QUARTZ PROPERTIES
All information provided in the links below was taken from the
GE® Quartz web site. Use the links below to access the corresponding attribute.
Semiconductor Grade Fused Quartz Tubing
In the semiconductor industry a combination of extreme purity and excellent high temperature properties make fused quartz tubing an ideal furnace chamber for processing silicon wafers. The material can tolerate the wide temperature gradients and high heat rates of the process. And its purity creates the low contamination environment required for achieving high wafer yields. The advent of eight inch wafers combined with today's smaller chip sizes has increased chip production by a factor of four compared to technology in place just a few years ago. These developments have impacted heavily on quartz produced, requiring both large diameter tubing and significantly higher levels of purity. GE Quartz. has responded on both counts. Quartz tubing is available in a full range of sizes, including diameters of 400mm and larger. Diameter and wall thickness dimensions are tightly controlled. Special heavy wall thicknesses are available on request. By finding new and better sources of raw material, expanding and modernizing our production facilities, and upgrading our quality control functions, GE has reduced contaminants levels in its fused quartz tubing to less than 25 ppm, with alkali levels below 1 ppm.
Grade 214LD
This is the large diameter grade of industry standard 214 quartz tubing. For all but the highly specialized operations, this low cost tubing offers the levels of purity, sag resistance, furnace life and other properties that diffusion and CVD processes require. For superior performance at elevated temperatures GE type 214 LD furnace tubing gives process engineers a better balance between the effects of higher temperatures and heavier wafer loads.
224LD
Low Alkali Quartz Tubing As the semiconductor industry moves toward higher densities, furnace atmosphere contaminant becomes an increasingly critical factor in controlling wafer yields. One potential contaminant is sodium, which occurs naturally in the silica sand used to make fused quartz. This highly mobile ion can effectively destabilize the electrical characteristics of MOS and bipolar devices if not removed. For these critical applications GE has developed Grade 224 low alkali fused quartz tubing. It is made in a special process that eliminates up to 90 % of the naturally occurring alkalis. The process achieves a typical sodium level of 0.1 ppm (vs. a normal 0.7 ppm), greatly reduces potassium, and virtually eliminates lithium
244LD
Low Alkali/Low Aluminum Quartz Tubing This grade has been specially developed for quartz users concerned about the aluminum level in fused quartz. 244 has a typical aluminum level of 8 ppm.
Low (OH-)
One reason that GE fused quartz tubing can withstand the wide thermal gradients and chemical environments of wafer processing operations is its (OH-) content of less than 10 ppm water in most grades. Low OH- minimizes the sag rate at diffusion temperatures, and effectively retards the progress of devitrification. Because of its low hydroxyl content, GE Quartz tubing does not require special coatings that could potentially release contaminants at elevated temperatures.
Fused Quartz Rod & Solids
GE supplies two forms of high purity fused quartz solid shapes for fabricators of quartz ware. Type 214 rod has the high purity, elevated temperature characteristics and low coefficient of thermal expansion required for wafer carriers and push rods used in semiconductor wafer processing. The material is available in diameters of 1 to 20 mm. Very tight quality control and special processing of raw materials is used to achieve low levels of trace element contamination. When larger sizes and different shaped starting materials are required, GE supplies fabricators with pieces cut from fused quartz ingots. They are up to 72 inches in diameter, two feet thick, and weigh up to 9000 pounds.
Large Ingots
GE Type 124 ingots have been the semiconductor industry's material of choice for fabricating diffusion and CVD furnace components for a number of years. The advent of larger wafer sizes, tighter device geometries, and the drive for lower contaminant levels has stimulated GE's development of an even higher purity grade. Type 144 is specially processed to reduce alkali content by up to 90%. Sodium is held to 0.2 ppm or lower, potassium is significantly reduced while lithium is about 0.2 ppm. Type 012 provides the ultra high purity of synthetic fused silica, while maintaining low (OH) at < 5 ppm.
Lamp Grade Tubing
GE Quartz is the world's leading producer of fused quartz for lighting applications. Four basic types of lamp grade quartz are available, each designed to fulfill specific performance requirements. Together, these materials cover a wide variety of applications. They include:
Type 214
The worldwide standard for clear fused quartz lamp tubing. GE 214 is a high purity, high transmittance, high temperature material with a low hydroxyl (OH-) content. It is suitable for a broad range of mercury, halogen and other quartz lamp applications.
Type 219
Known as "Ozone-Free" or "Germicidal" quartz tubing. GE 219 transmits UV-A and UV-B while blocking the deep, high energy wavelengths that cause ozone generation and pose the greatest exposure risks. Type 219 transmits the 253.7 nanometer mercury emission very efficiently, making it an ideal material for disinfection applications and various other UV treatments.
Type 254
A doped quartz material that blocks virtually all UV-B and UV-C radiation. Type 254 has a transmittance cutoff wavelength between 350 and 400 nanometers. It is ideal for lamps requiring maximum visible transmittance with nearly complete UV protection. Applications for GE 254 are those where UV exposure to people or property is undesirable, including some quartz halogen and metal halide lamps and other UV sources.
Type 021
This is a dry synthetic fused silica material providing high transmittance in the deep ultraviolet range. It combines the advantages of low hydroxyl content with ultra high purity to yield superior UV transmittance and resistance to solarization for a variety of UV lamp applications including water purification, ozone generation, paint and ink curing, and chemical processing.
Types 214A, 219A, and 254A
These are identical to the standard types but are produced with a lower hydroxyl content. "A" products contain <1 ppm (OH-) and are intended for metal halide lamps and other applications where the quartz must be devoid of hydroxyl as well as all dissolved gases.
Quartz Crucibles
In the manufacture of silicon metal for semiconductor wafer applications, polysilicon starting materials are placed in fused quartz crucibles, heated to high temperatures and pulled from the melt as a single crystal. Fused quartz is one of the few materials that can combine the high purity and high temperature properties required.
Other Compositions
To keep pace with the increasingly stringent purity requirements of the industry, GE now offers a variety of compositions in its quartz crucibles. Each type is designed to address specific micro-contamination concerns. However. other options are also available. GE's "Crucible Team" is prepared to work with you on your specific crucible designs
Fiber Optic Tubing
GE fused quartz series as deposition tubing for one of the major methods of producing optical waveguides, the Modified chemical vapor deposition (MCVD) process. For this application, GE offers high quality quartz tubing that is virtually airline free, with tight dimensional tolerances and low (OH-). This combination of characteristics translates into excellent attenuation for the fiber manufacturer. GE produces fiber optic tubing from either naturally occurring or synthetic quartz. The synthetic grades, combined with GE's unique continuous fusion process, produces fiber optic tubing with all the advantages found in natural occurring quartz, plus the higher tensile strength required for producing long length fibers. Along with waveguide material, GE offers high quality quartz tubing and handles required by the MCVD process. Each waveguide tube produced by GE is serialized, characterized and accompanied by a data slip showing the complete geometry of the tube. If desired, a computer disc can be supplied with the shipment for direct entry into our data bank.
Guidelines for Users of Fused Quartz
Like any material that is expected to provide a design life at high temperatures, fused quartz demands some care in handling and use to achieve maximum performance from the product.
Storage
Space permitting, fused quartz should be stored in its original shipping container. If that is not practical, at least the wrapping should be retained. In the case of tubing, the end coverings should be kept in place until the product is used. This protects the ends from chipping and keeps out dirt and moisture which could compromise the purity and performance of the tubing.
Cleaning
For applications in which cleanliness is important, General Electric recommends the following procedure: The product, particularly tubing, should be washed in deionized or distilled water with a degreasing agent added to the water. The fused quartz should then be placed in a 7% (maximum) solution of ammonium bifluoride for no more than ten minutes, or a 10 vol % (maximum) solution of hydrofluoric acid for no more than five minutes. Etching of the surface will remove a small amount of fused quartz material as well as any surface contaminants. To avoid water spotting which may attract dirt and cause devitrification upon subsequent heating, the fused quartz should be rinsed several times in de-ionized or distilled water and dried rapidly. To further reduce the possibility of contamination, care should be used in handling fused quartz. The use of clean cotton gloves at all times is essential. Washing of translucent tubing is not recommended because the water or acid solution tends to enter the many capillaries in the material. This may cause the quartz to burst if the pieces are subsequently heated rapidly to very high temperatures.
Rotation Procedures For Fused Quartz Furnace Tubes
The following procedure has been used to create an even layer of crystobalite on diffusion tubes in order to increase resistance to devitrification. Place the tube in a furnace at 1200øC, and rotate it 90ø every two hours for the first 30 hours. If the working schedule does not permit adherence to this procedure, the following suggestion is offered. Place the tube in a furnace at 1200øC and rotate it 90ø every two hours for the first 8 hours, then reset the furnace to operating temperature.
Solarization
Fused quartz made from natural raw material solarizes or discolors upon prolonged irradiation by high energy radiation (such as short UV, x-rays, gamma rays and neutrons). Resistance to this type of solarization increases with the purity of fused quartz. Hence, synthetic fused silica is highly resistant to solarization. Solarization in fused quartz can be thermally bleached by heating it to about 500øC.
Technical Support
An important consideration for today's users of fused quartz is the availability of technical product support. GE Quartz backs its products with fully equipped analytical and development lab oratories and a staff of materials and fusion experts available to support customer requirements. State-of-the-art analytical equipment assures optimal production quality and also enables certification and subsequent verification of GE Quartz product compliance with stringent industry standards. Physical properties and other information shown on pages 14 through 24 was developed from a number of sources, including GE's technical laboratories, text books and technical publications. While GE believes that this information is accurate, it is not an exhaustive review of the subjects covered and, accordingly, GE makes no warranty as to the accuracy or completeness of the data. Customers are advised to check references to ensure that the product is suitable for the customer's particular use or requirements. Additional technical assistance from our engineering team is available by calling or faxing our world headquarters.
| Type | Al | As | B | Ca | Cd | Cr | Cu | Fe | K | Li | Mg | Mn | Na | Ni | P | Sb | Ti | Zr | *OH |
| 214 | 14 | <0.002 | <0.2 | 0.4 | <0.01 | <0.05 | <0.05 | 0.2 | 0.6 | 0.6 | 0.1 | <0.05 | 0.7 | <0.1 | <0.2 | <0.003 | 1.1 | 0.8 | <5 |
| 219 | 14 | <0.01 | <0.2 | 0.4 | <0.01 | <0.05 | <0.05 | 0.2 | 0.6 | 0.6 | 0.1 | <0.05 | 0.7 | <0.1 | <0.2 | <0.003 | 100 | 0.8 | <5 |
| 254 | 14 | <0.1 | <0.2 | 0.4 | <0.01 | <0.05 | <0.05 | 0.2 | 0.6 | 0.6 | 0.1 | <0.05 | 0.7 | <0.1 | <0.2 | <0.003 | 500 | 0.8 | <5 |
| 214A | 14 | <0.002 | <0.2 | 0.4 | <0.01 | <0.05 | <0.05 | 0.2 | 0.6 | 0.6 | 0.1 | <0.05 | 0.7 | <0.1 | <0.2 | <0.003 | 1.1 | 0.8 | <1 |
| 214 Rod, 214 LD | 14 | <0.002 | <0.2 | 0.4 | <0.01 | <0.05 | <0.05 | 0.2 | 0.6 | 0.6 | 0.1 | <0.05 | 0.7 | <0.1 | <0.2 | <0.003 | 1.1 | 0.8 | 10 |
| 224/224 Rod | 14 | <0.002 | <0.2 | 0.4 | <0.01 | <0.05 | <0.03 | 0.2 | <0.2 | <0.2 | 0.1 | <0.03 | <0.2. | <0.1 | <0.2 | <0.003 | 1.4 | 0.8 | 10 |
| 224 LD | 14 | <0.002 | <0.2 | 0.4 | <0.01 | <0.05 | <0.01 | 0.2 | <0.2 | 0.001 | 0.1 | <0.05 | <0.1 | <0.1 | <0.2 | 0.003 | 1.1 | 0.8 | 10 |
| 244/244 Rod | 8 | <0.002 | <0.1 | 0.6 | <0.01 | <0.05 | <0.05 | 0.2 | <0.2 | <0.2 | <0.1 | <0.03 | <0.2 | <0.1 | <0.2 | 0.003 | 1.4 | 0.3 | 10 |
| 244 LD | 8 | <0.002 | <0.1 | 0.6 | <0.01 | <0.05 | <0.05 | 0.2 | <0.2 | 0.001 | <0.1 | <0.03 | 0.1 | <0.1 | <0.2 | <0.003 | 1.4 | 0.3 | 10 |
| 124 | 14 | <0.002 | <0.2 | 0.4 | <0.01 | <0.05 | <0.05 | 0.2 | 0.6 | 0.6 | 0.1 | <0.05 | 0.7 | <0.1 | <0.2 | <0.003 | 1.1 | 0.8 | <5 |
| 144 | 8 | <0.002 | <0.1 | 0.6 | <0.01 | <0.05 | <0.05 | 0.2 | <0.2 | <0.2 | <0.1 | <0.03 | <0.2 | <0.1 | <0.2 | <0.003 | 1.4 | 0.3 | <5 |
| 982 WGY | 14 | ** | ** | 0.4 | <0.01 | <0.05 | <0.05 | 0.2 | 0.6 | 0.6 | <0.1 | <0.03 | <0.2 | <0.1 | <0.2 | <0.003 | 1.4 | 0.3 | <5 |
| 098 WGY | 0.2 | ** | ** | <0.005 | <0.01 | <0.05 | <0.05 | 0.07 | <0.05 | <0.05 | 0.1 | <0.05 | 0.7 | ** | ** | ** | 1.1 | 0.8 | 3 |
| 095 WGY | 9 | ** | ** | <0.005 | <0.01 | <0.05 | <0.05 | 0.07 | 0.1 | <0.05 | <0.05 | <0.02 | <0.05 | ** | ** | ** | <0.02 | <0.02 | 10 |
| 095 WGY | 9 | ** | ** | <0.005 | <0.01 | <0.05 | <0.05 | 0.07 | 0.1 | <0.05 | <0.05 | <0.02 | 0.1 | ** | ** | ** | <0.02 | <0.02 | <10 |
| 510, 520, 530, 512, 522, 532 | 14, 8 | <0.01, <0.01 | <0.2, <0.1 | 0.4, 0.6 | <0.01, <0.01 | <0.05, <0.05 | <0.05, <0.05 | 0.2, 0.2 | 0.6, 0.5 | 0.6, 0.5 | 0.1, <0.1 | <0.05, <0.05 | 0.7, 0.7 | <0.1, <0.1 | <0.2, <0.2 | <0.003, <0.003 | 1.1, 1.4 | 0.8, 0.2 | 50, 50 |
| 567, 577, 587, 568, 578, 588 | 14, 8 | <0.01, <0.01 | <0.2, <0.2 | 0.4, 0.6 | <0.01, <0.01 | <0.05, <0.05 | <0.05, <0.05 | 0.2, 0.5 | <0.03, <0.03 | <0.01, <0.01 | 0.1, <0.1 | <0.05, <0.05 | <0.02, <0.02 | <0.1, <0.1 | <0.2, <0.2 | <0.003, <0.003 | 1.1, 1.4 | 0.8, 0.2 | 70, 70 |
Since fused quartz is utilized in applications involving internal pressures, it is sometimes helpful to know the maximum pressure which can be applied to a certain size tube. The following formula will approximate this information at room temperature.
CFQ Rupture Formula For Tubing
S = p*r/t
Where:
S = Hoop Stress in Pa
p = Working pressure (Pa)
r = Inside Radius of Tube (mm)
t = Wall Thickness (mm)
This formula is not applicable when the internal pressure
exceeds 7x10 5 Pa (100 psi).
CFQ Rupture Formula For Discs And Plates
Calculating pressure differential is also required for many applications of stressed fused quartz discs, plates, and sight glasses.
The formula which follows can be used for room temperature applications of circular parts with either clamped or unclamped edges.
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Clamped
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Unclamped
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Where:
p = Pressure differential, PSI
do = Unsupported disc diameter, mm (for plates substitute width)
Smax = Maximum stress (approx. 7 to 1 safety factor) 1000 PSI
t = Disc thickness, mm
However, the following factors will affect the strength of these parts and must be considered when using the formulae:
- Surface should be highly polished and free of scratches.
- Means by which a sample is clamped into a pressure device.
- The gasketing material used.
- The thermal gradients expected across the surface and between the surfaces.
- The rate of pressure increase which will be applied.
- Temperature of specimen.
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Temperature °C
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1450
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1400
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1350
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1300
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1250
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1200
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1100
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1000
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800
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500
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300
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Elements
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Na
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Mg
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Ca
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Ba
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B
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Al
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Ti
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Zr
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V
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Nb
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Ta
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Cr
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Mo
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W
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Mn
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Fe
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Co
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Ni
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Cu
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Ag
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Zn
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Cd
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Hg
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C
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Si
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Sn
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Pb
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As
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Sb
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S
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Si
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Ir
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Oxides
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H2O
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MgO
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CaO
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Al2O3
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SiO2
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P2O5
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MoO3
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WO3
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ThO2
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Hydroxides
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Sn(OH)2
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Ba(OH)2
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Carbonates
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CaCO3
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BaCO3
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Halides
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LiCl
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NaCl
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KCl
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RbCl
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CsCl
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NaBn
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KBn
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Nal
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Kl
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MgCl2
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CaCl2
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SnCl2
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BaCl2
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AlCl3
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Legend:
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| Permeation Time (@ 1000C) | |||||||||
| Ion | Diffusion Coefficient D(CM2/sec) | Covalent Radius A | Heat of Vaporization | Thickness or Quartz | |||||
| 5mm | 3mm | 2mm | 1mm | 0.5mm | 0.1mm | ||||
| aN | 7.00E-06 | 1.54 | 97.6 | 20 min | 7.2 min | 3.2 min | 48 sec | 12 sec | 0.5 sec |
| Li | 1.00E-06 | 1.23 | 145.9 | 2.6 hrs | 57 min | 25 min | 6.3 min | 1.6 min | 3.8 sec |
| Ag | 7.00E-07 | 1.34 | 250.9 | 3.7 hrs | 1.4 hrs | 36.2 min | 9 min | 2.3 min | 5.4 sec |
| Ca | 2.00E-08 | 1.64 | 153.6 | 5.5 days | 1.98 days | 21 hrs | 5.3 hrs | 1.3 hrs | 3.2 min |
| K | 1.00E-08 | 2.03 | 79.9 | 11 days | 3.96 days | 42.2 hrs | 10.6 hrs | 2.6 hrs | 6.3 min |
| Al | 1.00E-13 | 1.18 | 293.4 | 3010 years | 1084 years | 482 years | 120 years | 30 years | 1.3 years |
| P | 8.00E-14 | 1.06 | 12.12 | 3763 years | 1355 years | 602 years | 151 years | 38 years | 1.5 years |
| Ni | 1.00E-15 | 1.15 | 370.4 | 300K years | 110K years | 48K years | 12K years | 3K years | 120 years |
| Au | 5.00E-16 | 1.34 | 334.4 | 600K years | 220K years | 96K years | 24K years | 6K years | 241 years |
| As | 1.00E-16 | 1.2 | 34.7 | 3000K years | 1100K years | 480K years | 120K years | 30K years | 1204 years |
| B | 1.00E-17 | .82 | 489.7 | 11000K years | 11000K years | 4800K years | 1200K years | 300K years | 12K years |
Mechanical properties of fused quartz are much the same as those of other glasses. The material is extremely strong in compression, with design compressive strength of better than 1.1 x 109 Pa (160,000 psi).
However, surface flaws can drastically reduce the inherent strength of any glass, so tensile properties are greatly influenced by these defects. The design tensile strength for fused quartz with good surface quality is in excess of 4.8 x 107 Pa (7,000 psi). Taking into consideration safety features and fatigue, the common practice is to use 6.8 x 106Pa (1000 psi).
However, surface flaws can drastically reduce the inherent strength of any glass, so tensile properties are greatly influenced by these defects. design for fused quartz with good quality is in excess 4.8 x 107 Pa (7,000 psi). Taking into consideration safety features and fatigue, the common practice is to use 6.8 x 106Pa (1000 psi).
Since electrical conductivity in fused quartz is ionic in nature, and alkali ions exist only as trace constituents, fused quartz is the preferred glass for electrical insulation and low loss dielectric properties. In general, the electrical insulating properties of clear fused quartz are superior to those of the opaque or translucent types. Both electrical insulation and microwave transmission properties are retained at very high temperatures and over a wide range of frequencies.
Typical electrical property values for clear fused quartz include:
Electrical Resistance: 0.7 x 109 ohm-cm at 350oC
Dielectric Loss Factor: Less than 0.0004 at 20oC, 1 MHz
Dielectric Constant: 3.75 at 20oC, 1 MHz
Specific Resistivity: l018 ohm/cm3 at 20oC
Dissipation Factor: Less than 0.0001 at 20oC. 1 MHz
Optical transmission properties provide a means for distinguishing among various types of vitreous silica as the degree of transparency reflects material purity and the method of manufacture. Specific indicators are the UV cutoff and the presence or absence of bands at 245 nm and 2.73 micrometers. The UV cutoff ranges from about 155 to 175 nm for a 10 mm thick specimen and for pure fused quartz is a reflection of material purity. The presence of transition metallic impurities will shift the cutoff toward longer wavelengths. When desired, intentional doping, e.g., with Ti in the case of Type 219, may be employed to increase absorption in the UV. The absorption band at 245 nm characterizes a reduced glass and typifies material made by electric fusion. If a vitreous silica is formed by a "wet" process, either flame fusion or synthetic material, for example, the fundamental vibrational band of incorporated structural hydroxyl ions will absorb strongly at 2.73 micrometers.
UV Cutoff
As the transmission curve illustrates, GE Type 214 fused quartz has a UV cutoff
(1 mm thickness) at < 160 nm, a small absorption at 245 nm and no appreciable
absorption due to hydroxyl ions. Type 219, which contains approximately 100
ppm Ti, has a UV cutoff at about 230 nm for a 1 mm thick sample.
High IR Transmission
The IR edge falls between 4.5 and 5.0 micrometers for a 1 mm thick sample. Type
(214/124 electrically) fused quartz is a very efficient material for the transmission
of infrared radiation. Its infrared transmission extends out to about 4 micrometers
with little absorption in the "water band" at 2.73 micrometers. This makes GE
electrically fused quartz different than flame fused quartz (often referred
to as "wet" quartz). This difference is seen in the transmission for the IR
range. The IR Transmission figure illustrates this difference. Conversion to
other thicknesses can be accomplished with the following formula:
T = (1-R)2 e -at
Where:
T = percent transmission expressed as a decimal.
R = surface reflection loss for one surface.
e = base of natural logarithms
a = absorption coefficient, cm-1
t = thickness, cm
Annealing of Fused Quartz
When quartz is flame worked, the glass worker may induce thermal stress
in the piece. As in metals and other vitreous (glassy) materials, this thermal
stress is relieved by annealing. The principles of annealing is simple, but
can easily be misunderstood resulting in possible breakage of parts during use.
Before you can understand the principles of annealing, you need to understand
the some common terms used to describe the thermal properties of glass. Details
for the principles of Annealing Quartz are covered in the Annealing of Fused
Quartz PDF to the right.
Effects Of Temperature
Fused quartz is a solid material at room temperature, but at high temperatures,
it behaves like all glasses. It does not experience a distinct melting point
as crystalline materials do, but softens over a fairly broad temperature range.
This transition from a solid to a plastic-like behavior, called the transformation
range, is distinguished by a continuous change in viscosity with temperature.
Viscosity
Viscosity is the measure of the resistance to flow of a material when exposed
to a shear stress. Since the range in "flowability" is extremely wide, the viscosity
scale is generally expressed logarithmically. Common glass terms for expressing
viscosity include: strain point, annealing point, and softening point, which
are defined as: Strain Point: The temperature at which the internal stress is
substantially relieved in four hours. This corresponds to a viscosity of 1014.5
poise, where poise = dynes/cm2 sec. Annealing Point: The temperature at which
the internal stress is substantially relieved in 15 minutes, a viscosity of
1013.2 poise. Softening Point: The temperature at which glass will deform under
its own weight, a viscosity of approximately 107.6 poise. The softening point
of fused quartz has been variously reported from 1500°C to 1670 °C, the range
resulting from differing conditions of measurement.
Devitrification
Devitrification and particle generation are limiting factors in the high temperature
performance of fused quartz. Devitrification is a two step process of nucleation
and growth. In general, the devitrification rate of fused quartz is slow for
two reasons: the nucleation of the cristobalite phase is possible only at the
free surface, and the growth rate of the crystalline phase is low. Nucleation
in fused quartz materials is generally initiated by surface contamination from
alkali elements and other metals. This heterogeneous nucleation is slower in
non stoichiometric fused quartz, such as GE quartz, than in stoichiometric quartz
materials.
Cristobalite Growth
The growth rate of cristobalite from the nucleation site depends on certain
environmental factors and material characteristics. Temperature and quartz viscosity
are the most significant factors, but oxygen and water vapor partial pressures
also impact the crystal growth rate. Consequently, the rate of devitrification
of fused quartz increases with increasing hydroxyl (OH)- content, decreasing
viscosity and increasing temperature. High viscosity, low hydroxyl fused quartz
materials produced by GE Quartz, therefore, provide an advantage in devitrification
resistance. The phase transformation to Beta-cristobalite generally does not
occur below 1000°C. This transformation can be detrimental to the structural
integrity of fused quartz if it is thermally cycled through the crystallographic
inversion temperature range (250 °C). This inversion is accompanied by a large
change in density and can result in spalling and possible mechanical failure.
An Advantage
In certain applications, devitrification can be put to the user's advantage
since the cristobalite tends to inhibit sag of the fused quartz. For example,
if a diffusion furnace tube is to be used at high temperatures for extended
periods of time, and is not subject to thermal cycling below the beta to alpha
cristobalite transformation, rotation procedures have been found to be beneficial.
Contamination
Contamination in almost any form is detrimental. Alkaline solutions, salts,
or vapors are particularly deleterious. Handling of fused quartz with the bare
hands deposits sufficient alkali from perspiration to leave clearly defined
fingerprints upon devitrification. Drops of water allowed to stand on the surface
will collect enough contamination from the air to promote devitrified spots
and water marks. Surface contamination affects devitrification in two ways.
First, the contaminant promotes nucleation of the cristobalite. Second, it acts
as a flux to enhance the cristobalite to beta (high) tridymite transformation.
Under some conditions, the tridymite devitrification will grow deeply and rapidly
into the interior of the fused quartz. Heating fused quartz to elevated temperatures
(ca. 2000 °C) causes the SiO2 to undergo dissociation or sublimation. This is
generally considered to be: SiO2 -> SiO + 1/2 O2. Consequently, when flame-working
fused quartz, there is a band of haze or smoke which forms just outside the
intensely heated region. This haze presumably forms because the SiO recombines
with oxygen from the air (and perhaps water) and condenses as extremely small
particles of amorphous SiO2. The haze can be removed from the surface by a gentle
heating in the oxy-hydrogen flame. The dissociation is greatly enhanced when
the heating of fused quartz is carried out in reducing conditions. For example,
the proximity or contact with graphite during heating will cause rapid dissociation
of the SiO2.
Resistance To Sag
The most significant chemical factor effecting the sag resistance of fused quartz
is the hydroxyl (OH)- content. GE controls the (OH)- content in its quartz to
meet the specific needs of its customers. To maximize the performance of tubes
used in high temperature semiconductor processes, it is important to understand
the impact of changes in diameter and wall thickness. In one study using GE
214LD fused quartz tubing, it was found that the sag rate decreases as the wall
thickness of the tube is increased. Generally, as the wall thickness doubles,
the sag rate decreases by a factor of approximately 3. Also, it was shown that
with a fixed wall thickness, the sag rate decreases as the tube diameter decreases.
Cristobalite Thickness/Time Chart
Diffusion Tubing, Collapse vs. Time For Tube ID Chart
Diffusion Tubing, Collapse vs. Time For Wall Thickness Chart
