Atomic Layer Etching (ALE) is the precision material removal technique that removes exactly one atomic or molecular layer per cycle through a two-step, self-limiting process — analogous to ALD in reverse — enabling sub-nanometer etch depth control, atomic-level surface smoothness, and damage-free processing that conventional continuous plasma etch cannot achieve.

Why Conventional Etch Is Too Coarse

Plasma etch is a continuous process — turning off the plasma is the only way to stop etching, but process lag, chamber pressure decay, and plasma extinction dynamics make stopping within ±1 nm practically impossible. When the target etch depth is 3 nm (e.g., recessing a gate oxide or trimming a nanosheet), ±1 nm is a ±33% error. ALE provides the clock-like precision that continuous etch fundamentally lacks.

The ALE Cycle

1. Surface Modification: A reactive gas (Cl2, BCl3, or fluorocarbon) adsorbs onto or reacts with exactly the top monolayer of the target material, forming a weakly-bonded modified layer. The reaction is self-limiting — once the surface is fully covered, no further modification occurs regardless of exposure time. 2. Modified Layer Removal: A low-energy ion bombardment (typically Ar+ at 10-30 eV, below the sputter threshold of the unmodified material) selectively removes only the modified layer. The unmodified material underneath is too strongly bonded to be sputtered at this energy. 3. Purge and Repeat: Reaction byproducts are pumped away, and the cycle repeats. Each cycle removes exactly one monolayer (~0.3-0.5 nm depending on material).

ALE Variants

• Directional (Anisotropic) ALE: The ion bombardment step is directional (ions arrive vertically), so only horizontal surfaces are etched. This provides atomic-level depth control with anisotropic profile — essential for gate recess and spacer etch-back.
• Isotropic (Thermal) ALE: Both steps use thermal reactions (no plasma). The modified layer is removed by a second gas that reacts only with the modified surface. This achieves isotropic (all-direction) etching with monolayer precision — critical for the lateral SiGe recess in nanosheet inner spacer formation.

Materials and Selectivity

ALE has been demonstrated for Si, SiO2, Si3N4, Al2O3, HfO2, W, and TiN. By choosing the modification chemistry, selectivity between materials (e.g., etching SiN but not SiO2) is achieved through thermodynamic differences in the surface reaction — the modification step simply does not occur on the non-target material.

Atomic Layer Etching is the surgical scalpel of semiconductor manufacturing — removing material one atom at a time when the engineering tolerances are measured in individual atomic layers.