CVD Deposition Kinetics and Mass-Transfer Dynamics in EGS Production

01Gas-Phase Thermodynamics and Reaction Pathways

The deposition of silicon from trichlorosilane (${\text{HSiCl}}_3$, TCS) in a Siemens CVD reactor involves a coupled network of homogeneous gas-phase reactions and heterogeneous surface reactions.

Primary Reaction Mechanisms

1. Homogeneous Pyrolysis in Gas Phase:

Dichlorosilylene (${\text{SiCl}}_2$) is the primary reactive intermediate generated in the gas phase adjacent to the hot rod surface.

1. Heterogeneous Reduction at Rod Surface:

1. Competing Etch Back Reaction (Hydrogen Chloride Attack):

At high temperatures ($>1150^{\circ }\text{C}$) or elevated $\text{HCl}$ partial pressures, the reverse etch reaction accelerates, consuming deposited silicon and lowering net growth rates.

02Boundary Layer Mass Transfer and Growth Rate Equations

The linear growth rate (G, in $\mu \text{m/min}$) of the polysilicon rod is governed by the interplay between surface reaction kinetics ($k_s$) and boundary layer mass transport ($k_m$).

The overall deposition flux ($J_{\text{Si}}$, in ${\text{mol/m}}^2\cdot \text{s}$) is expressed by:

Where:

• $C_{\text{bulk, TCS}}$ is the concentration of TCS in the bulk gas flow.
• $C_{\text{eq, TCS}}$ is the thermodynamic equilibrium concentration at the rod surface.
• $k_m=\frac{D_{AB}}{{\delta }_{\text{BL}}}$ is the mass transfer coefficient ($D_{AB}$ is binary diffusivity, ${\delta }_{\text{BL}}$ is boundary layer thickness).
• $k_s=A_0\exp \left(-\frac{E_a}{RT}\right)$ is the surface reaction rate constant ($E_a\approx 110-140\text{ kJ/mol}$).

Temperature Regime Operating Windows

• Surface-Reaction Limited ($T<1000^{\circ }\text{C}$): Growth rate is strongly exponentially dependent on temperature. Yield is low, but surface morphology is exceptionally smooth and uniform.
• Mass-Transport Limited ($T>1150^{\circ }\text{C}$): Growth is constrained by species diffusion across the stagnant boundary layer. Unstable dendrite/popcorn morphology forms due to gas-phase nucleation.
• Optimal Siemens Operating Window ($1050^{\circ }\text{C}-1100^{\circ }\text{C}$): Operates in a balanced transition regime, maximizing growth rate ($1.0-2.0\text{ mm/hour}$) while maintaining high rod density and structural integrity.

03Impurity Segregation Thermodynamics in CVD Deposition

Although TCS feedstock is purified to 11N standards, trace levels of phosphorus ($\text{P}$), boron ($\text{B}$), arsenic ($\text{As}$), and carbon ($\text{C}$) remain in the gas feed. The degree to which an impurity incorporates into the growing silicon lattice during CVD is dictated by its gas-to-solid segregation coefficient ($k_{\text{CVD}}$):

04Morphological Stability and Popcorn Prevention

A major failure mode in Siemens reactors is popcorn growth: a severe surface roughening condition where dendritic silicon nodules sprout from the rod surface.

Prevention Metrics

Popcorn growth occurs when homogenous gas-phase nucleation produces silicon dust clusters that land on the rod. To suppress popcorn defectivity:

1. Maintain molar feed ratio $[{\text{H}}_2]/[{\text{HSiCl}}_3]\ge 6:1-10:1$.
2. Limit thermal boundary layer supersaturation by optimizing gas inlet nozzle velocity ($v_{\text{jet}}>30\text{ m/s}$).
3. Precisely stabilize rod surface temperature at $1080^{\circ }\text{C}\pm 5^{\circ }\text{C}$ via adaptive pyrometer control.

→Prerequisites & continue reading

Prerequisites: Advanced chemical thermodynamics, boundary layer mass transfer, CVD surface physics

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

Tier 1 · Achieving 11N Purity: Trichlorosilane and the Siemen…Tier 2 · Fractional Distillation Columns and Siemens CVD Reac…