Typically annealing of the grown crystal is carried at temperatures above 500°C and to remove these oxygen complexes. The oxygen can precipitate under normal device processing conditions, forming SiO2 regions inside the wafer. The precipitation arises because the oxygen was incorporated at the melt temperature.
Carbon is another impurity normally present in CZ grown silicon crystals. The carbon comes from the graphite components in the crystal pulling machine. For some applications, it is important to have even lower concentrations of impurity atoms like Oxygen and the Float Zone Crystal Growth is used.
Float Zone Crystal Growth
The basic idea in float zone (FZ) crystal growth is to move a liquid zone through the material as shown in Fig.3.11. In this process the end of a long polysilicon rod is locally melted and brought in contact with a single crystalline Silicon seed. The melted zone slowly migrates through the poly rod leaving behind the final perfect crystal. In the float zone process, dopants and other impurities are rejected by the growing silicon crystal. Impurities tend to stay in the liquid and refining can be accomplished, especially with multiple passes. Since the melt never comes into contact with anything but vacuum (or inert gases), there is no incorporation of impurities by dissolving the crucible material as in the CZ crystal growth method. FZ crystals therefore are always used when very low oxygen concentrations are required. One needs to keep the liquid Si from just collapsing by surface tension and this limit the maximum diameter of crystals. A summary of the CZ and FZ growth methods are given in Table 3.