1. Introduction

In classical optics manufacturing water is one of the most important and generally used operating materials. It is the basis for the lubricants employed for cutting, shaping, and grinding and for the suspensions applied during lapping and polishing [1]. The glass work piece is thus in permanent contact with water or aqueous media throughout the entire manufacturing process. This contact may cause severe damage or defects due to water-induced decomposition of the glass matrix [2], leading to an alteration of the chemical composition and thus optical properties of near-surface glass material. Extensive work on this effect was performed and reported for the single-component glass fused silica [3,4]. However, complex optical multi-component glasses are less considered in literature – even though such glasses are of higher interest and relevance in the manufacture of classical imaging optical systems. Most of these glasses are based on the same network former as fused silica, i.e. silicon dioxide. As well known for fused silica, every silica-based glass may thus generally form an outer modified region, the Beilby layer, a.k.a. polishing re-deposition layer or silica gel layer. This modified zone is generated by reactions of the aqueous polishing slurry [5] where water and hydrogen penetrate into the glass matrix, causing hydrolytic scission and the formation of hydrated silica or silanol [6,7]. In the course of growth, impurities from the manufacturing process such as residues from operating materials [5,8] or even wear debris from the used tools [9,10] are embedded in the formed silica layer. Being highly hygroscopic, this layer is also known to cause leaching of glass constituents from the bulk material [11]. Such leaching also occurs in the course of polishing where water induces an extraction of alkalis [12,13], i.e. alkali metals such as sodium and potassium or alkali earth metals as for example barium from glasses. This extraction can be attributed to the chemical interdiffsuion of alkalis with molecular water [14], hydrogen [15], or hydronium [16] where the latter mechanism can be described by the Doremus model [17]. Apart from the interdiffusion of alkalis, the contact with liquids can even give rise to the leaching of heavy metals from the glass as for example lead [18,19], especially in acid environment [20]. In the course of classical optics manufacturing, where acidic polishing slurries are often applied in order to enhance chemically-induced glass reactions and removal, the glass surface is thus modified and chemically altered by several mechanisms. Against this background, the aim of the present work is to contribute to the understanding of the formation of modified near-surface layers on multi-component glasses.

2. Materials and methods

2.1 Sample material and preparation

Measurements were performed on two different multi-component glasses, barium crown glass (N-BaK4 from Schott) on the one hand and heavy flint glass (SF5 from Schott) on the other hand. These glasses represent two glass families that are quite common and conventional media in optics manufacturing, for example for the realization of achromatic doublets, apochromatic lenses or even more complex imaging optical systems. According to the manufacturer’s data sheets, the indices of refraction of N-BaK4 and SF5 at a wavelength of 546.1 nm are 1.57125 and 1.67764, respectively. Apart from oxygen and silicon, the main constituents of the investigates glass are barium, sodium, tin, and potassium for the barium crown glass and lead, potassium, and sodium in case of the heavy flint glass [21].

From these glasses, plane plates with a thickness of about 5 mm were produced by classical optics manufacturing. The plates were first cut from a glass block and subsequently lapped where the lapping grain size was successively reduced (from initially 29 µm to at last 7 µm). Finally, the lapped surfaces were polished for 6 hours. The particular tools and operating materials used in the course of manufacturing are summarized in Fig. 1.

 

Fig. 1. Overview on tools and operating materials used for sample preparation via classical optics manufacturing.

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2.2 Surface analysis

The chemical composition of the polished glass samples was measured by a high-resolution X-ray photoelectron spectroscopy (XPS) apparatus (PHI VersaProbe II from Ulvac-phi). Prior to measurements, the spectrometer was calibrated to two reference lines, i.e. copper at 932.62 eV on the one hand and gold at 83.96 eV on the other hand. The minimum detector resolution of 0.79 eV was measured at the Ag3d5/2 silver peak with a pass energy of 46.95 eV. Depth-resolved analysis of the near-surface glass layer was realized by alternating measurements and material removal using the built-in argon ion gun. The applied X-ray source, a monochromatic Al-Kα source, has a photon energy of 1486.6 eV and a power of 100 W. The emitted beam with a size of 100 µm was scanned over a measuring surface of 1400 × 200 µm² whilst detail spectra for carbon, oxygen, silica, barium, sodium, potassium, lead, cerium, calcium, and lanthanum were recorded with a pass energy of 46.95 eV and a step size of 0.1 eV. The electron take-off angle was kept constant at a value of 45°. The measurements were performed at room temperature; the base pressure within the vacuum analysis chamber was 2 × 10^-6 Pa. All measurements were carried out with charge compensation using a cold cathode electron flood source and low energy argon ions in order to avoid disturbing charging effects of the sample surfaces. For depth-profiling, Ar⁺ ions with an energy of 2 keV were applied to an area of 3 x 3 mm². These sputter conditions result in a sputter rate of approx. 4 nm/min at SiO₂.

In addition to such XPS analysis, the index of refraction of the polished glass surfaces was measured using an ellipsometer (ep4 from Accurion) where the measurement wavelength was 546.1 nm.

3. Results and discussion

3.1 Chemical composition of the near-surface glass material

Apart from silicon and oxygen, the main major compounds of the investigated glasses are barium, lead, sodium, and potassium. As detected by the depth-resolved XPS measurements and shown in Fig. 2, the concentration of these glass-forming elements is notably lowered at the surface.

 

Fig. 2. Concentration of major glass-forming elements of the investigated barium crown (left) and heavy flint (right) glass vs. sputter depth.

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The observed gradient-like decrease in concentration towards the interface can be explained by different mechanisms. First, leaching of alkalis [12,13], alkali earth metals, and heavy metals [18,19] from the glass surface can occur as introduced in more detail in the Introduction. Such leaching is induced by the aqueous polishing suspension and supported by frictional heat that results from the tool-glass interaction. It is thus driven by interdiffsuion due to the presence of molecular water [14], hydrogen [15], or hydronium [16]. Unfortunately, the used analytical method, XPS is not capable of quantifying water within the sample material directly – even though the evaluation of oxygen peaks would allow to identify the presence of hydrogen. However, a slight increase in oxygen close to the surfaces was observed as shown in Fig. 3.

 

Fig. 3. Concentration of oxygen within the investigated barium crown and heavy flint glass vs. sputter depth.

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This increase could indirectly indicate a diffusion of water or hydronium (H₃O⁺) – particularly with regard to the well-known effect of an exchange of removed alkali cations by this aqueous cation [13,16].

In addition to leaching and interdiffusion, the decrease in glass-forming elements as shown in Fig. 2 can be attributed to the formation of the growth of a hydrogenous thin film, which usually occurs during polishing of silica-containing glass surfaces. This film is commonly referred to as silica gel or Beilby layer [5,22]. It arises due to direct interaction of water provided by the polishing suspension and silicon dioxide and can be described as hydrated silica or silanol, mainly consisting of H₃SiOH or Si(OH)₄ [23]. However, impurities that originate from operating materials used during manufacturing are usually embedded in the near-surface hydrated silica layer. Such impurities and especially calcium and lanthanum were also detected in the present work as shown in Fig. 4.

 

Fig. 4. Concentration of the contaminations calcium and lanthanum within the surface of the investigated barium crown (left) and heavy flint (right) glass vs. sputter depth.

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The first element, calcium, most likely originates from the tap water used for sample preparation by cutting, lapping and polishing. According to the supplier’s drinking water analysis, the calcium content in the tap water used for manufacturing is 33.0-38.2 mg/L. Moreover, it contains a certain amount of additional calcium carbonate. Calcium can thus be incorporated into the glass surface in the course of the polishing process in aqueous suspension. This also applies to the second element displayed in Fig. 4, lanthanum. The presence of this element at the outermost glass surfaces can be attributed to wear debris that occurs during the polishing process. Here, smallest fractions of polishing grains are rubbed off [24].The impurities generated in this vein such as cerium, aluminum, zirconium or iron – depending on the used polishing agent – are then embedded in the polishing-induced hydrated silica layer [5,25]. In the present case lanthanum was detected whereas the nominally applied polishing agent, i.e. cerium or cerium oxide, could not be measured. This is due to the fact that in spectra obtained via XPS, the cerium signal is superimposed by the barium-Auger-peak (Ba MNN), leading to a challenging or even impossible identification and quantification of this element. However, lanthanum provides well-isolated and evaluable signals. In this work, it was thus used as a tracer for the presence of residues from the polishing agent since it is commonly found in cerium oxide-based polishing suspensions [26]. In some commercial polishing suspension premixes the lanthanum content can even exceed 30 wt% [27,28]. As verified by auxiliary XPS-analysis, the lanthanum content of the polishing agent used in the present work was 4.45 ± 0.19 at% [8].

As a mathematical approach, both the decrease in major glass-forming elements (see Fig. 2) and the increase in manufacturing-induced contaminants (see Fig. 4) towards the interface could generally be described by converse exponential functions. This behavior supports the assumption that (i) leaching and (ii) a certain interdiffusion between the glass material and the polishing suspension and its constituents occurs. Moreover, a gradient-like growth of the hydrated silica layer can be assumed. As visualized in Figs. 2, the thickness of this gradient layer, where the concentration of actual glass components is lowered, is in the range from some nanometers for heavy flint glass to approximately 20 nm in case of barium crown glass. The differences can easily be explained by the different chemical compositions and thus reactivity of both glasses. However, the thicknesses are in good accordance with values reported in literature by various authors [5,26,29,30]. It can moreover be stated that the penetration depth of contaminants from the polishing suspension, see Fig. 4, is higher than the zone of depleted glass-forming element concentration. This indicates that such contaminants are incorporated into deeper regions of the glass surface during polishing as also reported in previous work [31]. As a possible explanation for this difference, the accumulation of impurities from the polishing suspension within digs and micro cracks of the lapped surfaces at the very beginning of the polishing process, i.e. prior to the growth of the hydrated silica layer, can be hypothesized.

3.2 Optical properties and imaging

The notable manufacturing-induced modification of the chemical composition of the near-surface glass material with respect to the bulk material inevitably results in an alteration of the optical properties of the glass surface. This chiefly applies to the index of refraction as verified by the results of the ellipsometric analysis, shown in Fig. 5. The displayed values and error bars result from measurements at 25 different positions on the sample surfaces. The nominal values, i.e. the reference values for the glass bulk material, were taken from the manufacturer’s data sheets where the error bars represent the specified tolerances in index of refraction.

 

Fig. 5. Comparison of the nominal and actually measured indices of refraction at a wavelength of 546.1 nm of the investigated barium crown and heavy flint glass.

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It turns out that in comparison to the nominal values for both glasses, the actually measured ones are generally lower. This result can be explained by the formation of a hydrated silica layer and the observed leaching of heavy glass constituents such as barium and lead, compare Fig. 2. Such removal of heavier elements or the replacement by lighter ones comes along with a reduction in index of refraction which is directly related to the mass density of a glass [32,33] – a behavior which is referred to as Gladstone-Dale law [34].

Regarding the final use of classically manufactured glass components such manufacturing-induced variation in the actual index of refraction of polished surfaces gives rise to several disturbing effects. First, the reflectance of a polished glass surface differs from the expected one as expressed by the Fresnel equations. In the present case, the percentage deviation amounts to 0.64% in the case of barium crown glass and to 0.55% for heavy flint glass. Although this change seems to be marginal, it may have a notable impact on the performance of functional coatings applied to the surface. Second, variations in index of refraction directly cause deviations in refraction of light according to Snell’s law. This circumstance may have mentionable implication on the imaging quality of optical systems that are calculated based on theoretical reference glass data as visualized by the comparison shown in Fig. 6. Here, a simple achromatic doublet was simulated using ray tracing software (WinLens Basic from Qioptiq and Optical Software Company) where merely on-axis rays were considered.

 

Fig. 6. Visualization of the impact of variations in surface index of refraction on imaging quality of an achromatic doublet by means of the spot diagram (a) and the modulation transfer function (b), calculated by applying the theoretical (‘nominal’, left) and actually measured (‘actual’, right) indices of refraction. The solid line-circle in (a) and the dashed line in (b) represent the diffraction limit.

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For a well-designed doublet calculated based on the nominal indices of refraction as specified for the glass bulk material, a quite high imaging quality and diffraction limitation is achieved as shown by the spot diagram and the modulation transfer function. When simulating the same system applying the above-reported measured manufacturing-induced change in index of refraction at the sample surface (see Fig. 5), imaging quality is notably degraded. It turns out that the spot diameter in the image plane is notably increased – the doublet is thus no longer diffraction limited for the given conditions. This effect can be explained by the increase in Seidel sum S_I, which occurs when considering modifications in index of refraction. For a doublet calculated on the basis of the nominal indices of refraction, S_I is 0.00015 whereas it amounts to 0.00019 when decreasing the index of refraction of a thin layer at each lens surface by the differences displayed in Fig. 5. This change in the first Seidel sum corresponds to an increase by about 27%. Apart from the accompanying increase in spot diameter, modulation transfer and thus contrast transfer is reduced for longer wavelengths. Moreover, the effect of chromatic aberration is increased since the condition for achromatism is actually no longer fulfilled due to the change in index of refraction. In the present simulation, this change causes a rise in longitudinal chromatic aberration from 78 µm to 81 µm, i.e. 4%. Finally, the entire opto-mechanical setup (distance doublet-sensor) suffers from slight variations in effective focal length. This essential value is increased by 13 µm in the simulated setup. This circumstance may lead to a remarkable positioning error and thus further blurring of the image.

As a consequence, a theoretically perfect lens or optical system design may not feature the expected performance in practice due to manufacturing-induced alterations in glass composition via leaching of glass-forming elements, ion exchange, implantation of contaminations etc. and index of refraction, respectively. It should finally be noted that the polishing-induced modification in index of refraction is most likely not uniformly distributed. Since the impact of the polishing tool varies over the surface, lateral variations in index of refraction may occur additionally as observed in previous work [4]. In the present case, this effect is also indicated by the comparatively large error bars shown in Fig. 5 that result from ellipsometric measurements on 25 different positions on the sample surface.

4. Conclusions

In this work two representatives of technically relevant multi-component glass families, i.e. crown and flint glass, were considered. Such glasses are the basis of any imaging optical system used for classical purposes in the visible wavelength range. The observed depletion in glass-forming elements and simultaneous accumulation of impurities during polishing are of mentionable relevance. For both glasses, this led to a modification in index of refraction. The resulting potential impact on imaging quality is thus originally caused by the use of water or aqueous solutions that are not exclusively applied during polishing but throughout the entire classical optics manufacturing process. This implies that the reduction or avoidance of such liquids might allow to increase the quality of imaging optical systems. Even though this suggestion is apparently in contradiction with the traditional, well-established and highly precise handcraft of optics manufacturing, such approaches were invented and developed in the last years and decades. Several techniques are in hand, reactive atomic plasma technology (RAPT), ion beam etching (IBE), reactive ion etching (RIE), laser polishing, or inert gas plasma polishing. These methods are mainly used for surface finishing, sub-aperture correction or aspherization, but could be combined in order to offer a water- or liquid-free process in optics manufacturing. Apart from a reduction in surface leaching and contamination, this approach would also reduce or even avoid the formation of micro cracks and subsurface damage.

For a better understanding of processes and mechanisms during conventional or classical optics manufacturing, the present study gives a first insight in the modification of near-surface layers of quite different optical multi component glasses. However, hydrogen as one of the most important of the involved elements cannot be detected via the used analytical method, XPS. Further investigations applying suitable techniques for this purpose as for example laser-induced breakdown spectroscopy (LIBS) or secondary ion mass spectroscopy (SIMS) will thus be carried out in ongoing work. Moreover, micro cracks and subsurface damages are currently investigated in order to identify the grade of contamination by operating materials and residues from water within such defects.