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.
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 1420°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.