Advanced Atomic Layer Deposition (ALD) encompasses the cutting-edge ALD techniques and applications at sub-5nm technology nodes — including area-selective deposition (ASD) that deposits material only on target surfaces without lithographic patterning, high-productivity spatial ALD, and novel precursor chemistries that enable conformal films on the most challenging 3D device geometries including gate-all-around nanosheet transistors.

ALD Fundamentals Review:

Cycle 1:
  Dose A: Precursor A (e.g., TMA - trimethylaluminum) → chemisorbs on surface
          → Self-limiting: reacts only with available surface sites
  Purge:  Remove excess precursor and byproducts with N₂
  Dose B: Co-reactant (e.g., H₂O) → reacts with adsorbed A layer
          → Forms one atomic layer of material (e.g., Al₂O₃)
  Purge:  Remove byproducts

Repeat N cycles → N atomic layers (~0.5-1.5 Å/cycle → ~1 nm per 10 cycles)

Area-Selective Deposition (ASD):

The most transformative ALD advancement for advanced nodes. ASD deposits material selectively on one surface type while avoiding deposition on another — enabling self-aligned patterning without lithography:

Target: deposit material on metal, not on dielectric

Approach 1 — Inherent selectivity:
  Some ALD precursors naturally nucleate on metals but not on SiO₂
  (e.g., Ru ALD on Cu but not on SiO₂ for ~20 cycles)
  Selectivity window: typically 2-5nm before loss of selectivity

Approach 2 — Surface modification (SAM blocking):
  Apply self-assembled monolayer (SAM) on surface to block
  e.g., octadecylphosphonic acid on oxide → blocks ALD on oxide
  ALD deposits on unmodified metal surfaces
  Achieve >10nm selective thickness

Approach 3 — Etch-back (super-cycle):
  ALD deposits on both surfaces but nucleation delay differs
  After N cycles: thin film on target, nuclei on non-target
  Mild etch removes nuclei from non-target while target film survives
  Repeat ALD + etch cycles for thicker selective films

Applications at Advanced Nodes:

Spatial ALD:

Conventional ALD cycles through gas doses in time (temporal ALD) — slow (1-10 Å/min). Spatial ALD separates precursor and reactant zones in space — the wafer moves between zones, achieving effectively continuous deposition:

Temporal ALD:  dose A → purge → dose B → purge (one cycle ~2-10 sec)
Spatial ALD:   wafer passes zone A → gas curtain → zone B → gas curtain
               Multiple cycles per rotation → 10-100× throughput improvement

Plasma-Enhanced ALD (PEALD):

Uses plasma (O₂, N₂, H₂) as the co-reactant instead of thermal reactants. Benefits: lower deposition temperature (50-200°C vs. 250-400°C for thermal ALD), enabling BEOL-compatible deposition and processing on temperature-sensitive substrates. Critical for depositing quality dielectrics at low temperatures.

Advanced ALD is indispensable at the most aggressive semiconductor technology nodes — as device dimensions shrink below 5nm, only ALD's self-limiting, conformal growth mechanism can deliver the atomic-scale thickness control and 3D conformality required for gate dielectrics, spacers, barriers, and self-aligned selective deposition in gate-all-around and future device architectures.