Crystal Pulling

EGS is placed in a rotating quartz crucible and melted at 1415ºC. A rotating seed crystal is slowly punched into the molten silicon, then slowly pulled out to obtain an ultra-pure, single-crystal silicon ingot. Single-crystal silicon wafers are obtained by sawing the round ingot into slices. Then wafers are lapped, cleaned, etched, polished, labeled, and shipped to IC chip manufacturers.

There are two methods commonly used in the semiconductor industry to generate single-crystal silicon, the Czochralski (CZ) method and the floating zone (FZ) method. The CZ method is cheaper and can produce larger size wafers whereas the FZ method can produce wafers with high purity.

CZ Method

· The majority of silicon wafers used in IC processing are made from the CZ method (Fig. 7). The process takes place in a sealed chamber with argon ambient to control contamination.

· In the CZ method, the high-purity EGS is melted in a slowly rotating quartz crucible at 1415ºC, just above the silicon melting point of 1414ºC, by radio frequency (RF) or resistive heating coils.

· A single-crystal silicon seed rod mounted on a slowly rotating chuck is gradually lowered into the molten silicon, and the surface of the seed crystal submerges in the molten silicon and starts to melt.

· The seed crystal temperature is precisely controlled at just below the silicon melting point. When system reaches thermal stability, the seed crystal is withdrawn very slowly, dragging some molten silicon to recondense around it with the same crystal orientation.

· The ingot, a whole piece of single-crystal silicon is formed after up to 48 hours of pulling.

· The diameter of the crystal in the CZ method can be controlled by the temperature and pulling rate.

· Single-crystal silicon ingots pulled by the CZ method have trace impurities of oxygen and carbon, which come from the crucible materials.

Fig. 7 Schematic of the Czochralski (CZ) crystal-pulling method [3]