Electronic Devices and Circuits Engineering Sciences 154

Purification:

See slide on Zone Refining

Crystal Growth:

References:

The Science and Art of Si Crystal Growth
Silicon Crystal Growth and Wafer Production
Crystal Growth from CAESAR

Czochralski (CZ) single silicon crystal growth

"The most common technique used for growing crystals for the development of wafers is the Czochralski growth.  There are other methods of growing crystals but the Cz is most common. The material used in growing a single crystal silicon ingot is electronic grade silicon(EGS), which is refined from MGS and must have 99.999999999% purity.

"When the polysilicon,defined as containing many crystals, is transported to the wafer fabrication station or wafer Fab., it is placed in a fused quartz crucible that would dissolve during the crystal growing process. After the polysilicon is loaded in the crucible, it is then
placed in an evacuated chamber that would be filled with an inert gas such as Argon, when sealed. While the inert gas is released in the chamber, the crucible is heated to about 1500°C causing the EGS material to melt. As the polysilicon is melting, a seed crystal about .5cm in diameter and 10cm in length is placed in a rotating shaft. The seed crystal would then be rotated in the molten silicon to develop a silicon ingot or "boule."

"When the polysilicon(EGS) is melted to a right consistancy, the seed crystal is then lowered into the melted material and the tip just penetrates below the surface of the molten silicon. The shaft is then rotated counterclockwise and the crucible is rotated clockwise.  As the shaft is rotating, the crystal seed is slowly pulled away from the molten, developing an ingot. The size of the ingots are usually 1 to 2 meters long and can achieve a maximum diameter of 200mm. By carefully controlling the temperatures and rotating speeds of the crucible and rod, that a precise diameter of the crystal is maintained. While the ingots are pulled, it is cooled to form it into a solid state. The length of the ingot is determined by the amount of molten silicon there is in the crucible.

"During crystal growing, an amount of dopant is added for the desired charge it is needed for a device. As the silicon is melted, a specific amount of dopant is added, depending if a P or N device is needed. In the CZ method, the impurities are usually added and dissolved in the melt using solid impurities. For donors, elements such as phophorous and arsenic is added.  For acceptors, boron is usually the additive. By adding these dopants, they increase the concentration for mobile carriers and that increases the conductivity of the device."  (source)

(source)
(source)

Floating Zone (FZ) single silicon crystal growth