Subsurface damage (SSD), especially photoactive impurities, degrades the performance of high energy optics by reduction in the laser induced damage threshold. As the polishing defects are trace content and lie beneath the surface, they are difficult to detect. We herein present a biological method to measure impurities on polished fused silica, based on the intense inhibiting ability about trace level of ceria on enzyme activity. And the enzyme activity is measured in the individual etching solutions of a sequential etching process. Results show that detectability of the biological method satisfies the needs of trace impurity detection with low cost and simple apparatus. Furthermore ceria can be used to tag SSD in lapped and polished optics.
© 2014 Optical Society of America
The lifetime of optical components subjected to high fluences is a particular problem in UV or high power laser systems. Improving optics damage thresholds for their operation under high-energy irradiations has been major research field of optical manufacture. Laser-induced damage on optical surfaces is often associated with absorbing contaminants introduced by the polishing process. It is well known that photoactive impurities, such as ceria, embedded in polished surfaces and subsurface cracks produced in prior grinding or lapping processes are the possible damage precursors. Even nano-scale impurities and defects can lead to significant threshold reduction . Liu  considered that cerium element had a strong influence on damage threshold while subsurface damage (SSD) had a strong influence on damage density. Recent results have shown that the cracks is likely to be the main cause of laser damage [3,4], and fracture-induced subbandgap absorption is found to be the dominant source of laser damage initiation .
For high power laser applications, SSD under polished surfaces must be eliminated entirely so accurate and efficient SSD measurements is a key factor in developing new manufacture techniques or optimizing existing polishing processes. SSD includes fractures and scratches produced during both grinding or lapping and polishing that get partially or totally hidden by the polishing redeposition layer . The defected layer and redeposition layer overlapped successively in bulk material. And from the subsurface layer to the surface layer, entropy exhibits an ascending trend . Therefore, SSD distinguishes itself from other regions in both composition and microstructure, which contains hydrated silica induced by dissolution process, embedded impurities in polishing slurry, fractures, and scratches. Furthermore, the depth of the redeposition zone and defected zone were found to be very process dependent, extending from 5nm below to as much as 140 nm, and from 100 nm to 500 nm, respectively [8,9].
Optical microscope measurements, such as polishing-etching method, tapering method, ball dimpling method, magnetorheological finishing (MRF) spot technique, and MRF wedge technique [10–14], are in fact the most widely used techniques for characterizing subsurface cracks depth so far. The quasi-Brewster angle technique , nanoindenter technique , and light scattering measurement technique  were adopted to comprehensively characterize the SSD in polished optical surfaces. For specific damage types, Secondary Ion Mass Spectroscopy (SIMS) [2,17,18], Inductively Coupled Plasma-Atomic Emission Spectrometer (ICP-AES) , and Inductively coupled plasma optical emission spectrometer (ICP-OES)  had been used to profile the distribution of impurities in the polished subsurface region of fused silica, and peak concentration of cerium element on the surface was 23 ppm . Kozlowski’s SIMS data showed an exponential decay of contaminant concentration to a depth of 100-200 nm . This depth is consistent with a polishing redeposition layers formed during the chemo-mechanical polishing of fused silica. More recently, small amount of Barium, nano-sized fluorescent particles, and oil based coolants were selected as a tracer for cracks detection, then, ICP-AES and confocal fluorescence microscopy were employed to achieve relative concentration of the marks with depth [20–22]. However, these impurities sensitive measurements are highly dependent on special and expensive apparatus, which limits the application in optical fabrication and post treatment processes.
In this work, we have used a biological rule about enzyme activity inhibiting to detect trace ceria in polished fused silica. Polishing redeposition layer was gradually removed by chemical etching method and mixed with enzyme solution. Then residual activity of enzyme in the mixture was measured with spectral photometer. The ceria concentration distribution with depth was finally achieved according to the established calibration curve about ceria concentration to residual activity of enzyme. We also compared different SSD characterization techniques, constancy of chemical etch rate method, HF bath dilution technique, and SIMS method, applied to polished fused silica samples. There exists good concordance between the different measurements.
2.1 Sample preparation methods
Two different samples were analyzed: (1) classical planetary polished fused silica and (2) classical planetary polished fused silica with an additional MRF polishing step. Substrate materials of the samples were fused silica (JGS2-1, SiO2>99.99%, Kinglassquartz Co., Ltd, Beijing, China), and size of the samples were 100 mm in diameter and 10 mm in thick. The samples were previously lapped with 20 μm SiC abrasive grains using cast iron plate, and the last trace of cracks depth is about 8 μm detected with MRF wedge technique . Then the lapped sampled were full aperture polished sufficiently by a classical planetary polisher developed by our laboratory using polyurethane pad with a diameter of 30mm, shown in Fig. 1, such that the residual subsurface cracks induced in Lapping process were completely eliminated. Polishing pressure, velocity, and ceria (PF-1-1, size range 1~3 μm, CeO2>99.95%, Yunnan Opto-Electro Auxiliary Material Co., Ltd, Kunming, China) slurry concentration were 2.8 kPa, 2 m/s, and 10 wt%, respectively. Then, an MRF machine developed by ourselves named UPF700-7 (using (a) PF-1-1 CeO2 based fluid maintained at a viscosity of 350 cp, (b) an 1 l/min flux, (c) 5 A field intensity, and (d) a 300 mm wheel at 100 rpm) was used to polish the classical polished samples with a one-dimensional scan mode. Furthermore, X-ray photoelectron spectroscopy (XPS, K-Alpha 1063, Thermo Fisher Scientific, U.K.) and scanning electronic microscope (SEM, S4800, Hitachi Co., Japan) were used for characterizing the size evolution of ceria and slurry components after polishing processes.
2.2 Ceria measurement by biological detection method
In contrast of transition metal ion inhibitors, rare earth ions especially its oxides exert severely inhibit ratio on several enzymes’ activity under trace level . Such as Ce4+ can inhibit superoxide dismutase activity and horseradish peroxidase activity was reduced by 14% by 0.01 mmol/L Ce4+ . Its mode of inhibition is one of typical competitive inhibition, which is the common type of inhibition of enzymatic activities where in the inhibitor competes with the substrate for the same binding site, thus preventing the binding of the substrate to the enzyme. Its kinetics is characterized by an increased apparent Michaelis constant (Km) with no change in maximum velocity of the enzymatic reaction (Vmax).
CeO2 is one of the most interesting oxides industrially because oxygen vacancy defects can be rapidly formed and eliminated, giving it a high “oxygen storage capacity” . The structure-function relationship of biological effect of ceria is in progress . The nature of scavenging superoxide anion reminded us its ability to severely affect oxidase activity than Ce4+ .
Enzyme activity can be characterized by unit time and per unit volume reduction of substrate or increase of product. Laccase, typical of multicopper oxidases, is a robust catalyst largely studied for its potential uses in industrial processes and in organic synthesis applications reducing dioxygen into water . Commercial laccase (Sigma-Aldrich Co. LLC, USA, 10 U/mg) is isolated from Trametes versicolor with molecular weight of about 70,000, as determined by Schiff staining of sodium dodecyl sulfate-polyacrylamide gels. Laccase catalyzed oxidation and polymerization of phenols always leads changes in products’ optical absorption properties, which are positive correlation with their concentrations. Guaiacol is one of the optimal substrate for laccase activity assay. Thus absorbance spectrum measurement of laccase-guaiacol reaction system was developed in 5 mL reaction volume (pH was adjusted by 1 M succinic acid to 4.5) containing the indicated concentration of laccase and 1 mM guaiacol for 5, 10, 15, 20, 25, and 30 minutes in 25 °C thermostat water bath. And the control group was inactivated enzyme, which was mixed with boiling water for 30 minutes. Then visible absorbance spectrum of laccase mediated process from 310 nm to 600 nm was recorded with spectral photometer (UV-5800PC, Metash Corp, China), shown in Fig. 2. As oxidative products of guaiacol by laccase exhibited maximum absorption peak at 465 nm according to Fig. 2, laccase activity was determined by the absorbance increase at 465 nm. The laccase activity was measured in a 5.0 mL reaction volume containing the indicated concentration of laccase, 1 mM guaiacol, and 50 mM succinic acid-NaOH buffer (pH 4.5) One unit was defined as the amount of the laccase that oxidized 1 uM of substrate per minute. Assays were carried out independently in triplicate.
Eight concentration (10−8 ppm, 10−7 ppm, 10−6 ppm, 10−5 ppm, 10−4 ppm, 10−3 ppm, 10−2 ppm, and 10−1 ppm) calibrated solutions were prepared with ceria, 4 mg/L laccase, and de-ionized water. The laccase activity was measured with spectral photometer mentioned above. And the relationship between ceria concentration and laccase residual activity (ratio of measured and standard enzyme activity) over five measurements and the fitting curve are shown in Fig. 3(a). Na, Cu, Mg, Ca, Fe, Al, and Mn are the common impurities imbedded in the polishing surface coming from glass components, machine parts, processing mediators, and surrounding circumstance [1,2,17–19], which has little influence on laccase activity comparing with ceria, shown in Fig. 3(b). Consequently disturbance of metal elements on the biological method is negligible.
Fitting equation between laccase residual activity and ceria concentration is given by
After sufficient washing with de-ionized water in ultrasonic environment, each MRF and classical polished fused silica sample was etched during 1 and 2 minutes in a alkaline solution (5% NaOH in 100°C), respectively. Then about 10 nm and 20 nm of MRF and classical polished silica were detected during this preparation, measured with optical interferometer (New View 200, Zygo Co., USA). This etching cycle was repeated up to 10 times which was generally enough to probe through the subsurface damage layer. After the controlled sample detection, each etchant was mixed with equal volume laccase solution and pH value was adjusted to 4.5. The laccase residual activity was measured with spectral photometer after 30 minute 25 °C thermostat water bath.
Then, relationship between ceria concentration of etching solution and surface material removal in each etching increment is given by
3. Results and discussion
3.1 Trace impurity and SSD measurement results
Samples detailed in section 2.1 were detected using the technique detailed in section 2.2. In the biological measurements, volume of etching solution, Vs, is 50 ml, values of a, b are 36.6 and −6.1, according to the fitting curve shown in Fig. 3. Etching increments in depth, δ, are measured with optical interferometer. Substituting this value and detected laccase residual activity with spectral photometer, Lra, into Eq. (3) gives the ceria concentration distribution along depth over five measurements, illustrated in Fig. 4. Correlation between ceria concentration and etch depth shows that impurities embedded depth of classical and MRF polished samples are in the range of 91.3 nm~107.8 nm and 49.3 nm~57.5 nm, respectively. An exponential decay of contaminant concentration to the etch depth is presented in Fig. 4, which is similar to the Kozlowski’s SIMS results . In the case of small particles (diameters<50 μm) electrostatic van der Waals forces are the primary acting forces. And the attractive forces between the impurities and the workpiece are so high that they are effectively redeposited back on to the workpiece and they remain on the surface even after sufficiently washing with de-ionized water in ultrasonic environment when polishing process is stopped . Therefore, the maximum errors of measurements results exist in the classical and MRF polished surface layer, which are 6% and 13%, respectively, shown in Fig. 4.
The slurry components after classical and MRF polishing were characterized by XPS. XPS spectra were recorded using Al Kα line with pass energy of 12 kV × 6 mA. Both the classical and MRF polished slurry give peaks of elements of Ce, Si, and O, while added peak of element Fe and Na appears for the latter, shown in Fig. 5. Moreover, the atomic concentration of Ce is 3.15% and 3.39% in element composition analysis of classical and MRF polished slurry, respectively. Therefore, disturbance of impurities on the biological method is negligible according to Fig. 3(b).
Surface and subsurface quality were verified by measurement with the atomic force microscope (AFM, Solver P47-PRO, NT-MDT Co., Russia) with a scanning size of 50 μm × 50 μm. The subsurface structure of classical and MRF polished samples are shown in Fig. 6, in which redeposition layer are eliminated completely with etching method.
Etch rate and surface roughness (Ra) of each etch depths were obtained with 10 measurements equally placed radially on the classical polished surface by AFM, shown in Fig. 7. Redeposition layer depth of polished samples are between 90.7 nm and 108.2 nm deduced from the etch rate and surface roughness curve, which were detected by constancy of chemical etch rate method  and HF bath dilution technique . There are obvious fluctuating regularity of surface roughness with etch depth similar to the Neauport and Carr’s results [22,31], which attributes to hydrated silica, re-deposition impurities embedded in polishing scratches, and etch rate change with contact area between substrate and etchant.
Second ion mass spectroanalyzer (Model 2100 PHI Trift II TOF−SIMS, Physical Electronics, USA) was used to analyze the concentration depth profile for potential contaminants embedded in classical and MRF polished samples. Contaminants detected include the major polishing compound components (Ce from ceria), and other metal (Fe) incorporated during the MRF polishing, shown in Fig. 8. Correlation between sputtering account of Ce and sputtering depth clearly indicates that impurities embedded depth of classical and MRF polished samples are in the range of 90.3 nm~104.2 nm and 46.2 nm~55.5 nm, respectively. MRF material removal is mainly dominated by shear load, where classical finishing processes are dominated by normal load, thus, MRF induces very low SSD . However, the peak concentration of Fe is higher by over one order of magnitude than that of Ce in MRF polished samples, shown in Fig. 8(b), which will further reduce laser damage threshold for Fe element has much more absorbing ability to laser . Some process parameters and surface/subsurface quality are detailed in Table 1. Because of its global surface detection capabilities similar to ICP-OES  and ICP-AES [19,20], the biological method is of great interest compared to local (100 μm × 100 μm) SIMS analyses. However the analyzable surface can be reduced and localized to particular zones if necessary .
3.2 Structure characterization and embedding mechanism of ceria
Ceria polishing consists of the passage of particles under load across the silica surface which possesses a chemical tooth that expedites both bond shearing and transport of reaction products away from the surface faster than their rate of redeposition. The magnitude of the tensile force is determined by the local pressure of the grain, the extent of bonding between the particle and the silica surface, and the bond strength of the polishing grain relative to the Si–O bond .
SEM was used for measuring the shape of ceria before and after classical polishing, which can be simplified to multipyramidal, spherical or elliptical structure, shown in Fig. 9(a). The glass-polishing process is also a milling process for the polishing agent changing the average particle size and shape with continuous fracturing and wearing effects, shown in Fig. 9(b). Cumbo  considered that the median ceria particle size was reduced to 50% of the initial value after 6 h of polishing.
For the case of nanosized particle, shown in Fig. 9(b), Si–O–Ce linkage is formed between particle and silica surface by a hydroxyl in polishing slurry. As the particle moves away from the bonding site, the formed Si–O–Ce bond is strained. Considering the Ce–O bond strength (61.4 kcal/mol) is lower than that of Si–O (102.5 kcal/mol), the oxygen will remain with the silica, leading to accretion or redeposition of particle on the silica surface . Williams  estimated the magnitude of the forces attracting and holding quantum dots (7.8nm) on the surface by comparing the electrostatic van der Waals forces (1.0 × 10−10 N nm) and hydrodynamic force (3.3 × 10−12 N nm) acting on the quantum dots respectively. Therefore the nanosized quantum dots can remain in the vicinity to interact with the workpiece surface.
Cook  considered that impurities in the polishing solution can repolymerize back onto the glass surface via surface hydroxylation of glass and polishing particle. That is scission and release of surface species having only one bridging oxygen linkage initially is the most probable material removal processes. Then adjacent molecular water and hydroxyl will rapidly react with the free silica intermediate to form a free silicic acid molecule. On dissociation, the silicic acid molecule can rebond to the glass surface, or form a complex with adjacent metal ion sites on the polishing particle surface via hydroxyl exchange. We consider that trace ceria particles and silicic acid molecule are uniform mixing in the redeposition layer. And there should exist rough relationship between trace impurity concentration distributions in depth with freshly polished surface structure. Referring to the optical grinding process, the calculated cumulative crack depth distribution was exponential distribution according to fundamental instantaneous distribution of cracks, the summation of the cracks distribution, and their continued shortening with material removal .Therefore, during polishing SSD detection, trace ceria impurity can be selected as the tracer of SSD distribution along depth.
Rare earth oxides exert severely inhibit ratio on laccase activity under trace level, which promises the validation of laccase subject to determination of rare element in polished surface. In this work, we have used above biological rule associating with gradually etching removal of redeposition layer to detect trace ceria in polished fused silica. Results show that detectability of the biological method satisfies the needs of trace impurity detection with low cost and simple apparatus. And there exists good concordance between different common measurements with the biological method. Furthermore we can use ceria to tag subsurface damage in lapped and polished optics.
This work performed under the auspices of the National Natural Science Foundation of China (NSFC) through grants 60908022 and 51205400.
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