Fused silica glass, with its large transparency range, low thermal expansion, high resistance to optical damage, high refractive index homogeneity, and ease of finishing has found use as an optical material in applications as varied as space shuttle windows, optical fiber, and deep UV lens elements. Its widespread role as an optical material has been enabled by manufacturing the glass from pure liquid precursor compounds, thereby minimizing impurities, and maximizing refractive index homogeneity. As a technical material, silica glass exhibits anomalous properties with respect to volume as a function of temperature and cooling rate, among other properties; these anomalies remain under investigation to this day. This article describes the manufacture of silica glasses and some of their varied optical applications. Silica is often used in demanding environments, an example of which is its use as a lens material for lithography systems using high photon excimer lasers. Static and dynamic properties of silica glass are discussed here.
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Fused silica (SiO2) has been called “the clearest glass in the world” for good reasons [1]. It has a window of high transmission extending from the deep ultraviolet to the infrared. It also has high radiation damage resistance, low birefringence, and high index homogeneity along with excellent chemical and thermal stability [2]. Add the ability to economically manufacture it in large sizes, and it is easy to see why fused silica is the material of choice for many of today’s most demanding optical applications [2]. As an optical material, fused silica has played a key role in the development of deep UV optical lithography, optical fiber, high energy laser systems, and space exploration.
From a scientific standpoint, fused silica has a “deceitful simplicity” [3]. Its simple chemistry belies a strange behavior which is, more often than not, the exact opposite that of common silicate glasses [4]. Furthermore, it cannot be manufactured by traditional glass-melting methods, and novel processes have had to be developed just to make it [2,5–9]. Because of these properties, fused silica has captured the imagination of scientists for over two centuries and continues to do so. Since the beginning of 2022 alone, over 150 articles have been published with the keyword “fused silica”. This article offers a brief review of fused silica as an optical material.
Although often used interchangeably, the terms “fused silica” and “fused quartz” refer to two different types of silica glass. Fused quartz (Type I/II) is made by melting natural or purified crystalline silica, while fused silica (Type III/IV) is a synthetic material made from a chemical precursor. For optical applications, the key difference between fused quartz and fused silica is their levels of impurities that contribute to absorption. Fused quartz typically contains 20-50 ppmwt of impurities – primarily Al, alkalis, and transition metals – while fused silica has < 1 ppmwt. Fused silica can be made by a variety of methods [2,5,10], but only flame hydrolysis is described here since it is the principal process by which commercial fused silica is manufactured.
Table 1. Properties of commercial fused silica and fused quartz glasses
View TableDr. James Franklin Hyde made the first fused silica glass in 1934 using flame hydrolysis [6]. In this process, a silicon-containing chemical precursor is vaporized and injected into a methane-oxygen or hydrogen-oxygen flame where it is hydrolyzed to form amorphous silica. If SiCl4 is used as the precursor, the reaction in the flame is:
The silica produced by Reaction 1 is in the form of ∼100 nm glass particles, commonly referred to as “soot” [9]. In modern manufacturing processes, halogen-free precursors, such as siloxanes, are often used to mitigate pollution.
In the “direct laydown” process, the soot is collected on a refractory substrate and simultaneously sintered to a transparent glass (Fig. 1).
This kind of fused silica, commonly referred to as Type III, has a high water content, 800-1200 ppmwt OH, as a consequence of hydrogen and water being incorporated into the glass during the laydown process. Commercial Type III silica glasses include Corning HPFS 7980, Suprasil 1/2, Spectrosil 2000, and KU-1.
Alternatively, the silica soot can be collected on a mandrel to form a porous soot preform which is subsequently sintered into a transparent glass. The advantage of the soot-to-glass process is that the porous preform can be dried and doped prior to sintering. In the outside vapor deposition (OVD) process, the rotating mandrel is oriented perpendicular to the traversing flame while soot is collected on it (Fig. 2(a)) [9]. The mandrel is removed from the soot preform after deposition is complete. In the vapor axial deposition (VAD) process, the mandrel acts as a “seed rod” off of which the soot preform is grown (Fig. 2(b)) [11,12].
Drying is commonly done by flowing Cl2 or a chlorinated gas through the heated soot preform [13]. Dopants can be added during soot deposition or by exposing the preform to a gas in the sintering furnace [14,15]. For example, fluorine can be added to the glass using SiF4 or CF4 during the sintering process [15,16]. High purity silica glasses with no detectable OH (dry fused silica) can be obtained using the soot-to-glass process. Commercial fused silica glasses of this type include Corning HPFS 8655 and AGC AQ2. OVD and VAD are also used in the manufacture of optical fiber (see Section 6.4).
Table 1 compares the properties of the two types of fused silica to those of a fused quartz glass. Notice that the ultraviolet transmission of the fused silica glasses (Ti, 193 nm) is higher than that of the fused quartz glass. In addition, the anneal points of the low water silica glasses are more than 100°C higher than that of Type III fused silica.
Any introduction to glass structure inevitably begins with Zachariasen’s famous drawing of the atomic structure of silica glass as a disordered network of corner-sharing SiO4 tetrahedra. (Figure 3(a)) [19].
Fig. 3. Structure of silica glass. (a) Zachariasen’s model (Reprinted with permission from J. Am. Chem. Soc. 54, 3851 (1932). Copyright 1932, American Chemical Society). (b) Evans-King 3D ball-and-stick model. (c) Molecular dynamics simulation (Ref. [20], Fig. 1(b)). (d) ADF-STEM image of 2D amorphous silica supported by graphene. Scale bar 0.5 nm. (Adapted with permission from Nano Lett. 12, 1081 (2012). Copyright 2012, American Chemical Society).
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Over the years, the continuous random network (CRN) model, as it is now known, has been supported by numerous x-ray diffraction, neutron diffraction, and NMR studies as well as computer simulations [22–27]. These studies have provided information on the bond lengths and bond angle distributions in silica glass and enabled the construction of three-dimensional models [28,29]. An early ball-and-stick type model by Evans and King is shown in Fig. 3(b). Such physical models have now been replaced by computer simulations, an example of which is shown in Fig. 3(c) [20,26,27]. In 2012, Huang et. al. published the first atomic-scale image of a silica glass which bears a remarkable resemblance to Zachariasen’s original drawing (Fig. 3(d)) [21].
Modifications to the CRN model (e.g., co-existing structures [30,31], polyamorphism [32–34]) have been proposed to explain the anomalous properties of fused silica compared to multi-component glasses: for example, an increase in density with increasing fictive temperature (Tf) [4]. While explaining some of these properties, however, the basic structural features of the CRN have been retained [35].
One intrinsic feature of the CRN model is the assemblage of silica tetrahedra into rings of various sizes. Rings comprising five to eight tetrahedra can be identified in Zachariasen’s drawing (Fig. 3(a)). Statistical analyses have shown a preponderance of five-, six-, and seven-membered rings in the silica glass structure (Fig. 4) [36,37].
More significant is the presence of three- and four-membered rings. While not “defects”, per se, the Si-O-Si bonds in these small rings are more strained than those in larger rings and, as a result, are more chemically reactive and photosensitive [38]. Two distinct bands in the Raman spectrum of silica glass at 495 cm-1 (D1) and 606 cm-1 (D2) have been associated with four- and three-membered rings, respectively [39]. Studies have shown that the concentration of small rings increases with increasing Tf, and a minimum in D2 was found at 950°C corresponding to a minimum in density [40,41]. Similar studies using the 2260 cm-1 Si-O-Si stretching vibration in the infrared spectrum have shown that the average bond angle in the silica structure decreases with increasing Tf [42]. Both Raman and IR can be used to measure the Tf of silica glasses [42,43] and have been widely used in studies of structural relaxation kinetics [44–48].
Most importantly for transmission, real silica glasses always contain structural defects not depicted in Zachariesen’s drawing (Fig. 3(a)). An intrinsic, or non-stoichiometric, defect occurs where an Si4+ is either under- or over-coordinated with oxygen. Examples include the E’ center (≡Si•), non-bridging oxygen hole center, NBOHC (≡Si-O•), and oxygen deficient centers, ODC (≡Si-Si≡, =Si••) [49,50]. Extrinsic, or compositional, defects are created when other elements (e.g., H, Cl, F) are incorporated into the glass during the manufacturing process. These elements form terminating bonds that break up the silica network: ≡Si-X (X = H, OH, Cl, F) [51–53]. Alternatively, they can dissolve in the glass as gases: H2, O2, Cl2, F2 [54]. Defects can also be created or destroyed by exposure to radiation, as described in Section 5. Comprehensive reviews of optical defects in silica glass can be found in [49,50].
Due to its wide band gap, fused silica can be used in a range of applications where optical transparency is required. Applications span the range from the UV for lithography stepper machines operating at wavelengths less than 300 nm to the infrared portion of the spectrum where its use in optical elements and waveguide devices is critical. Two of the most important properties that impact glass transmission, absorption and Rayleigh scattering, are reviewed here. Glass structure and defects play an integral part in determining these properties. Refractive index is another important optical property but will not be covered here, and the reader is directed to other publications for information on this topic [17,55–57].
The UV edge of silica glass is formed by the Urbach tail of the 10.4 eV SiO2 absorption band [2,58]. The fundamental edge is located at 153 nm, but its position is strongly affected by structural disorder and defects [52,54,58–61]. Figure 5 illustrates the effects of some compositional defects on the vacuum ultraviolet (VUV) transmission of fused silica.
Fig. 5. Transmission of fused silica in the vacuum ultraviolet. A: 1270 ppmwt OH. B: 1300 ppmwt Cl. C: 2 ppmwt OH. D: 5000 ppmwt F. 10 mm pathlength. Spectra include reflection and scattering losses.
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Both SiOH and SiCl introduce electronic transitions with energies near the UV absorption edge that cause it to shift to longer wavelengths: Glasses A and B in Fig. 5. The 157 nm absorption cross-sections (σ157nm) for SiOH and SiCl have been calculated as 16.8 × 10−20 cm2 and 6.3 × 10−20 cm2, respectively, consistent with [52,54]; however, these compositional defects have little or no effect on transmission at 193 nm. Reducing the hydroxyl concentration moves the UV edge closer to the fundamental edge: Glass C in Fig. 5. The highest 157 nm transmission was obtained from dry, fluorinated silica: Glass D in Fig. 5. Fluorine doping has several beneficial effects on the ultraviolet transmission of fused silica [47]. First, fluorine displaces hydroxyl to form ≡SiF units. Second, unlike SiOH and SiCl, SiF does not introduce absorption near the UV edge. Third, fluorine decreases structural disorder by preferentially reacting with small rings that normally create excess absorption near the UV edge [38,62]. As a result, internal transmissions greater than 80%/cm at 157 nm have been achieved in F-doped silica glasses [60,63,64].
Many non-stoichiometric defects act as color centers in the ultraviolet. The ODC1 oxygen-deficient center (≡Si-Si≡) has an absorption band centered at 163 nm [49], and interstitial O2 creates excess absorption from about 155-175 nm [54]. Both of these defects impact the VUV transmission of fused silica. The E’ center has an absorption band centered at 215 nm with a tail that extends to 193 nm. The NBOHC has an absorption band at 260 nm with a tail extending to 248 nm [49], and the ODC2 oxygen-deficient center (=Si••) has an absorption band that peaks at 248 nm [65]. It is, therefore, imperative that the creation of both non-stoichiometric and compositional defects be minimized during manufacturing and laser exposure in fused silica glasses used for UV optics.
The infrared absorption edge of silica glass is formed by the tails of structural and water-related vibrational bands [66,67]. Silica has four fundamental vibrational bands located at 9.1, 12.5, 21.3, and 36.4 µm [66], with overtone and combination bands located at 3.0, 3.2, 3.8, 4.2, and 4.5 µm as can be seen in the infrared transmission spectrum of HPFS 8655: Glass E in Fig. 6. The fundamental SiOH stretching band lies at 2.73 µm with overtone bands at 2.24 and 1.37 µm [67,68]. These bands are present in the transmission spectrum of HPFS 7980, Glass F in Fig. 6, but are essentially absent in that of HPFS 8655.
Fig. 6. IR transmission of (E) Corning HPFS 8655, and (F) Corning HPFS 7980. 10 mm pathlength. Spectra include reflection and scattering losses. (Also see Table 1)
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Optical fibers must be essentially OH-free in order to transmit 1310 or 1550 nm light over long distances. The exchange of deuterium for hydrogen is sometimes used to shift the OH absorption bands to longer wavelengths [69].
While it is well-known that alkalis and transition metal impurities have strong absorption bands in the UV, visible, and near IR [70,71], they are not a concern for fused silica glasses because of their exceptionally low impurity levels.
Rayleigh scattering is caused by fluctuations in the glass structure arising from density or compositional inhomogeneities that are smaller than the wavelength of the light (λ). Scattering losses due to density fluctuations (αρ) are directly proportional to the fictive temperature of the glass [72], while scattering losses due to concentration fluctuations (αc) are proportional to (dn/dc)2, where n = refractive index and c = dopant concentration [73]. The total Rayleigh scattering loss (αRS) can be expressed as:
Rayleigh scattering has a 1/λ4 wavelength-dependence and is, therefore, stronger for ultraviolet light than for infrared light. Nevertheless, Rayleigh scattering is the largest source of loss in telecommunications optical fiber because of the very long optical pathlength.
In dry, undoped, fused silica, only density fluctuations contribute to Rayleigh scattering, and low scattering loss is directly correlated with low Tf. Slow cooling through the glass transition region is a typical way of obtaining glasses with low Tf [74]. However, structural relaxation in dry, undoped fused silica is so slow that it is difficult to obtain a low Tf by simply controlling the cooling rate [45,72]. The slow relaxation is a result of the high rigidity of the silica network which is also responsible for the high viscosity (Tg > 1200°C) and low fragility (m∼20) of the glass [75–77]. Consequently, decreasing αRS by decreasing Tf involves speeding up the relaxation process.
Dopants that break up the silica network can accelerate the structural relaxation process but can also produce concentration fluctuations. If the dopant has a strong effect on the refractive index of the glass, (dn/dc)2, any decrease in αρ may be offset by additional αc loss. Studies have been conducted on the effects of different dopants on Rayleigh scattering in fused silica. Lines showed that alkali additions can reduce Rayleigh scattering by up to 20% compared to dry fused silica [73]. Schroder, et. al. found that OH reduces the Rayleigh scattering of silica glass: that is, wet silica glass exhibits lower Rayleigh scattering than dry silica glass [78]. Meanwhile, Kakiuchida et. al. found that αc increases with fluorine concentration but not with chlorine concentration [79,80]. All of these dopants accelerate the structural relaxation process. In particular, researchers have found that with sufficiently high OH, F, or Cl doping, “sub-relaxational processes” are activated that have significantly shorter relaxation times at low temperatures than the main (α) relaxation process [45–47]. Both controlled cooling rates and dopants have been effectively used to decrease Rayleigh scattering loss in optical fibers [81,82].
Recently, it was reported that the Rayleigh scattering of a low OH (50 ppmwt) fused silica was reduced by up to 30% using a hot compression method that resulted in the shrinkage of structural voids [83]. A subsequent modeling study predicted that Rayleigh scattering loss could be reduced by > 50% if the glass were quenched at an “appropriate pressure” [84].
This section describes some of the effects of laser-induced changes in silica. Under certain exposure conditions the propensity for the glass to undergo transmission, refractive index, and structural changes becomes more pronounced. These changes can be particularly troublesome for lithography stepper applications, where imaging and throughput can be adversely affected. On the other hand, these changes can be used to advantage to produce gratings and waveguides, using ultra short, pulsed lasers operating in the IR or by using UV excimer laser exposure in Ge-doped silica optical fiber. While there has been significant work on these topics, they are out of the scope of this review; however, references can be found in [85–89]. Here the processes that are discussed are limited to the effect of UV exposures with a particular emphasis on those relevant to short wavelength lithography.
In the study of UV-induced changes in silica glass one point becomes clear: complicating a full understanding of these laser-induced changes and their dependence on composition and structural variables, is that there is also an incident power dependence both for color center formation and density changes. This presents challenges in revealing mechanisms that occur under different exposure conditions and also in translating responses obtained in more convenient, high-power exposures to other exposure regimes. Nonetheless, some fundamental processes have become evident.
Color center formation in silica glass is a well-studied phenomenon [90–95]. A comprehensive review of defects in silica is given in reference 49. For the topic at hand, the radiation-induced defects of consequence are the result of breaking Si-O bonds in the glass structure. This leads to a pair of defects referred to as the E’ center, centered at 215 nm, and the non-bridging oxygen hole center, NBOHC, centered at 260 nm. The absorptions are sufficiently broad that they can impact transmission at 248 and 193 nm, the exposing wavelengths used in lithography applications. In the lithographic application not only is the photon energy of the exposing source high (greater than 5 eV) but the incident power is delivered in short time bursts. These conditions can promote multi-photon absorption events that promote valence band electrons to the conduction band, with subsequent bond breaking and trapping of the defects, resulting in the absorptions described above. Interestingly, the color centers that are formed can have a dynamic response, in that the optical absorption is higher while the light is on, with transmission largely recovering in the dark. The transient response has a practical implication in that the end user requires minimal induced absorption during exposure. As will be shown, this necessitates an understanding of that dynamic process and forces experimental work to be accomplished in a way that captures that response.
As described earlier in the article, there are different silica compositions, largely differentiated by their hydroxyl (SiOH) contents. However, it had also been known from other work [96,97] that molecular H2 dissolved in silica could ‘heal’ the E’ and NBOHC centers, producing SiH and SiOH species, that do not absorb in the visible to UV spectral range. Consequently, significant study was directed to understanding the role of H2 in the optical processes and ultimately using that chemistry to improve the optical response of the glass with respect to absorption.
The 193 nm laser-induced absorption processes of silica under UV irradiation, specifically the behavior of the E’ center centered at 215 nm, were studied using an in-situ exposure and measurement set up, shown in Fig. 7. [95]
Fig. 7. Experimental arrangement to measure induced absorption on-line. The probe beam is a collimated D2 beam passing through the sample orthogonal to the exposure beam. Detection was accomplished using photomultiplier tubes (Reprinted with permission from [95], Optica).
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This arrangement permitted the measurement of the optical absorption during exposure and after the exposing laser was turned off. The role of molecular H2 was explicitly examined in this study by preparing high hydroxyl glass with 3 different H2 levels, as determined by Raman spectroscopy. Figure 8 illustrates the response of the color center with time.
Fig. 8. (a) Measured fading of the 193nm-excimer laser-induced absorption for samples with different molecular H2 contents: Curve A: 3 × 1017 molecules/cc; Curve B: sample heated to 1000°C to remove H2; Curve C: 2 × 1019 molecules/cc. (b) normalized curves of fading of the absorption for the 3 glasses (Reprinted with permission from [95], Optica).
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The data shown in Fig. 8 illustrate two important behaviors: 1. The H2-free glass exhibits the highest amount of induced absorption. Further, as the molecular H2 concentration in the glass is increased, the induced absorption is decreased. 2. Recovery of the 215 nm transmission is absent in the H2-free glass and increases as the molecular H2 increases. These responses are consistent with the reaction of H2 with the E’ center, shown in [Eq. (2a)], proposed by Faile and Roy [96]. The glass also undergoes redarkening when the irradiation is resumed (Fig. 9). The light-induced darkening is attributed to photolysis of the SiH bond, which regenerates the E’ center [Eq. (2b)].
Fig. 9. Representative fade (laser off) and redarkening (laser on) curve illustrating the return of the absorption to its previous level (Reprinted with permission from [95] Optica)
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A kinetic model that seeks to explain this and related behaviors can be found in [98].
Work on 193 nm exposures of silica investigated the roles of hydroxyl, H2, and an intrinsic defect produced during manufacturing, the oxygen-deficient center, ODC, on induced absorption [99]. The glasses used in that study came from different commercial sources and consequently have a range of the species of interest. Because of the different manufacturing processes used to produce these glasses, it is difficult to draw general conclusions regarding the impact of any specific variable on the observed optical changes from this study. Further, significant differences in behavior were observed depending on the incident fluence, perhaps illustrating the competition of the different variables involved in the darkening processes, as mentioned previously. There is, however, a clear correlation between changes in absorption and the ODC concentration of the glasses that could be explained by the reaction shown below [Eq. (3)]:
Further additions to the understanding of the behavior of fused silica under excimer laser irradiation have been reported, using the correlation between 193 nm induced absorption and photoluminescent-related defects. Again, commercially available fused silica glasses were used for the study [100]. In this work, the linear absorption at 193 nm was correlated with photoluminescence at 550 nm while non-linear absorption processes appear to be related to luminescence at 650 nm; luminescence at 650 nm is known to be associated with the NBOHC defect [49]. The identity of the structural species responsible for the 550 nm luminescence is not known at this time.
Along with color center formation under high energy radiation, fused silica also undergoes structural changes referred to as compaction (or densification) and rarefaction (or expansion). While compaction is a density increase of the glass, rarefaction is manifested as a density decrease. In these cases, the result is a refractive index change as a consequence of the laser-induced density change. While small, on the order of ppm density changes and nm of birefringence, these effects do accumulate in the long pathlengths necessary for UV and DUV imaging systems and can adversely affect the final image. Characterization of the changes has been accomplished using measurements of stress-induced birefringence and interferometry, coupled with finite element calculations.
Compaction of silica has been studied and characterized [101–105] and is often described as evolving by a power law representation where ρ is the density change associated with compaction. In the case of laser-exposed glass I is incident intensity and N is number of pulses:
The value of the exponent is found to be on the order of 0.6 for 193 nm exposures [102] and is in good agreement with Primak’s foundational work on radiation-induced compaction of silica [106].
The complicated fluence dependence of the laser-induced processes is particularly highlighted by the compaction and rarefaction response. Exposures using a 157 nm excimer laser revealed that the initial response of the glass is a decrease in refractive index (rarefaction) with a transition to compaction with continued exposure [107]. In experiments using the same glass, with high OH and containing H2, a crossover point from rarefaction to compaction was found to occur at fewer pulses with higher fluence (Fig. 10(a)).
Fig. 10. (a) Induced refractive index changes of a high OH, high H2 glass exposed at different fluences, plotted as a function of pulse count. (b) Induced refractive index change of H2-free glasses with different OH contents plotted as a function of pulse count (Reprinted with permission from [107], Optica).
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Using the same exposure fluence and H2-free glasses with a range of OH contents, Fig. 10(b) shows the crossover dependence on hydroxyl content. Here the crossover trends with OH content, where the higher OH glasses exhibit the crossover at longer pulse counts. This result suggests that a mechanism involving photolysis of the hydroxyl group, with structural rearrangement leading to a volume increase, is operative in the rarefaction process. Coincident and competing with that process is the structural rearrangement of the lattice itself that is responsible for compaction. In another study, infrared spectroscopy was used to probe the light-induced changes in the band associated with the hydroxyl species [108]. The results show an increase in the H-bonded OH species as a consequence of exposure.
The combined effects of induced absorption and compaction can result in the formation of micro-channels (extended microscopic voids) in the glass [109]. The process is shown in Fig. 11. Localized areas of increased refractive index form in the glass under laser exposure. These areas turn into waveguides through a process of accumulated self-focusing. Continued exposure leads to waveguide build-up and mode field diameter collapse. If the light intensity in the waveguide exceeds the damage threshold of fused silica, a plasma spark is ignited and creates a micro-channel, MC. The highest resistance to micro-channeling is obtained in fused silica glasses with low compaction rates and high index homogeneity.
Fig. 11. (a,b) Bright spots on exit surface of the glass sample in the advanced stage of self-focusing. (c) Side view of sample during the active phase of MC formation. Red fluorescence is from NBOHC defects. Yellow-green fluorescence is from already formed MC. (d) Glass sample fractured close to an MC and HF-etched to expose the MC and the waveguide leading to the MC (Reprinted from Appl. Phys. Lett. 105, 244110 (2014) with the permission of AIP Publishing).
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The first application of fused silica was ultrasonic delay lines in 1952. Today, fused silica is used in a wide range of commercial optical applications that includes windows, mirrors, lenses, and optical fiber. Some significant applications enabled by fused silica are shown in Fig. 12.
Fig. 12. (a) Schematic of a UV lithography tool with fused silica lenses highlighted in blue. (b) NIF fused silica focus lenses in cleaning process (Courtesy of Lawrence Livermore National laboratory). (c) View of earth from the Earth observation (nadir) window in the Destiny laboratory on the International Space Station. Portions of the space shuttle Atlantis and the Canadarm2 are visible through the window (Courtesy of NASA). (d) Self-portrait of NASA's Curiosity rover on Mars (Courtesy of NASA).
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Microlithography is a key process in the fabrication of integrated circuits. In optical projection lithography, the pattern on a mask is projected through a series of lenses onto a photoresist layer that is subsequently developed and etched in order to reproduce the pattern in the underlying semiconductor material. Figure 12(a) shows a schematic of an optical step-scan lithography tool. The beam enters at the bottom left; the fused silica illumination and projection optics are highlighted in blue. The smallest feature that can be printed with projection lithography is equal to kλ/NA, where k is a factor determined by the system, λ is the wavelength of the light, and NA is the numerical aperture of the imaging system.
The momentum of Moore’s law, which states that the number of transistors per integrated circuit doubles every two years, has driven the development of new lithographic tools with illumination sources of ever shorter wavelengths that can write ever smaller features [110]. The transition to UV lithography in the late 1980’s and 1990’s using KrF (248nm) and ArF (193nm) excimer laser sources was enabled by the development of special grades of fused silica with superior index homogeneity, low birefringence, and a high resistance to laser damage (see Section 5). Next generation, extreme ultraviolet (EUV) lithography tools operate with 13 nm light and have reflective lenses made from Corning EUV grade ULE glass, a titania-doped silica glass with zero thermal expansion.
High energy laser systems throughout the world are being used to study inertial confinement nuclear fusion (ICF). These systems depend on high quality fused silica optical components for lenses, windows, phase plates, and debris shields [118]. High energy laser systems are located at The National Ignition Facility (NIF), the OMEGA laser facility at the University of Rochester (USA), the US Naval Research Laboratory NIKE, and the Laser Mégajoule in France. In August 2021, the National Ignition Facility delivered a 1.9 MJ shot of light that yielded a record 1.3 MJ of fusion energy, raising expectations for nuclear fusion as a future source of alternative energy [119]. Figure 12(b) shows some of the wedged focus lenses used to direct NIF’s 192 UV laser beams to the center of the target chamber. The lenses are made of fused silica because of its high ultraviolet light transparency, high damage threshold, and excellent surface quality.
Fused silica has been used for terrestrial telescope mirrors since the early 1960’s due to its high thermal stability, polishability, and manufacturability in large sizes. Corning has supplied fused silica mirror blanks ≥ 2.6 m in diameter since 1964, including the 3.6 m mirror for the telescope operated by the European Southern Observatory (ESO) at the La Silla Observatory [120]. The COSMO Large Coronagraph Telescope with its 1.5 m fused silica primary lens will be the largest refracting telescope in the world when completed [121]. The Laser Interferometer Gravitational Wave Observatory (LIGO) that detected gravitational waves for the first time in 2015 has low-OH fused silica mirrors [122].
The high radiation damage resistance of fused silica makes it suitable for space-based applications, and it has been an integral part of our exploration of outer space. Every U.S. space vehicle having service personnel has been equipped with fused silica windows (Corning HPFS 7940 or 7980) including the Apollo 11 Lunar Module, the first crewed vehicle to land on the moon, and the space shuttle [2]. The highest quality optical window ever installed in a crewed spacecraft, the nadir window of the Window Observational Research Facility (WORF) on the International Space Station (ISS), is made of fused silica specially designed to eliminate optical distortions (Fig. 12(c)) [123]. Fused silica corrective lenses were installed on the Hubble Space Telescope in 1993 [124]. Fused silica is even on Mars in the camera optics of NASA’s Curiosity rover which landed on Mars in August 2012 (Fig.12d) [125].
Optical fiber is, perhaps, the most important innovation to arise from the invention of fused silica and the flame hydrolysis process. Optical fiber is composed of a core glass surrounded by a clad glass (Fig. 13), where the core glass has a higher refractive index than the clad glass. A polymer coating protects the fiber from mechanical damage. The composite structure confines light propagating down the fiber to its core through a process of total internal reflection [111,112]. In telecommunications optical fibers, the core glass is typically a doped fused silica, and the clad glass is pure fused silica. The fibers are made by soot-to-glass processes because the silica needs to be free of OH and trace metal impurities to have high enough transmission to send infrared light over kilometers-long pathlengths.
The first low loss optical fiber was made in 1970 using a type of flame hydrolysis process [113,114]. The fiber, composed of a TiO2-SiO2 core glass and a SiO2 clad glass, had an attenuation of 16 dB/km at 632.3 nm [1]. Today, most optical fibers have a GeO2-SiO2 core glass, which has lower IR absorption than titania-silica [81,82,115], allowing the operating wavelength to be shifted to either 1310 or 1550 nm where Rayleigh scattering is lower. Recent improvements in optical fiber transmission have come from reductions in Rayleigh scattering using methods described in Section 3.2 [82]. Today’s ultra-low loss fibers have pure silica cores containing a relaxation-enhancing dopant and are annealed during fiber draw. Attenuations <0.15 dB/km at 1550 nm have been reported in the literature [115–117].
Fused silica has enabled some of the world’s most critical technical achievements – from high-density integrated circuits to optical fiber to space travel – due to its exceptional purity, ease of manufacture, and unique thermal and optical properties. While an essential material for these very practical reasons, silica also holds a significant place in glass and materials science. As a ‘simple’ glass it is often thought of as the model for amorphous materials, yet its properties are often anomalous. This combination has inspired generations of scientists to contemplate the structure and properties of this fascinating material. As this article suggests, finding new applications for fused silica and its study will continue well into the future.
Corning Incorporated; Corning Research and Development Corporation.
The authors acknowledge the collaboration of co-authors cited, Dr. Slava Khrapko, Dr. Ken Hrdina, Dr. Dana Bookbinder, Rich Fiacco, Roger Welch, Mike Wasilewski, Scott Aldrich, Greg Bucher, Brad Ackerman, Michelle Pierson-Stull, Dr. Bill Rosch, Larry Sutton, Bart Lanahan, Harita Machiraju, MK Cornfield, and Will Angell.
LAM, CMS: Corning Research & Development Corporation (E,P).
No data were generated or analyzed in the presented research.
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