Polysilicon Production
title: "Polysilicon Production" description: "How metallurgical silicon is refined into ultra-pure polysilicon via modified Siemens and FBR processes, and the distinction between solar-grade and semiconductor-grade material." section: "upstream"
From Metallurgical to Electronic Purity
Metallurgical silicon at 98.5–99.5% purity must be refined by several orders of magnitude to serve solar cells and semiconductor devices. The key process converts solid silicon into a gaseous chlorosilane intermediate, purifies by fractional distillation, then reduces the gas back to solid silicon — eliminating metallic, carbon, and dopant impurities along the way.
The two dominant commercial routes today are the Modified Siemens Process and Fluidized Bed Reactor (FBR).
Modified Siemens Process
The industry workhorse, accounting for roughly 80% of global polysilicon output:
- Hydrochlorination — metallurgical silicon reacts with HCl at 300°C: Si + 3HCl → SiHCl₃ (trichlorosilane, TCS)
- Distillation — TCS is purified by multi-stage fractional distillation to remove boron, phosphorus, and metal chlorides
- CVD deposition — purified TCS decomposes on slim-rod silicon seeds at 1100°C in a bell-jar reactor: SiHCl₃ + H₂ → Si + 3HCl
- Rod harvesting — polysilicon rods (15–20 cm diameter) are broken into chunks and classified by size
Energy intensity is high — typically 50–70 kWh/kg — but the process produces the highest-purity material (11N for semiconductor grade).
Fluidized Bed Reactor (FBR)
FBR produces granular polysilicon (1–3 mm beads) by decomposing silane (SiH₄) or TCS on silicon seed particles fluidised at 700–900°C. Advantages: continuous operation, lower energy use (~20 kWh/kg), easier handling for Czochralski pulling. Disadvantage: higher metallic contamination surface area; FBR granules are blended with Siemens chunks for semiconductor-grade ingots.
Solar-Grade vs Semiconductor-Grade
| Parameter | Solar-grade (UMG/FBR blend) | Semiconductor-grade (Siemens) |
|---|---|---|
| Purity | 6N–9N (99.9999–99.9999999%) | 9N–11N (≥99.9999999%) |
| Boron (ppba) | <1 | <0.01 |
| Phosphorus (ppba) | <1 | <0.05 |
| Carbon (ppma) | <1 | <0.2 |
| Bulk resistivity | 1–300 Ω·cm | >300 Ω·cm |
| Primary use | PV cells, modules | IC wafers, power devices |
Solar-grade now dominates volume; semiconductor-grade commands a significant price premium.
Industry Context
Global polysilicon capacity is overwhelmingly concentrated in China (>90% by 2024), particularly in Xinjiang, Inner Mongolia, and Sichuan. This concentration has prompted supply-security concerns in European and US PV supply chains, driving investment in alternative production sites.
See the Photovoltaic industry page for how polysilicon feeds into the solar module supply chain, and Semiconductor for the IC wafer pathway.