Atomic Layer Deposition (ALD) — depositing ultra-thin films one atomic layer at a time through self-limiting sequential chemical reactions, providing angstrom-level thickness control.

Process Cycle

1. Pulse A: First precursor adsorbs on surface (self-limiting — only one monolayer sticks) 2. Purge: Remove excess precursor and byproducts 3. Pulse B: Second precursor reacts with adsorbed layer, forming one atomic layer of film 4. Purge: Remove excess 5. Repeat cycles for desired thickness (~1 angstrom per cycle)

Key Properties

• Self-limiting: Film thickness determined by number of cycles, not time or flow
• Conformality: Perfect step coverage in high-aspect-ratio features (>100:1)
• Uniformity: Excellent across 300mm wafer
• Thickness control: Sub-angstrom precision

Applications in CMOS

• High-k gate dielectric (HfO2): 1-2nm precision critical
• Metal gate work function layers
• Spacers and liners in FinFET/GAA
• Barrier layers in advanced interconnects

Trade-off: ALD is slow (~1 A/cycle, ~1 sec/cycle) compared to CVD, so it's used only where atomic precision is essential.

ALD is indispensable at advanced nodes — you cannot build a 3nm transistor without it.