Thermal Decomposition Chemistry
Thermal CVD relies on heat-induced pyrolysis to decompose precursor molecules. The balance between gas-phase and surface reactions determines film quality and deposition rate.
Pyrolysis Reaction - Silane
SiH₄ → Si + 2H₂ (overall reaction)
Decomposition pathway:
SiH₄ → SiH₂ + H₂ (primary step)
SiH₂ → Si + H₂ (surface or gas-phase)
Ea_decomp ≈ 50-60 kcal/mol
Homogeneous vs Heterogeneous Nucleation
Homogeneous (gas-phase):
J_homo = A × exp(-ΔG*/kT)
ΔG* = (16πσ³)/(3(Δμ)²)
σ: surface energy, Δμ: supersaturation
Heterogeneous (surface):
J_hetero = n_sites × ν × exp(-Ea_ads/kT)
n_sites: adsorption site density
Ratio: J_homo/J_hetero ∝ exp(ΔG*/kT)
Gas-Phase Nucleation - CNT
Critical nucleus radius:
r* = 2σV_m/(RT ln(S))
S: supersaturation ratio = P/P_eq
Nucleation barrier:
ΔG* = (16πσ³V_m²)/(3(RT ln(S))²)
Higher T → lower S → higher barrier
Temperature Gradient Effect
Vertical temperature profile:
T(z) = T_substrate - ∇T × z
∇T: temperature gradient (°C/cm)
Decomposition occurs at:
z_decomp ≈ (T_substrate - T_decomp) / ∇T
High ∇T → gas-phase decomposition
Low ∇T → surface-dominated
Particle Formation
Particle growth:
d(r_p)/dt = D × n_Si / r_p
D: diffusion coefficient
n_Si: Si atom concentration
Final size: r_p = √(2Dt × n_Si)
Larger particles at high T, high P
Deposition Rate
R_dep = k_surface × [SiH₄]_surface + k_condensation × Φ_particles
k_surface = k₀ × exp(-Ea/RT)
Φ_particles: particle flux to substrate
High T: more homogeneous, particulates
Low T: heterogeneous, smooth films
Impurity Incorporation
Sources of contamination:
1. Incomplete decomposition → SiH_x
2. Carrier gas reactions → Si-H bonds
3. Residual O₂/H₂O → SiO₂ inclusions
[H] (at.%) ∝ 1/T_substrate
[O] (ppm) ∝ P_O₂ × 10⁶
Decomposition Pathways
Primary pathway (T < 800°C):
SiH₄ → SiH₂ + H₂ → Si_film + H₂
Secondary pathway (T > 800°C):
SiH₄ → SiH₃• + H• (radical)
2SiH₃• → Si₂H₆ (disilane)
Si₂H₆ → 2Si + 3H₂ (gas-phase)
Byproducts affect film morphology