Sunday, 14 April 2013

Crystal Pulling (Part-2)

Crystal Pulling: Crystal Pulling is the next step in the Manufacturing of a Silicon wafer.  In this process the Polysilicon chunks or granules are loaded into the Quartz crucible of the Crystal pulling furnace along with a small amount of Boron, Phosphorus, Arsenic or Antimony dopant. The Polysilicon is then melted at a process temperature of 1400° C in a high purity Argon gas ambient. Once the proper "melt” is achieved a "seed" of single crystal Silicon is lowered into the melt. Then the temperature is adjusted and the seed is rotated as it is slowly pulled out of the molten Silicon. The surface tension between the seed and the molten Silicon causes a small amount to rise with the seed, as it is pulled and cooled into a perfect mono-crystalline ingot with the same crystal orientation as the seed. The following view is of the lower portion of the crystal pulling furnace, these machines can stand 12-15 feet high and are quite complex in nature. These furnaces also must be very stable and vibration free since the process takes hours and the slightest jarring of the furnace can break the Ingot from the seed. 
                                                     CZ Crystal Puller Furnace

                          An actual picture of a "Seed" and Ingot inside of a crystal pulling furnace.

Next the finished Ingot is ground to a rough size diameter (a little larger than a finished wafer) and it is either notched or flatted along its length to indicate the orientation of the Ingot. This is also a point when many inspections are made on the ingot to catch any major flaws or problems with resistivity etc.

                                                              Silicon Crystal or Ingot

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