Atomic Layer Deposition (ALD)
Atomic Layer Deposition (ALD)
ALD deposits films one atomic layer at a time through self-limiting sequential reactions:
- Pulse A: First precursor enters and reacts with the surface until all available sites are occupied — then the reaction stops (self-limiting)
- Purge: Excess precursor and byproducts are removed
- Pulse B: Second precursor enters and reacts with the surface created by Pulse A — again self-limiting
- Purge: Remove excess and byproducts
- Repeat: Each cycle deposits ~0.5–1 Å (about half an atomic layer)
ALD advantages:
- Angstrom-level thickness control: Thickness = number of cycles × growth per cycle
- Perfect conformality: Coats the inside of deep trenches uniformly (100% step coverage)
- Pinhole-free films: Self-limiting nature prevents gaps
Key Concept: High-k Gate Dielectrics
ALD's precision made it indispensable for depositing high-k gate dielectrics (HfO₂) that replaced SiO₂ at the 45nm node. A 1–2nm HfO₂ film deposited by ALD provides the same capacitance as a much thicker SiO₂ film, while reducing leakage current.
Epitaxial Growth
Epitaxial Growth
Epitaxy grows a crystalline film that matches the crystal structure of the substrate — essentially extending the single crystal. This is critical for advanced transistors:
- Silicon epitaxy: Growing pure Si layers with precise doping profiles
- SiGe epitaxy: Growing silicon-germanium alloys to create strain in the channel, boosting carrier mobility by 50–100%
- Selective epitaxy: Growing crystalline material only on exposed silicon, not on oxide — used for raised source/drain in FinFETs
Analogy: Growing a Crystal
Epitaxy is like adding more rows of bricks to a perfectly aligned wall. Each new atom aligns itself with the existing crystal pattern below, continuing the single-crystal structure. If the new atoms are slightly larger (Ge in SiGe), they strain the lattice — and that strain actually improves transistor performance.
Knowledge Check
Knowledge Check
1 / 2What makes ALD 'self-limiting'?