ALD Fundamental Chemistry
Atomic Layer Deposition uses sequential, self-limiting surface reactions to grow films one atomic layer at a time with precise thickness control and excellent conformality.
Langmuir Adsorption - Self-Limiting
θ = (K × P) / (1 + K × P)
θ: surface coverage (monolayers)
K: adsorption equilibrium constant
P: precursor partial pressure (Torr)
At high P: θ → 1 (saturation, self-limiting)
Growth Per Cycle (GPC)
GPC = (θ_max × d_ML × ρ_sites) / M_precursor
Typical values:
Al₂O₃ (TMA/H₂O): 1.0-1.2 Å/cycle
HfO₂ (HfCl₄/H₂O): 0.8-1.0 Å/cycle
TiN (TiCl₄/NH₃): 0.4-0.6 Å/cycle
ALD Reaction Sequences
Al₂O₃ ALD:
Step 1: Surface-OH + Al(CH₃)₃ → Surface-O-Al(CH₃)₂ + CH₄
Step 2: Purge unreacted TMA
Step 3: Surface-Al-CH₃ + H₂O → Surface-Al-OH + CH₄
Step 4: Purge unreacted H₂O
HfO₂ ALD:
Step 1: Surface-OH + HfCl₄ → Surface-O-HfCl₃ + HCl
Step 2: Purge
Step 3: Surface-HfCl + H₂O → Surface-Hf-OH + HCl
Step 4: Purge
Temperature Window
T_low: Insufficient thermal energy, precursor condensation
T_optimal: Self-limiting, maximum GPC
T_high: Precursor decomposition, loss of ALD behavior
Typical windows:
Al₂O₃: 150-350°C
HfO₂: 200-400°C
TiN: 350-450°C
Pulse Time Optimization
t_pulse,min = (L × AR) / (v̄ × α)
L: feature depth (cm)
AR: aspect ratio
v̄: mean molecular velocity = √(8kT/πm)
α: sticking probability
For AR=20:1, typical t_pulse = 1-5 seconds
Conformality
Conformality (%) = (t_bottom / t_top) × 100
ALD achieves >95% even at AR > 50:1
CVD typically <80% at AR > 10:1
Film Composition Control
[M]/[O] ratio controlled by:
- Pulse ratio adjustments
- Temperature (affects reaction completeness)
- Purge efficiency (prevents CVD-like reactions)
Typical purity: >99% for optimized processes
Nucleation Delay
N_delay = number of cycles before linear growth
Caused by:
- Low initial site density
- Substrate-film lattice mismatch
- Island coalescence
Typical: 5-20 cycles depending on substrate