Quartz Glass and Opaque Quartz

Vitreous silica, made from fusing or vitrifying naturally occurring materials, can appear clear or opaque depending on whether clear crystalline quartz or white quartzite sand is used. Both naturally occurring raw materials are highly pure forms of silica or silicon dioxide. These materials can also be used to manufacture, through distillation, the highly pure chlorosilanes such as silicon tetrachloride which in turn are the raw material for a form of vitreous silica commonly called "synthetic quartz glass”.

Quartz and the quartz glass form of vitreous silica have the same basic chemical composition but are, of course, structurally very dissimilar, the one being crystalline and the other a glass. For this reason a piece of crystalline quartz cannot withstand a sudden quench cooling from a high temperature without cracking due to temperature dependent phase changes, whereas quartz glass can withstand such processes. Vitreous silica in all its various forms has a variety of properties rarely found in a single material; extremely low thermal expansion, excellent elastic and thermal shock characteristics, high transformation and softening points, low thermal conductivity, low dielectric losses, good optical transmission from ultra-violet to infra-red, chemical purity, corrosion resistance and a trace impurity diffusion barrier. The technology for welding and shaping the material is well advanced as is that for cold working and laser cutting.

Manufacture/ Quality Assurance/ Application

Raw Materials

Quartz glass for use in the semiconductor industry is made from the purest naturally occurring raw materials.

Despite high initial purity levels, major additional processing steps are included in raw material preparation in order to produce a glass product which is suitable for modern requirements.

The raw materials are usually rock crystal and pegmatitic quartz. Various chemical purification processes involving wet etching and high temperature treatments are used and the final raw material is a granulate of specific crystalline shape and size and virtually free from trace impurities.

Vitrification Process

Tubes, rods and plates are the basic products from the vitrification process. The first and most important stage is the fusing of the crystalline grains to an amorphous glass structure which is, even in the initial process stages, largely bubble and inclusion free and physically isotropic and homogeneous.

Manufacturing Process and Analysis

Production Methods

There are 2 principal production methods for continuous grain fusing:

• Oxy-hydrogen flame fusing;
• Fusing in an electrically heated crucible from which the glass, in the form of a tube, can be drawn.

Both methods allow the controlled, high volume production of large size tubes.

By allowing the grains to dwell in the oxy-hydrogen flame the crucible-free flame fusing method enables the production of a bubble free and extremely homogeneous glass.

On the other hand electrical fusing can eliminate the hydroxyl (OH) content within the crystal raw material, a factor which is significant for the production of glasses with mechanical stability at high temperatures and for increased infra-red optical transmission.

The initial basic glass shapes produced by these production methods are transformed by subsequent forging processes into semi-manufactured shapes with:

• Low trace element contamination
• Low bubble content
• High homogeneity
• Low OH-content
• High optical transmission

Chemical Analysis

Trace element contamination in quartz glass can be divided into elements which have a high and low diffusivity at operational temperatures. To the former belong the alkali metals and copper, and through specific raw material preparation steps it is possible to reduce their concentration to ppm or even to fractions of this.

It is difficult to determine the precise influence such trace impurities have in semiconductor processes. However, in view of the other sources of contamination present in high temperature processing, the quartz glass can be considered as extremely pure.

Aluminium concentrations in quartz glass can exceed 10 ppm but since it is bound strongly in the silicon-oxygen lattice it is very immobile.

No semiconductor process has been known to have been affected by aluminium contamination from quartz glass. An exception to this however is in the use of quartz glass crucibles in the CZ-process for producing single crystal silicon. Here molten silicon reacts with the crucible wall resulting in aluminium (+ oxygen) being introduced into the melt.

QCS materials are subjected to permanent incoming and production controls for contamination, using standard AAS/NAA methods.

Chemical Behaviour towards other Materials

Transparent fused silica is outstandingly resistant to water, salt solutions and acids. It is therefore always at the top of the DIN Standard list for the chemical stability of glasses, i. e. its stability is second to none; the same applies to its resistance to alkaline solutions.

In contrast to ordinary glass, fused silica is not hygroscopic and therefore does not effloresce. It is only attacked by hydrofluoric acid. Metals which are free from oxide, with the exception of alkali and alkaline-earth metals, cannot react with fused silica, and can be distilled and melted in vessels made of fused silica.

Fused silica is sensitive to all alkali and alkaline-earth compounds, because even slight traces of them hasten devitrification at high temperatures. It is therefore always advisable to wipe off fingerprints (traces of alkali) from fused silica equipment, with alcohol, before heating it to over 900°C.

The following tables indicate, as far as possible, the behaviour of the various elements and compounds towards transparent and opaque fused silica. These tables were drawn up from information contained in technical literature, so that the conclusions are not always exactly comparable.

Hydrolytic resistance as per DIN 12111

• 1. Hydrolysis class: Base discharge < 0.01 mg Na₂O (2g Grains)

Resistance to acids as per DIN 12116

• 1. Acid class: Weight loss < 0.1 mg/dm² (surface area)

Resistance to alkaline solutions as per DIN 52322

• 1. Alkaline solution class: Weight loss approx. 50 mg/dm² (surface area)

Peculiarities and Properties

Peculiarities

• Purity: Basins and plants in the high-purity field under consideration of the hydrolytic resistance of < 0,01 mg Na₂O according to DIN 12111 for medicine, analysis and chemistry.
• Chemical resistance: to the most mediums no matter if solid, liquid or gaseous. For production of reagents and the therefore needed plants.
• High resistance to sudden changes of temperature: is granted by the coefficient of expansion of α 0...900°C, K⁻¹ 0,48 x 10⁻⁶ and is requested for many chemical-physical processes.
• UV- and IR-permeability: in the area of 200 to 3500 nm makes the material very useful for heating or radiation of materials of every physical condition.
• Electrical isolation property: of 10¹⁸Ω cm at 20°C offers a wide application for insulators in electric and electronics.

Physical Properties

Optical Properties

Electrical Properties

Dielectric Loss Angles

Dielectric loss angle is practically constant at a frequency of 1 MHz up to 200°C, but then increases as the temperature rises. At 10¹⁰ Hz the dielectric loss angle falls off slowly with rising temperature up to 350°C, but then, as the temperature rises still further, it begins to climb again slightly.

Note: The diagrams for modulus of elasticity, internal damping, sound velocity, tensile strength, viscosity, coefficient of expansion and change in length are not shown as graphics here, but their values are summarized in the tables above.

Typical Trace Elements

Reactions of diverse Elements and Connections to Quartz glass

Behaviour of alkaline solutions towards transparent and opaque fused silica

The first line of this table thus predicates that a 10% solution of NH₄(OH) at 20°C will in 100 hours resolve 0.019 mg transparent or opaque fused silica from a surface of 1 cm².

In addition there is chemical resistance to most of electroplating baths. More information on request.

Partner for Laboratory and Industry

• Hydrogen-Oxygen Combustion Apparatus V5
• Distillation Apparatus for pure water production
• Surface Evaporators for evaporation of liquids
• Immersion Heaters for the heating of aggressive media
• Crucibles, Beaker, Test Tubes, Basins and Flasks as well as
• Ground joints, Stopcocks, Screw Threads made out of quartz glass
• Quartz Glass Wool, Filtering disks
• Rods, Tubes, Capillary Tubes made out of quartz glass
• Special products acc. to customer specifications
• Quartz glass products for the chemical Industry
• Products made for Laboratory and Industry
• Quartz products for Optics
• Quartz glass for the communication Industry
• Quartz glass for photo voltaic
• Quartz glass for Infrared Applications
• Spirals made of Quartz Glass and opaque fused silica