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Abstract
Impurities on glass surfaces, such as metallic trace contaminations induced by manufacturing processes, can cause severely disturbing effects as for example, a reduction in laser resistance or optical performance. Both the amount and nature of such impurities was thus investigated in the present work. For this purpose, fused silica surfaces were produced by classical optics manufacturing consisting of cutting, grinding or lapping and polishing with different pad materials. After each production step, the amount and the chemical binding state of the trace contaminations of interest–calcium, cerium and sodium, originating from the used operating materials–were determined via X-ray photoelectron spectroscopy. It is shown that in the course of manufacturing the chemical bonds of these elements and its compounds are modified. The polished fused silica optics feature the trace elements sodium, cerium and calcium bound in the form of NaOH, Ce2O3 and CaF2. Such surfaces moreover feature the lowest grade of contamination in the range of 0.2–0.5 atom-%.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In the course of optics manufacturing, a glass surface is in direct contact with a number of quite different tools and operating materials. For cutting and grinding, abrasives such as diamond or silicon carbide particles, embedded in metallic or plastic matrices are used where a cooling lubricant consisting of water and mineral oil is added to the process. In some cases, optics surfaces are ground with loose abrasives as for example aluminum oxide or silicon carbide that are mixed with water. For final polishing of ground or lapped glass surfaces, polishing pads made of synthetic or natural media in combination with a polishing suspension are applied. The suspension basically consist of water, eventual additives and the actual polishing agent as for example cerium oxide, iron oxide, aluminum oxide etc. Consequently, polished glass surfaces are to some extend contaminated by residues from the manufacturing process – and especially polishing – as investigated and reported by several authors [1–6] and in own previous work [7–9].
Even though the main contamination of glass surfaces can be attributed to the polishing process it has been shown that earlier manufacturing steps also contribute to surface and subsurface impurities. During grinding and lapping, micro cracks and subsurface damage occur [10–14] where the damage depth depends on the grit size of the used abrasive grains on the one hand and the roughness of the ground glass surface on the other hand. Neauport and co-workers showed that residues of oil-based coolants can accumulate within subsurface damages of ground fused silica surfaces [15]. Apart from liquid operating materials, wear debris of the used grinding tools can moreover be incorporated in surface and subsurface damages as indicated by results presented by Kozlowski et al. where iron and copper were measured on polished surfaces [16]. One has to consider that even though such manufacturing-induced contamination results in trace impurities, relevant characteristics such as the laser resistance or optical performance of polished glass surfaces can be notably affected [3,17,18].
Against this background, traces from tools and operating materials used in optics manufacturing where detected and analyzed in this work. Since the impact of manufacturing steps prior to polishing is not extensively reported or discussed in literature such investigation was carried out for each manufacturing step applied in classical production of optics, i.e. cutting, grinding, lapping and polishing.
2. Materials and methods
2.1 Sample preparation
The glass investigated in the present study was a standard fused silica (Tosoh Corp., type Clear Silica Glass N). A reference sample surface was generated by simple mechanical breaking to avoid surface contamination by contact with tools. Further samples were prepared by classical optics manufacturing, consisting of cutting, grinding or lapping, respectively, and final polishing [11] as shown schematically in Fig. 1.
Fig. 1. Schematic of the sample preparation process including the particular manufacturing steps (rectangles) and produced types of samples (parallelograms). For detailed information on the used tools and operating materials, see Table 1.
Table 1. Sample designations and the corresponding tools and operating materials applied for the realization of the particular sample surface.
After each of those manufacturing steps, the chemical composition of the generated surfaces was measured as described in detail in Section 2.2. For polishing, two different standard pad materials, polyurethane (PU) and pitch, were applied. Polishing was performed after both bound abrasive grinding and loose abrasive grinding, also referred to as lapping. The particularly used tools and operating materials as well as the corresponding sample designations used throughout this work are listed and defined in Table 1.
One has to notice that the samples were merely washed with tap water without any mechanical action as for example brushing after each production step. Before the XPS-measurement the samples were cleaned with isopropanol (analytical standard) and a tissue.
2.2 Chemical surface analysis and measurement of index of refraction
After each manufacturing step specified in Table 1 the chemical composition of the glass surfaces was determined via high-resolution X-ray photoelectron spectroscopy (XPS). The X-ray source of the used apparatus (Ulvac-phi, Inc., model PHI VersaProbe II) was a monochromatic Al-Kα source with a photon energy of 1486.6 eV and a power of 100 W. Scanning of the sample surfaces was carried out by an X-ray beam with a size of 100 µm where the total scanned area was 1400 × 200 µm2. High-resolution spectra were detected with a pass energy of 46.95 eV and a step size of 0.1 eV at a constant electron take-off angle of 45 °. This angle was chosen since for such setting, deviations in measured elemental concentration caused by differences in surface roughness can be assumed to amount to maximum 10% [19]. Initial calibration of the spectrometer was accomplished using the reference lines of copper at 932.62 eV on the one hand and gold at 83.96 eV on the other hand. The minimum detector resolution measured at the silver peak Ag (3d5/2) was 0.79 eV with a pass energy of 46.95 eV. During measurement, the temperature was kept constant at room temperature and the base pressure was 2·10−6 Pa. Moreover, the charge compensation with a cold cathode electron flood source and low energy argon ions was realized in the course of measurement in order to avoid disturbing charging effects of the sample surfaces. For data evaluation where the software MultiPak was used, all detected spectra were shifted to the carbon peak (C1s) of 284.6 eV. In order to represent the chemical composition in the best possible way and to cover differences in chemical compositions over the sample surfaces, 3 different measuring points were examined on 4 different samples from each production step. Each data shown below and the corresponding error bars are thus based on 12 single measurements. In the course of statistical analysis, a Kolmogorov–Smirnov normality test was applied in order to ensure a normal distribution. The data were analyzed with a Tukey honestly significant difference test to check the obtained results for statistical differences between the variants [20]. For both tests, the level of significance was 5%.
In addition to XPS analysis, the index of refraction of polished silica surfaces was determined with the aid of an imaging ellipsometer (Accurion GmbH, model nanofilm_ep4). All measurements were performed at a wavelength of 386 nm where the angle of incidence ranged from 45° to 65° with a step size of 1°.
3. Results and discussion
The glass reference only consisted of the glass-forming elements silicon and oxygen in the expected stoichiometric ratio of almost two. No impurities could be detected except a small amount of so-called adventitious carbon, i.e. surface contamination from the surrounding air that was reported in previous work [21]. In comparison to the untreated glass reference the trace elements calcium, sodium and nitrogen were detected after each grinding, lapping and polishing step. In addition to these elements, cerium was detected after polishing. Furthermore, zinc was detected in the lapping process. In the present work, further evaluation and analysis were focused on the metallic elements or trace impurities calcium, sodium and cerium that occur in each preparation process. Figure 2 shows the particular concentration of these elements of interest after each single production step.
Fig. 2. Atomic concentration of the trace elements calcium, cerium, and sodium detected after each manufacturing step on lapped and polished (left) and bound abrasive ground and polished (right) fused silica surfaces. For detailed information on the sample denomination, see Table 1.
It can be seen that a certain amount of the first element of interest, calcium, is observed after lapping and grinding where the concentration is approximately 0.31 - 0.51 atom-% for lapping and about 0.24 - 0.67 atom-% in the case of bound abrasive grinding. During the lapping-based process, merely slight differences in calcium content between the cut, lapped, and polished surfaces are observed. For bound abrasive grinding, the polished samples differ quite notably from the cut and ground ones, featuring a mentionable lower calcium content. However, no difference between the both particular polishing processes – PU polishing and pitch polishing – could be determined.
The second element of interest, sodium, is present with a content of about 0.53 atom-% after cutting. In the course of lapping and subsequent polishing, this proportion is reduced to 0.13 - 0.25 atom-% and can be regarded as constant over the entire process. In contrast, a constant and significant increase of sodium of up to 2 atom-% is observed in the course of bound abrasive grinding. This could be explained by the accumulation of wear debris from the sodium-containing bronze matrix of the grinding tools – which would also clarify the generally elevated amount of sodium after cutting where the same matrix material was used for the saw blade. In the final polishing step, this sodium contamination is removed and its proportion drops by a factor of 10 to 0.2 atom-%, being consistent with the atomic concentration detected on lapped and polished samples. Again, no significant differences between PU and pitch polishing are observed.
Even though a certain amount of sodium may be introduced by wear debris during bound abrasive grinding, both impurities considered so far most likely originate from the tap water used for mixing the cooling lubricant as well as the lapping and polishing suspension [7]. In terms of the third element of interest, cerium, it can be seen that this impurity is only incorporated in the course of polishing due to the use of cerium oxide as polishing agent. The contamination by cerium is quite constant at a value of 0.24 atom - 0.4 atom-%; there is no significance between the different applied polishing processes.
The comparison shown in Fig. 2 shows that apart from the behavior of sodium in the bound abrasive grinding process, the different applied manufacturing steps apparently induce a rather constant amount of impurities and contaminants. However, significant differences in chemical state can be determined when analyzing the particular signals of the detected elements of interest. Figure 3 shows the peak position of calcium for the different manufacturing steps. It can be seen that the average position of the Ca2p3/2 peak of the cut, lapped and bound abrasive ground samples is found quite close to a binding energy of 347.2 eV. This value can be attributed to calcium carbonate (CaCO3) which is described by quite different values in literature, ranging from 346.5-347.3 eV [22–24]. This compound most likely originates from the tap water that was used for sample preparation where the calcium carbonate content is 1.14-1.21 mmol/L as specified by the provider of the used water (Stadtwerke Göttingen AG) and given in [25].
Fig. 3. Peak position of the calcium signal (Ca2p), expressed by the binding energy, on lapped and polished (left) and bound abrasive ground and polished (right) fused silica surfaces. For detailed information on the sample denomination, see Table 1.
The polishing process leads to a considerable shift by about 0.5 eV to 1 eV from the Ca2p3/2 peak to higher binding energies, which can be attributed to calcium fluoride (CaF2) [22,26]. If a fluorine-containing compound was present on the glass surface, it should also be possible to detect fluorine (e.g. the F1s peak). However, this element could not be measured on polished fused silica surfaces via XPS. Fluorine has a notably lower emission cross section than calcium. Since the amount of detected calcium was < 0.5 at%, the fluorine signal can be assumed to be lower than the limit of detection. It can thus be assumed that this element cannot be discriminated from the underlying noise.
A fluorine-containing compound could be formed or accumulated by two different mechanisms: First, the reaction of calcium cations and fluorine anions during polishing could be driven by high local temperature that can arise during polishing. Both calcium and fluorine are present in the used water and – as verified via XPS and listed in Table 2 – in the polishing agent applied in the present work. Second, the polishing agent might provide CaF2 directly. This assumption is supported by the XPS-analysis of the pure polishing agent, where the measured calcium peak position was found at higher binding energies, i.e. 347.8 eV, identical to CaF2. This value was also measured on the polished glass surfaces, but not on lapped or ground ones. Hence, calcium fluoride from the polishing agent could be embedded within micro cracks, subsurface damages and the silica gel layer formed in the course of polishing as also observed and reported for another element from the polishing agent, cerium [4]. The molecular CaF2 may simply accumulate but also bond to the silica network by the formation of ionic bonds as already reported in the case of calcium hydroxide [27].
Table 2. Chemical composition of the used polishing agent Ceri 3000 (dry powder) as measured via XPS.
The high proportion of fluorine of about 21 atom-% in the polishing agent as listed in Table 2 indicates that apart from calcium also other elements and mainly cerium must be present as a chemical compound with fluorine. Based on this assumption the oxidation state of cerium within the polishing agent on the one hand and on the polished fused silica surface on the other hand was determined. A comparison of the particular cerium detail spectra is shown in Fig. 4.
Fig. 4. Cerium detail spectra of the used polishing agent Ceri3000 (top) and a polished fused silica surface generated by bound abrasive grinding and pitch polishing (bottom).
It can be seen that the measured curves differ quite significantly. For the pure polishing agent, the Ce3d5/2 and 3d3/2 doublets consist of three main structures which could be identified as Ce3+ and Ce4+ [28,29]. Due to the high fluorine content in the polishing material and the possible binding of fluorine or oxygen to cerium, it is difficult to distinguish between the binding state of cerium from literature [30]. However, the peak at 916.15 eV is a clear indicator for cerium(IV)-oxide [28]. It can thus be stated that cerium may be present in a mixture of Ce2O3, CeF3, CeO2, or CeOxFy in the polishing agent [29,30].
In contrast to the pure polishing agent, the polished glasses display four peaks (Pa – Pd, see Table 3) that could be identified as Ce2O3 [31]. Hence, CeO2 or CeF3 from the polishing agent is most likely converted to Ce2O3 at the glass surface in the course of the polishing process as also indicated by the fact that after polishing, merely the Auger signal, but not the direct electron signal of fluorine was detected. This observation suggests that original cerium compounds from the polishing agent are not only embedded mechanically in cracks, digs, and the formed hydrated silica layer, but also chemically bonded to glass-forming oxides.
Table 3. Peak positions of the cerium Ce3d peak of the polished fused silica surfaces.
Such chemical modification was also observed for sodium as shown in Fig. 5. The position of the Na1s peak of the cut, lapped and ground samples are found within a binding energy range from 1071.6 eV to 1071.8 eV which can be assigned to Na+ [32]. We assumed that the Na+ ions are present in the chemical compound Na2CO3 [33]. After the polishing process, the Na1s peak shifts to 1072.6 eV which most likely corresponds to NaOH [34,35]. This observation suggests that sodium is chemically bonded within the hydrated silica layer that – to a considerable extend – consist of hydroxyl groups. Such layers are known to be highly hygroscopic [36], thus attracting water including its impurities such as sodium.
Fig. 5. Peak position of the sodium signal (Na1s), expressed by the binding energy, on lapped and polished (left) and bound abrasive ground and polished (right) fused silica surfaces. For detailed information on the sample denomination, see Table 1.
To summarize, the results show that the chemical state of all considered elements – calcium, sodium and cerium – is notably modified by the polishing process. This observation supports the well-established hypothesis that glass polishing is also due to chemical reactions, apart from fretting, abrasion and flow of the glass material. A quite interesting point is that the chemical modification of the particular elements does not depend on the used pad polishing material. It can thus be attributed to direct glass-polishing slurry interactions induced by leaching and diffusion. It should finally be noted that the presented manufacturing-induced formation and accumulation of Ca-, Ce-, and Na-compounds – and most probably further compounds not considered here – at the polished surface leads to an alteration of the optical properties. As verified via ellipsometric measurements, the index of refraction was increased from a reference value of 1.47 up to 1.51 in the case of lapped and pitch-polished fused silica.
4. Conclusions and outlook
It was shown that during classical production of optics surfaces applying cutting, grinding or lapping and final polishing, different trace elements are embedded in the fused silica surface. These elements originate from the water used for mixing the used lubricants and lapping or polishing suspension as well as from the applied tools and operating materials. It was moreover detected that polished surfaces feature quite low impurities in comparison to cut, lapped or ground ones. The nature or type of bonding of the particular trace elements to the glass material was not clarified, but several locations or mechanisms can be assumed: First, the observed impurities are most likely embedded or chemically bound in the hydrated silica layer which is formed in the course of manufacturing – mainly during polishing, but to a certain extend also during any production step where aqueous solutions are used, i.e. cutting, grinding and lapping. Second, impurities can accumulate within micro cracks, scratches, digs and holes. Third, the measured elements and bonds may adhere to the surface as precipitates.
During the final polishing process, the trace elements sodium and calcium, undergo a modification in chemical binding state, resulting in the formation of NaOH and CaF2. This modification can be explained by the higher temperatures present during the polishing process and by the chemical compounds contained in the used polishing agent, namely cerium oxide. In comparison to the pristine polishing agent, the cerium is transformed to Ce2O3 and consequently bond to the glass surface.
The obtained results contribute to a better understanding of chemical-mechanical glass polishing mechanisms and tool-glass interactions. In order to expand such investigations and to address the effect of glass surface deterioration in terms of chemical composition and optical properties the determination of the proportion of trace elements over polishing time as well as the chemical stability of glass-adherent trace compounds and oxides over a longer storage period will be studied in ongoing work. Moreover, the determination of modifications in binding states of other operating materials or polishing agents such as iron oxide or zirconium oxide is of great interest. This also applies to different suspension concentrations that will be investigated in future work in order to gain further information on chemical processes and interactions at glass surfaces during optics manufacturing.
Acknowledgments
The authors thank Lutz Müller and Daniel Tasche from the University of Applied Sciences and Arts for the sample preparation and the support during surface analysis.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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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