Achieving 11N Purity: Trichlorosilane and the Siemens Process

01Introduction: The 11N Requirement

In the realm of semiconductor manufacturing, "pure" has an entirely different meaning than in traditional chemistry. While $99.9\%$ purity is considered exceptionally pure for commercial chemicals, an integrated circuit built on such material would fail instantly. At sub-2nm transistor nodes, a single metallic impurity atom per billion silicon atoms can cause gate oxide breakdown or severe current leakage.

Semiconductor-grade silicon requires a purity of 99.999999999%, universally referred to as 11N purity (eleven nines). This means that for every 100 billion silicon atoms, there is at most one non-silicon impurity atom.

02Step 1: Synthesis of Volatile Trichlorosilane (TCS)

Solid Metallurgical-Grade Silicon ($\text{MG-Si}$) cannot be purified directly by thermal melting alone because metallic impurities like iron, titanium, and boron remain trapped in the liquid lattice. To separate silicon from its impurities, solid $\text{MG-Si}$ is converted into a volatile liquid compound called trichlorosilane (${\text{HSiCl}}_3$, or TCS).

Inside a Fluidized Bed Reactor (FBR) operating at $300^{\circ }\text{C}-350^{\circ }\text{C}$, crushed $\text{MG-Si}$ powder reacts with anhydrous hydrogen chloride ($\text{HCl}$) gas:

Trichlorosilane is an ideal chemical vehicle because it has a relatively low boiling point of $31.8^{\circ }\text{C}$ at atmospheric pressure, making it easy to vaporize, condense, and purify via fractional distillation.

03Step 2: Fractional Distillation

Once TCS is synthesized, it passes through a series of tall distillation columns. Fractional distillation exploits the differing boiling points of TCS and its impurity chlorides:

By repeating distillation through multiple columns in series, impurity concentrations are suppressed below parts-per-trillion ($\text{ppt}$) levels.

04Step 3: Siemens Chemical Vapor Deposition (CVD)

The ultra-pure TCS liquid is vaporized, mixed with high-purity hydrogen gas (${\text{H}}_2$), and injected into a sealed Siemens CVD Reactor. Inside the reactor, slim filament rods of pure silicon are electrically heated to $1000^{\circ }\text{C}-1100^{\circ }\text{C}$.

When the gas mixture contacts the red-hot silicon filaments, TCS undergoes chemical reduction, depositing pure elemental silicon onto the filaments:

Over several days, the slim filaments grow into thick, dense rods of high-purity Electronic-Grade Polysilicon (EGS) measuring up to $20\text{ cm}$ in diameter and $2\text{ meters}$ in length.

05Key Takeaways & Summary

1. 11N Standard: Semiconductor fabrication demands $99.999999999\%$ pure silicon to prevent microelectronic defects.
2. Volatile Vehicle: MG-Si is reacted with $\text{HCl}$ gas to form trichlorosilane (${\text{HSiCl}}_3$), a volatile liquid with a boiling point of $31.8^{\circ }\text{C}$.
3. Distillation: Fractional distillation separates TCS from metal chlorides (${\text{FeCl}}_3,{\text{AlCl}}_3$) and dopants (${\text{BCl}}_3,{\text{PCl}}_3$).
4. Siemens CVD: Purified TCS gas decomposes on heated silicon filaments at $1100^{\circ }\text{C}$ to yield ultra-pure EGS polysilicon rods.

→Prerequisites & continue reading

Prerequisites: Understanding of MG-Si production (Topic 1.1) and basic chemical distillation

Also in Topic 1.2, Chemical Purification & the Siemens Process:

Tier 2 · Fractional Distillation Columns and Siemens CVD Reac…Tier 3 · CVD Deposition Kinetics and Mass-Transfer Dynamics i…