Atomic Layer Deposition (ALD) is the vapor-phase thin film deposition technique that builds films one atomic layer at a time through self-limiting surface reactions — alternating exposures to two (or more) precursor gases, each of which reacts only with the surface-adsorbed previous layer, providing angstrom-level thickness control, perfect conformality on complex 3D structures, and composition uniformity that make ALD the indispensable deposition technology for gate dielectrics, barrier layers, and spacers at the 10 nm node and below.

The ALD Cycle

1. Precursor A Pulse: First precursor gas (e.g., TMA — trimethylaluminum for Al₂O₃) flows into the chamber and chemisorbs on the surface, reacting with available surface sites (hydroxyl groups). Reaction is self-limiting — once all sites are occupied, no further adsorption occurs regardless of exposure time. 2. Purge: Inert gas (N₂ or Ar) purges excess precursor and byproducts. 3. Precursor B Pulse: Second precursor (e.g., H₂O for oxide) reacts with the adsorbed first precursor, forming one monolayer of the target film and regenerating surface sites for the next cycle. 4. Purge: Remove excess precursor B and byproducts.

One cycle deposits 0.5-1.5 Å of film. A 20 Å HfO₂ gate dielectric requires ~20 cycles.

Why Self-Limiting Is Revolutionary

• Thickness Control: Film thickness = number of cycles × growth per cycle (GPC). No dependence on gas flow uniformity, precursor concentration, or exposure time (once saturation is reached). Angstrom-level precision across entire 300mm wafers.
• Conformality: Every surface point (including inside deep trenches and around nanosheet channels) receives equal coverage because precursor molecules reach all surfaces and react identically. Step coverage >99% in aspect ratios >100:1 — impossible with CVD or PVD.
• Uniformity: Within-wafer thickness variation <0.5% achievable — limited only by temperature uniformity, not gas flow patterns.

ALD Variants

• Thermal ALD: Reactions driven by substrate temperature (200-400°C). The standard for high-quality dielectrics (HfO₂, Al₂O₃, ZrO₂).
• Plasma-Enhanced ALD (PEALD): Precursor B is a plasma (O₂ plasma, N₂ plasma, H₂ plasma). Enables lower deposition temperature (25-200°C, compatible with BEOL thermal budgets) and access to materials difficult to deposit thermally (TiN, TaN, SiN).
• Spatial ALD: Instead of temporal cycling in one chamber, the wafer moves through spatially separated precursor zones. Dramatically higher throughput (10-100× faster) suitable for display and photovoltaic manufacturing.
• Area-Selective ALD: Preferential deposition on one surface chemistry (e.g., metal) while inhibiting growth on another (e.g., oxide). An emerging technique for self-aligned patterning that could reduce lithography steps.

Critical ALD Applications

• High-k Gate Dielectric: HfO₂ (0.8-2 nm) — the most critical ALD application. Gate oxide uniformity directly determines transistor threshold voltage uniformity.
• Work Function Metals: TiN, TiAl — deposited by ALD to control NMOS/PMOS threshold voltage.
• Barrier/Liner Layers: TaN/Ta barriers for copper interconnects. ALD conformality ensures complete sidewall coverage preventing copper diffusion.
• GAA Nanosheet Fill: ALD is the only deposition technique capable of conformally coating the interior surfaces of released nanosheets with sub-10 nm spacing.

Atomic Layer Deposition is the atomic-precision manufacturing tool of semiconductor fabrication — the deposition technique that converts the abstract concept of "one atom at a time" into a practical, high-volume manufacturing capability that enables the 3D device architectures driving continued transistor scaling.