Atomic Layer Etching (ALE) is the self-limiting removal technique that etches exactly one atomic layer of material per cycle — analogous to ALD in reverse — using alternating steps of surface modification (chemical adsorption) and removal (low-energy ion bombardment or thermal desorption) to achieve sub-nanometer depth control, extreme selectivity, and damage-free processing that is essential for the most dimensionally critical steps at sub-3nm CMOS nodes.

Why ALE Is Needed

Conventional plasma etch is a continuous process — etch rate depends on plasma conditions, and stopping precisely at a specific depth requires real-time monitoring. At advanced nodes, the margin between "enough etch" and "too much etch" is 1-2 atomic layers. For processes like gate recess, spacer thinning, and channel release in GAA, the etch must remove material with atomic-layer precision while stopping without damaging the underlying film.

How Directional (Anisotropic) ALE Works

1. Modification Step: A reactive gas (Cl₂, fluorocarbon, or other halogen) is introduced. It chemisorbs on the surface, forming a thin modified layer (~1 monolayer). Adsorption is self-limiting — once all surface sites react, no more adsorption occurs regardless of additional exposure time. 2. Purge: Excess gas and byproducts are removed. 3. Removal Step: Low-energy inert ions (Ar⁺ at 15-30 eV) are directed at the surface. The energy is sufficient to sputter the weakened modified layer but insufficient to sputter unmodified material. The modified monolayer is removed while the underlying bulk is untouched — this is the self-limiting removal. 4. Purge: Byproducts removed. One ALE cycle complete — exactly one atomic layer removed.

The low ion energy is critical: it must exceed the sputtering threshold of the modified layer (~10-15 eV) but remain below the sputtering threshold of the unmodified bulk material (~25-50 eV). This energy window provides the self-limiting behavior.

Isotropic (Thermal) ALE

For applications requiring isotropic removal (equal etch in all directions): 1. Modification: Surface is fluorinated using low-energy plasma or gas exposure. 2. Removal: A ligand exchange reaction — a second gas (e.g., TMA, Sn(acac)₂) reacts with the fluorinated surface, forming volatile metal-organic products that desorb. No ions needed.

Isotropic ALE is essential for the GAA nanosheet channel release step — selectively removing SiGe sacrificial layers from between silicon nanosheets with atomic precision and perfect conformality, without any ion bombardment damage to the delicate suspended nanosheets.

Key Applications

• Gate Recess Control: Precise thinning of dummy gate or gate oxide with ±0.5nm accuracy.
• Spacer Thinning: Reducing spacer width by exactly the desired amount to tune overlap capacitance.
• Channel Release (GAA): Isotropic selective removal of SiGe between Si nanosheets.
• Surface Smoothing: ALE can reduce surface roughness by preferentially removing protruding atoms.

Atomic Layer Etching is the surgical counterpart to atomic layer deposition — removing material one atom at a time with the same digital precision that ALD uses for building, providing the etch control that makes sub-3nm transistor architectures manufacturable.