Crystal Growth

The first, and most critical step in the manufacturing of silicon wafers, is the "growth" of single crystal silicon. To begin, the raw material "polysilicon" is carefully stacked by hand inside a quartz crucible, which in turn, rests inside the furnace tank of a Crystal Puller. A small amount of "dopant" (electrically active elements) such as arsenic, boron, phosphorous or antimony is added to the polysilicon. This dopant predetermines the electrical properties, or resistivity of the wafers sliced from the crystal. The machine is tightly sealed, purged to eliminate gas impurities, then fired with a "recipe" for that particular customer specification.

As the furnace heater ramps to temperature, the crucible begins clockwise rotation. Once the melt has reached the desired temperature, a counter-clockwise, rotating silicon "seed" crystal is lowered into the molten polysilicon. The melt is slowly cooled to the recipe's temperature as crystal growth begins around the seed. The seed is then slowly raised or "pulled" from the melt - giving the appearance of controlled freezing. The temperature of the melt and the speed of extraction govern the diameter of the crystal. Once the desired diameter is achieved, the crystal-puller's advanced control systems maintain the diameter throughout the growth stage. A gradual and tapered cone finishes the crystal growing cycle in order to ensure the crystal's structural integrity. The crystal is allowed to cool before it is extracted from the crystal-puller for further processing.

 

 

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