What is Polysilicon
Polysilicon (or polycrystalline silicon or poly-Si) is the unique and unchangeable raw material for all semiconductor and photovoltaic (solar energy conversion) processes and it consists of multiple small silicon crystals.
Being the raw material and key component of the photovoltaic and also of the integrated circuit / semiconductor industry, for both industries a high purity level is necessary:
- The semiconductor industry needs silicon with 9N to 11N purity level (the “N” indicates the numbers of 9s behind the dot, example: 8N means 99.999999%);
- For the photovoltaic industry 7N to 8N is sufficient.
26 % of the upper crust of our planet consists of silicon: it appears most common as its oxide (SiO2) known as quartz. The best known form of quartz is sand.
How is Polysilicon produced?
The original and traditional production process had been set up in the beginning of 50’s by Siemens and it is still called “Siemens C process”. Along the years this process had been utilized by all main WW Producers as well as fully optimized for quality and manufacturing cost.
This section is devoted to the technical, scientific and technological description and explication of the Siemens Poly Silicon production process, i.e. the production of Poly Silicon chunks with 7/8N purity starting from the raw materials through deposition.
The traditional Poly Silicon production process can be, on a macroscopic level, broken down into six major process steps (shematized in fig.1):
- Trichlorosilane (TCS) synthesis
- TCS and H2 purification
- Poly Silicon deposition
- Off- gas recovery
 figure1. Major process steps of the full Siemens type Polysilicon
At the end of the batch the poly rods are downloaded from the reactor by taking care to avoid silicon contamination. Poly rod is a form that cannot be utilized in the semiconductor and solar field. For that reason the rods are crushed and transformed in “chunks or nuggets” more suitable for the following use.
After the rods are grown, the reactor has to be evacuated and then the rods can be downloaded from the reactor for further treatment in clean environments.
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