Polycrystalline silicon (poly, polysilicon) is multi-crystalline, high purity silicon, greater than 99.99999% pure, used as the starting material for the growth of high purity single crystal silicon to be used for integrated circuits, other semiconductor devices, and solar cells.
The traditional process for producing polysilicon is to react metallurgical grade silicon, (MGSi), at 98% purity, with hydrogen chloride, HCl, to produce trichlorosilane, SiHCl3. This trichlorosilane is then purified and passed into large chemical vapor deposition reactors, where it is decomposed to form high purity polysilicon in the form of large rods. The large rods are then broken into chunks of specific sizes, packaged in clean rooms, and shipped to the users.
The process requires expert knowledge of chlorosilane chemistry, experienced handling of hazardous gases at high temperatures and pressures, comprehensive safety programs, state-of-the-art chemical and electrical evaluation techniques, expert knowledge of vapor phase deposition technology, and operators experienced in chemical operations, high voltage electricity, high purity gas distribution systems, and mechanical systems for corrosive gases.
A supplier may classify the polysilicon produced into 50 or more different grades with different specifications specific to the user and the application. In general, the supplier may offer:
- Float Zone Grade, poly rods designed for very high purity single crystal float zone growth of silicon ingots for semiconductor power devices
- Electronic Grade (Prime Grade, VLSI Grade, Virgin Poly, Semiconductor Grade), poly chunks designed for high purity single crystal Czochralski growth of silicon ingots for advanced IC applications
- IC Grade or Standard Grade, poly chunks designed for single crystal Czochralski growth for standard, less advanced IC applications
- Secondary or Test Wafer Grade, poly chunks with lower purity values for Czochralski growth of silicon test wafers or solar cell applications
- Solar Grade or Casting Grade, poly chunks and rod sections with lower purity values for silicon cast crystal growth for solar cell applications
- Reject or Scrap Grade, poly chunks, rod sections, small chips, with lower purity values often sold to the recycling/recovery companies
Within each product grade are detailed specifications for rod size (diameter, length), chunk size (large, medium, small), acceptor/donor/carbon purity, bulk and surface metals purity, surface texture, and appearance. Sales prices are based on industry demand, product quality, and cost of production.
When supply and demand have been in balance, prices range from ~$100/kg for high purity fabricated Float Zone poly rods to ~$ 5/kg for waste/scrap polysilicon. In general, polysilicon products are identified by form (rod, chunks, chips) and purity. Purity specifications for solar-grade (SOG) polysilicon are much lower than for Electronic Grade (EG) poly. Table 1 lists general purity specifications for SOG and EG poly.
User A, B, and C are summaries from dozens of user specifications for solar cell applications. Standard Cz and Advanced IC are summaries from dozens of user specifications for Electronic Grade polysilicon. Acceptor elements, primarily boron, and donor elements, primarily phosphorus, are measured in parts per billion, atomic. Carbon and oxygen are measured in parts per million, atomic. Bulk metals and surface metals are measured in parts per billion, by weight.
Most solar cells are fabricated on silicon wafers that have been cut from ingots at about 0.1 ohm-cm p-type, so the polysilicon can have relatively large amounts of boron, compared to EG poly. An important difference between SOG and EG poly is the metals content. Reduction of metals content increases production costs. Metals content is not usually specified for SOG.