etching effect on the OIC’s LID performance should be clearly seen. LIDT measurements have revealed, that anti-reflective and polarizing OIC‘s, deposited on etched substrates, had 2.4 and 2.0 times higher LIDT, respectively, than OIC’s deposited on non-etched substrates (Fig. 7).
Characteristic damage morphologies of non-etched and etched FS substrates, coated with AR<0.2%@355 AOI = 0 deg were investigated by optical microscope. From these observations, it is clear that the damage behavior of different samples is significantly different, as shown in Fig. 8.
The breakdown of AR samples deposited on non-etched substrates starts from point defects. This phenomenon could be clearly seen while observing sequence of damage sites, exposed with constantly increasing laser fluence. These point defects could be related to polishing particles, embedded within “Beilby” layer  or within the subsurface damages  and therefore are acting as damage precursors. To confirm this assumption completely, further deep analysis of damage behavior is necessary. Damage morphology of AR coatings made on etched FS samples had very few visible point defects. Based on those observations we hypothesize that thermal effects are mostly involved in damage processes of both, etched and non-etched samples due to absorption of laser radiation in those point defects.
LIDT (1-on-1) measurements at 355 nm wavelength, showed 8.4 times higher threshold values for bare FS substrates, having etching depth of (50 ± 3.5) nm and (100 ± 7) nm compared with non-etched samples. LIDT values difference between non-etched and etched substrates depend significantly on substrate vendor. The etching depth from around 50 nm to 200 nm enhanced performance of FS surface quality (in terms of RMS, flatness and LIDT). Anti-reflective and polarizing optical interference coatings, deposited on etched substrates, had 2.4 and 2.0 times higher LIDT than those deposited on non-etched substrates. Presented argon plasma etching principle has led to considerable LIDT improvement of uncoated and also coated FS substrates.
The research leading to these results has been partially supported by project DEMOS (Grant No. E!10232) based on Eurostars-2 program and received from Lithuanian Agency for Science, Innovation and Technology.
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