Quartz Crucible Performance: Comparing with Alumina, Zirconia, & Graphite

In high temperature applications, selecting the right crucible material is crucial to ensure process efficiency and product quality. Quartz crucible is widely praised for its excellent thermal shock resistance and high purity, but in order to fully understand its performance, it is necessary to compare it with other mainstream crucible materials such as alumina, zirconia and graphite. The advantages and limitations of these materials will be discussed in detail in order to provide guidance for material selection in specific application scenarios.

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Comparison: Alumina Crucible vs. Quartz Crucible

Alumina crucibles are known for their excellent thermal stability and are particularly suitable for applications that require resistance to chemical reactions. Its high thermal stability ensures long-term stable operation under extreme temperature conditions. However, compared to quartz, alumina crucibles are slightly less resistant to thermal shock, which limits their use in processes involving rapid temperature changes. In addition, alumina can introduce impurities at high temperatures, which can be a major drawback for applications where purity is very high.

Zirconia Crucible: Benefits and Challenges in High-Temp Applications

Zirconia crucible, with its extremely high melting point and excellent durability, is ideal for extreme high temperature conditions. Its excellent high temperature resistance ensures long-term reliability of the material in high temperature environments. However, zirconia crucibles typically cost more than quartz crucibles and, in some analytical applications, are less transparent than quartz, which limits their applicability in applications where internal reaction processes need to be observed. In addition, zirconia may react with certain materials, resulting in contamination problems, which require special attention when used.

Graphite Crucible: Application Scope and Limitations

Because of its high thermal conductivity and ability to withstand extreme temperatures, graphite crucible has been widely used in specific fields such as metal melting. Its high thermal conductivity helps to heat the material quickly and evenly, improving the melting efficiency. However, the interaction between graphite and molten metal can lead to contamination problems, which is unacceptable in sensitive applications. Therefore, although graphite performs well in some specific scenarios, for applications requiring a high purity and inert environment, quartz crucible is more popular due to its purer and inert properties, becoming the first choice of many researchers and manufacturers.

In summary, quartz crucible, alumina crucible, zirconia crucible and graphite crucible each have their own unique performance advantages and application scenarios. When selecting crucible materials, factors such as process requirements, cost effectiveness, material purity and potential contamination risk should be considered comprehensively. Because of its excellent thermal shock resistance, high purity and inertness, quartz crucible performs well in environments requiring high purity and sensitive applications, making it the material of choice in many fields. However, for specific application scenarios, such as extreme high temperature conditions or cost-sensitive projects, other crucible materials may also be a suitable choice. Therefore, in practical applications, the most suitable crucible material should be selected according to the specific needs of the trade-offs.

It is well known that an important part of Cz growth is the extraction of semiconductor single crystals from a melt contained in a high temperature crucible. Why choose quartz for crucible material? Only a simple analysis is done here.

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Czochralski silicon does not use solvents, that is, the melt is composed of the same elements as the growing crystal. For silicon, the melt is almost pure elemental silicon, around °C. Both the crucible and the crystal are rotated, and the crystal is slowly pulled upwards, resulting in a cylinder of the desired diameter. However, when a transverse magnetic field is utilized, the rotation of the crucible is usually close to zero.

Molten silicon reacts with almost all known materials, so that there are very few potential crucible materials. The only crucible material available for high quality crystals is silica in the amorphous state. Looking at the periodic table, most elements, even in trace amounts, can have a detrimental effect on the quality of the silicon material, which results in a narrow range of choices. Almost all metals were excluded because the allowable concentrations were only a few ppt concentrations or less. Groups III and V elements are electroactive dopants and can generally tolerate higher levels, typically in the several ppba range. But this concentration is also too low to make crucibles from compounds of these elements.

Ceramic materials are also excluded, either because they contain one of the above elements or because they contain other elements in very low concentrations. Nitrogen (eg, derived from silicon nitride crucibles) is poorly soluble in crystals, and growing crystals tend to strongly repel it, and the same applies to carbon. The concentration of these elements in the melt is highest near the crystallographic interface, and when the solubility is close to saturation, there will be nucleation of small particles, destroying the single crystal structure of the growing crystal. Furthermore, while nitrogen is sometimes intentionally introduced into materials, neither nitrogen nor small amounts of carbon, even close to the level of its solubility, are allowed.

Fortunately, the situation with oxygen is different. Oxygen in crystals can tolerate considerable concentrations, typically in the part-hundred-thousandth of an atom (ppma) range; in most silicon wafer applications, oxygen is a required element and controlling oxygen levels has clear beneficial effects . Furthermore, oxygen is not easily repelled by the crystals; that is, its segregation coefficient is close to 1, and there is no risk of oxide particles being generated near the crystallographic interface. In addition to this, in contrast to carbides and nitrides, silicon oxides are volatile: in an oxygen-deficient environment at high temperatures, silicon tends to form silicon monoxide rather than silicon dioxide.

Silicon monoxide is easy to volatilize, and the vapor pressure is about 12 mbar at the temperature of silicon melt. In fact, most (98-99%) of the growth produced by the reaction between the highly active silicon melt and the quartz crucible wall will evaporate into the growth atmosphere. and is forcibly carried away by the flowing inert gas. Typically, only 1-2% of the dissolved oxygen eventually grows into the crystal, but process conditions, such as the use of a magnetic field and the size of the crystal relative to the crucible, may change this ratio. The silicon monoxide produced by the reaction flows with the inert gas and is the main cause of harmful reactions at the thermal interface between the crucible and the melt. The flow design of the gas must take into account the adverse effects caused by oxides, especially in today’s large furnaces, where the amount of oxygen released into the gas during a single growth process can be in the range of hundreds of grams.

Of course, quartz crucible is a material that is easily deformed by heat, and it is difficult to use alone. At present, Czochralski monocrystalline silicon and graphite crucible are used together, and the outer graphite crucible plays the role of support and heat transfer.