Fused Silica

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

Link to RuiQi Optics

Definition: amorphous silicon dioxide

Alternative terms: fused quartz, quartz glass, silica glass

More general term: optical materials

More specific terms: infrared grade silica, UV-grade silica

Category: optical materials

DOI: 10./d1v   Cite the article: BibTex plain textHTML   Link to this page   LinkedIn

Fused silica is amorphous silicon dioxide. It can be obtained e.g. by melting silica powder such that the grains are fused together, and cooling it down fast enough to avoid crystallization. Fused silica in a purified form belongs to the most important optical glasses, or more generally optical materials, both for a wide range of bulk optical components and in fiber optics.

There is also a variety of silicate glasses, which have fused silica as their main component but contain additional substances such as soda, alumina, germania or lime. They generally have much lower glass transition temperatures and also differ from pure silica in many other respects, e.g. in terms of the transparency range and the thermal expansion coefficient.

Fused silica is sometimes called fused quartz or quartz glass. However, it should be kept in mind that it is an amorphous material, while quartz is crystalline. (When lamps are said to have a quartz envelope, it is always fused silica; the same holds for most 'quartz tubes'.) Other common names are silica glass and vitreous silica.

While silica has a very wide range of industrial and other applications, this article focuses on optical properties and applications in optics.

Key Properties of Fused Silica

Fused silica has several remarkable features both concerning its mechanical, thermal, chemical and optical properties:

• It is hard and robust, and not too difficult to machine and polish. (One may also apply laser micromachining.)
• The high glass transition temperature makes it more difficult to melt than other optical glasses, but it also implies that relatively high operation temperatures are possible. However, fused silica may exhibit devitrification (local crystallization in the form of cristobalite) above °C, particularly under the influence of certain trace impurities, and this would spoil the optical properties.
• The thermal expansion coefficient is very low ' about 0.5 · 10'6 K'1. This is several times lower than for typical glasses. Even far weaker thermal expansion around 10'8 K'1 is possible with a modified form of fused silica with some titanium dioxide, introduced by Corning [4] and called ultra low expansion glass.
• The high thermal shock resistance is a result of the weak thermal expansion; there is only moderate mechanical stress even when high temperature gradients occur due to rapid cooling.
• Silica can be chemically very pure, depending on the fabrication method (see below).
• Silica is chemically quite inert, with the exception of hydrofluoric acid and strongly alkaline solutions. At elevated temperatures, it is also somewhat soluble in water (substantially more than crystalline quartz).
• The transparency region is quite wide (about 0.18 μm to 3 μm), allowing the use of fused silica not only throughout the complete visible spectral region, but also in the ultraviolet and infrared. However, the limits substantially depend on the material quality. For example, strong infrared absorption bands can be caused by OH content, and UV absorption from metallic impurities (see below).
• As an amorphous material, fused silica is optically isotropic ' in contrast to crystalline quartz. This implies that it has no birefringence, and its refractive index (see Figure 1) can be characterized with a single Sellmeier formula.

• The nonlinear index of fused silica is one of the lowest of all optical materials.
• It also exhibits relatively low chromatic dispersion and thus belongs to the crown glasses.
• For some applications, the high radiation resistance of pure fused silica is relevant.
• The high phonon energies (resulting from the light elements Si and O) lead to strong non-radiative transitions of integrated rare earth ions, which is beneficial in some cases but excludes the use of fused silica in other cases.

Fabrication of Fused Silica

Fused silica can made by melting some solid form of silica and cooling the melt sufficiently fast to avoid crystallization. A quite high temperature around  to °C is needed ' far above the glass transition temperature of many common optical glasses. The required heat can be provided by an electrically heated furnace or by a flame (Verneuille process) obtained with some combustion gas mixed with pure oxygen. For obtaining high quality material as needed in optics, contamination with unwanted impurities, which is particularly likely due to the high temperature, should be minimized with a suitable choice of materials, e.g. for crucibles.

One can use natural quartz crystals as the raw material, but this will generally lead to a relatively low material quality because quartz can contain a range of impurities (e.g. aluminum and sodium), which affect the optical properties, in particular the transmissivity in certain spectral regions. Therefore, one normally uses some chemically refined silica, which can exhibit a very low concentration of impurities.

Highly purified silica, e.g. for fiber fabrication (more precisely, the fabrication of fiber preforms), can be obtained in a chemical reaction. For example, one can burn silica tetrachloride (SiCl4) in a hydrogen'oxygen flame, where the oxygen combines with the silicon and the chlorine escapes in the form of HCl. The resulting synthetic silica is deposited in the form of a very fine powder (dust), which then can be fused to obtain solid material. It may exhibit substantial OH content, but a very low level of metallic impurities. In order to minimize OH content of the obtained silica for application in infrared optics, one needs to avoid hydrogen by using a vapor-free plasma flame.

Fused Silica Grades

There is not simply fused silica of higher or lower quality; it depends on the intended application (see below) what aspect of quality is relevant:

• Trapped air bubbles and other inclusions of course need to be avoided for any optical applications, except if one wanted to fabricate an optical diffuser. High optical homogeneity is also usually required.

• For application in infrared optics, it is essential to have a low content of hydroxyl (OH) ' often somewhat inappropriately called water content, since it is wrong to assume that the material would contain H2O molecules. An OH content below 10 ppm is typically required for IR-grade fused silica. Substantial absorption bands related to hydroxyl content are around 2.2 μm and 2.7 μm wavelength, but there are also overtone bands e.g. in the 1.4-μm region, which are relevant for the 1.5-μm telecom wavelength band.
• For applications in the ultraviolet spectral region, other properties are relevant. The UV transmission can be limited by various metallic impurities, which therefore need to be carefully minimized for UV-grade fused silica (while they do not matter much for IR applications). Also, it is important that the material is not substantially degraded by UV irradiation; one requires good solarization resistance, which implies low radiation-induced absorption through color centers. Another possibly important feature is to have low UV-induced fluorescence and phosphorescence.

The fabrication method (see above) must be chosen accordingly. For example, ordinary flame processing would often lead to a too high hydroxyl content for UV applications.

Various trade names are related to the type and application area. For example, Herasil, Homosil, Optosil and Vitreosil are fabricated with flame fusion, exhibit high OH content (around 150 to 400 ppm) and are thus suitable for visible and ultraviolet applications, but usually not in the infrared. Suprasil and Spectrosil are variants made with flame hydrolyzation of SiCl4, having a much lower content of metallic impurities, but also having a high OH content. Very low OH content (possibly well below 1 ppm) is achieved for materials like Infrasil, Suprasil W and Spectrosil WF, made with a water vapor-free plasma flame. Often, such trade names comes with additional numbers for different variants which are optimized for specific applications.

Of course, the surface preparation is another aspect of quality, as generally in optics. Various kinds of specifications can be relevant, e.g. surface flatness and scratch'dig specifications.

Applications of Fused Silica

Fused silica is used for a wide range of optical components, such as lenses, prisms, optical flats, mirror substrates and diffraction gratings. Key advantages are the broad spectral transmission range, the hardness and low thermal expansion ' e.g. for large telescope mirrors, where the possibility to fabricate large pieces is also vital. Fused silica is also used for optical windows, when a high pressure difference between both sides and/or a limited window thickness leads to the requirement of high mechanical strength. For photomasks, the high UV resistance can be important.

Fused silica is also widely used for the envelopes of various kinds of lamps, if those are exposed to high temperatures or high temperature gradients. For example, halogen lamps and various kinds of gas discharge lamps (particularly high intensity discharge lamps) need to be operated with a very hot envelope to avoid depositions which would diminish the light output. In some cases, the high ultraviolet transmissivity of fused silica is required; that is particularly the case for excimer lamps. In the case of halogen lamps, the high UV transmission is actually often unwanted, and makes necessary the use of additional filter glasses.

Acousto-optic modulators are often based on a piece of fused silica, particularly for high-power laser applications.

In dielectric coatings, fused silica is often used as the low-index material. It can be deposited in a vacuum chamber with electron beam evaporation or ion beam sputtering, for example.

Another important application area is fiber optics. Most optical fibers, including nearly all telecom fibers, are silica fibers. Here, one usually does normally not use pure silica throughout because an optical fiber usually contains a waveguide structure. A common option is to use pure fused silica for the fiber cladding while having some kind of silicate glass (e.g. germanosilicate) for the fiber core. Particularly for large-core multimode fibers, one may alternatively have a pure-silica core (exhibiting particularly low propagation losses) and a 'depressed cladding', which is typically doped with fluorine to obtain a reduced refractive index. Most photonic crystal fibers are made from pure silica.

Because the transmission distances in fibers are often very long (e.g. dozens of kilometers), sufficiently low propagation losses are generally needed, and this requires highly purified forms of silica. Indeed, the development of low-loss fibers, suitable for example for optical fiber communications, first required the identification of relevant impurities and the careful optimization of fabrication processes. See the article on silica fibers for more details.

One can fabricate various other types of waveguides on silica surfaces (or somewhat below). This is important in the context of photonic integrated circuits.

More to Learn

Bibliography

[1]I. H. Malitson, 'Interspecimen comparison of the refractive index of fused silica', J. Opt. Soc. Am. 55 (10), (); https://doi.org/10./JOSA.55. [2]R. Brückner, 'Properties of structure of vitreous silica. I', J. Non-Crystalline Solids 5, 123 (); https://doi.org/10./-(70)-0 [3]R. Brückner, 'Properties of structure of vitreous silica. II', J. Non-Crystalline Solids 5, 123 (); https://doi.org/10./-(71)-9 [4]C. L. Rathmann, G. H. Mann and M. E. Nordberg, 'A new ultralow-expansion, modified fused-silica glass', Appl. Opt. 7 (5), 819 (); https://doi.org/10./AO.7. [5]T. Olivier et al., 'Nanosecond Z-scan measurements of the nonlinear refractive index of fused silica', Opt. Express 12 (7), (); https://doi.org/10./OPEX.12. [6]Q. Feng et al., 'Strong UV laser absorption source near 355 nm in fused silica and its origination', Opt. Express 29 (20), (); https://doi.org/10./OE. [7]R. Schiek, 'Nonlinear refractive index in silica glass', Opt. Mater. Express 13 (6), (); https://doi.org/10./OME.

(Suggest additional literature!)

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Silicon Dioxide ' Glass ' Quartz ' Fused Silica

Silicon Dioxide (SiO2) is the simplest chemical composition of glass. Quartz is the most stable crystal modification at normal temperature and pressure conditions. Quartz is one of the most common minerals in the earth's crust. Glass (from 'glasa', Germanic for amber, the shiny or shimmery) also consists of silicon and oxide, but is a uniform amorphous solid material. Many glass varieties are clear and transparent respectively. This means transmissibility for the visible spectrum of light. In general, such material is associated with the term glass. Transparent materials allow light to pass through them without diffusing (scattering) the light.

Most common types of glass

At least years ago humans learned how to lower the glass softening temperatures by adding lime and soda before heating, which resulted in a glass containing sodium and calcium oxide.

Glass ' Additives and the industrial use of glass

The use of glass as one of the oldest, but also very important materials for the industry is linked with the application of additives. Chemicals like soda (sodium carbonate, Na2CO3) and in the past also potash (potassium carbonate, K2CO3), manganese oxide and metal oxides influence the properties of glass. Manufactured glass is a material formed by heating a mixture of sand, soda and lime to a high temperature and stays in a molten, liquid state. Glass can be made from pure silica, but quartz glass (also referred as quartz) has a high glass transition point at around °C, which makes it difficult to form into panes or bottles.

Quartz glass is the purest form of SiO2 and therefore the most valuable and sophisticated variety. Extremely clear glass can be used for optical fibers. Therefore synthetic quartz glass is used to transmit light across many kilometers. Many glasses block ultraviolet radiation, but only pure fused silica (only SiO2) is transparent for wavelengths < 350 nm (UV). Quartz glass also exists as an opaque variety and with different coloration to change the physical and chemical properties like transmission or absorption for specific wavelengths (filter glass). The opaque material at Heraeus, OM® 100, is also used as a heat barrier or for diffuse scattering of IR radiation.

At first glance, quartz glass appears very simple, both chemically and structurally, since it is made from a single oxide component (silicon dioxide ' SiO2).

Chemical structure

Silica, as it is also known, is found throughout the earth's crust. However, only a small fraction has sufficient purity (> 99.98 %) to be suitable as raw material for quartz glass. Sand at the beach is also SiO2, but isn't suitable for use in the semiconductor industry.

Structure of quartz and fused silica

In the quartz glass structure all atoms are bonded to at least two others. The strength of the silicon-oxygen (Si-O) chemical bond gives quartz glass high temperature stability and chemical resistance. But the structure is also rather open with wide spaces (interstices) between the structural units. This accounts for higher gas permeability and a much lower thermal expansion coefficient for quartz glass relative to other materials.

Purity is crucial for most industrial applications and processes. Fused silica has an outstandingly high purity and therefore is indispensable in the fabrication of high-tech products.

Despite existing at very low levels, contaminants have subtle yet significant effects. Purity is mostly determined by the raw material, the manufacturing method and subsequent handling procedures. Special precautions must be taken at all stages of manufacture to maintain high purity. Additionally, Heraeus uses different purification steps to improve the quality of the quartz sand as raw material even further.

The most common impurities are metals (such as Al, Na and Fe among others), water (present as OH groups) and chlorine. These contaminants not only affect the viscosity, optical absorption and electrical properties of the quartz glass but they also influence the properties of material processed in contact with the quartz glass during the final use application.

The purities of fused quartz and fused silica are outstandingly high. Synthetic fused silica from Heraeus contains total metallic contamination below 1ppm. For fused quartz the amount is approximately 20 ppm and consists primarily of Al2O3 with much smaller amounts of alkalis, Fe2O3, TiO2, MgO and ZrO2.

OH content

In addition to metallic impurities, fused quartz and fused silica also contain water present as OH units. OH content influences the physical properties like attenuation and viscosity. Generally, high OH content means lower use temperature. Typical values are given in the table. Electrically fused quartz has the lowest hydroxyl content (< 1 ' 30 ppm) since it is normally made in vacuum or a dry atmosphere. Hydroxyl content in this range is not fixed in the glass structure. It can go up or down depending on the thermal treatment and amount of moisture the quartz glass is exposed to at elevated temperature. Flame fused quartz has significantly more hydroxyl (150 ' 200 ppm) since fusion occurs in a hydrogen/ oxygen flame. Due to the production method, synthetic fused silica has similar high OH contents of up to ppm.

One of the most attractive features of quartz glass is its very low thermal coefficient of expansion (CTE). The average CTE value for quartz glass at about 5.0 × 10 -7/ °C is many times lower than that of other common materials. To put this in perspective, imagine if 1 m3 blocks of stainless steel, borosilicate glass and quartz were placed in a furnace and heated by 500 °C. The volume of the stainless steel block would increase by more than 28 liters and that of the borosilicate block by 5 liters. The quartz block would expand by less than one liter. Such low expansion makes it possible for the material to withstand very severe thermal shock.

It is possible to rapidly quench thin particles of quartz glass from over °C by plunging them into cold water without breakage. However, it is important to realize that the thermal shock resistance depends on factors other than CTE such as surface condition (which defines strength) and geometry. The various types of fused silica and fused quartz have nearly identical CTE's and thus can be joined together with no added risk of thermally induced breakage.

Mechanical properties, strength and reliability

The theoretical tensile strength of silica glass is greater than 1 million psi. Unfortunately, the strength observed in practice is always far below this value. The reason is that the practical strength of glass is extrinsically determined rather than being solely a result of an intrinsic property like density. It is the surface quality in combination with design considerations and chemical effects of the atmosphere (water vapor in particular) that ultimately determine the strength and reliability of a finished piece of quartz glass. Because of stress concentration on surface flaws, failure most always occurs in tension rather than compression.

In other words: 'reliablility depends on the chance'.

This could also be stated as the probability that the piece will experience a mechanical stress greater than the strength of any existing flaws. As a result of this dependence on probability, reliability decreases as the size of the glass article increases. Similarly, if the number of pieces in service increases, so does the chance of experience a failure.

Surface condition is very important. For example, machined surfaces tend to be weaker than fire polished ones. Also, older surfaces are usually weaker than younger ones due to exposure to dust, moisture or general wear and tear. These factors have to be considered thoroughly when comparing the strengths of different 'brands' of quartz glass.

This is because these tests in reality often turn out to be just comparisons of surface quality resulting from sample preparation, small differences in which easily overwhelm any differences in intrinsic strength.

Controlled heat management and sustained high temperatures is crucial in many industrial processes, especially in semiconductor industries.

Fused silica is a good electrical insulator, retaining high resistivity at elevated temperatures and excellent high-frequency characteristics. The large band gap inherent in the electronic structure of the silicon-oxygen bond results in electrical conduction being limited to current carried by mobile ionic impurities. Since the level of these impurities is very low, the electrical resistivity is correspondingly high.

Since ionic conduction is related to the diffusion coefficient of the ionic carriers, the resistivity also has a strong exponential temperature dependence. Hence, unlike typical conductors such as metals, the resistivity decreases with increasing temperature.

The dielectric constant of quartz glass has a value of about 4 which is significantly lower than that of other glasses. This value changes little over a wide range of frequencies. The reason for the low dielectric constant is, once again, the lack of highly charged mobile ions but it also results from the stiffness of the silicon-oxygen network which imparts a very low polarizability to the structure.

Because it has very low absorption down into the vacuum ultraviolet spectral range (the cutoff is at about 160 nm for a 1 mm thickness), synthetic fused silica is used for optical lenses in high-energy laser applications and as envelope material for ultraviolet light sources such as excimer or deuterium lamps. Depending on the exact experimental conditions, such as the wavelength, energy density and peak intensities in pulsed laser applications, various kinds of damage can occur to the glass.

At very high laser intensities, photoionization and plasma generation can take place locally at certain positions in the glass. This mechanical damage typically appears at the front or rear surface of the optical element. A related type of mechanical damage which can occur is the generation of fine microchannels within the glass along the propagation direction of a laser beam.

In addition to these visible damage phenomena, a more subtle damage mechanism can take place as defect centers (sometime called color centers) are generated in a photochemical process under irradiation of the glass. These centers cause absorption at characteristic wavelengths. Examples are the E' center with an absorption maximum at 215 nm and the non-bridging oxygen hole (NBOH) center at 265 nm. The NBOH hole also emits red fluorescence at about 650 nm when excited in its absorption band. These defects also interact with dissolved hydrogen in the glass. Hydrogen can passivate E' centers to create SiH groups, and NBOH centers to create SiOH groups, thus mitigating the transmission loss at the absorption wavelengths of these defects. Therefore, the hydrogen concentration is often controlled precisely during production processes, and measured by Raman spectroscopy in the analytical lab.

A third type of damage which can take place is a change in the refractive index of the silica glass due to restructuring of the glass network under irradiation. The refractive index can either increase (called compaction), or it can decrease (rarefaction), depending on the silica glass type and the irradiation conditions.

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