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Silicon University / Pillar 06 · Plasma Etching & Atomic-Scale Material Removal / Topic 6.2 · Atomic Layer Etching

Tier 1 · Core Fundamentals 12 min read

Atomic Layer Etching (ALE): Core Principles & Sub-Nanometer Precision

Explains how ALE achieves sub-nanometer etching precision by separating the chemical reaction into self-limiting surface modification and low-energy ion bombardment displacement steps.

StudentsProcess EngineersSemiconductor Enthusiasts
Dimitri AlbinoProject Leader, Advanced Wet Processing Equipment Manufacturing & Deployment

01The Scaling Imperative: Beyond Continuous Plasma Etching

As transistor dimensions approach sub-2nm nodes (GAA Gate-All-Around Nanosheets, FinFETs, 3D NAND), conventional continuous Reactive Ion Etching (RIE) hits fundamental physical limits. In continuous RIE, simultaneous chemical flux and ion bombardment create stochastic roughness, aspect-ratio dependent etching (ARDE), and lattice damage extending several nanometers deep into the substrate.

To pattern feature dimensions measured in individual atomic layers, the industry transitioned to Atomic Layer Etching (ALE). ALE is the digital counter-part to Atomic Layer Deposition (ALD), removing material layer-by-layer through self-limiting chemical reaction cycles.

Process schematicSchematic · source diagram
SubstrateCONTINUOUS RIE ETCHINGATOMIC LAYER ETCHING (ALE)Simultaneous Ion + Chemical FluxSequential Self-Limiting CyclesChem + IonsStep 1: Modification (Self-limiting)Step 2: PurgeResult: Roughness, Sub-surface DamageStep 3: Removal (Self-limiting)Step 4: PurgeResult: Atomic Flatness, Zero Damage

02The Universal 4-Step ALE Cycle

Every ALE process operates by splitting material removal into separate, self-limiting half-reactions:

Process schematicSchematic · source diagram
STEP 1: REAGENT MODIFICATIONChemical reactant adsorbs and modifiesonly the top atomic surface layerSTEP 2: PURGEInert gas purge removes unreacted chemicalsfrom the chamberSTEP 3: REACTION REMOVALLow-energy ions or thermal ligands reactexclusively with the modified layerSTEP 4: PURGEPurge clears volatile etch byproducts

Self-Limiting Behavior

  • Step 1 (Modification): Adsorption terminates automatically once all available surface bonding sites are saturated.
  • Step 3 (Removal): The removal energy is tuned strictly below the bulk material sputtering threshold. Once the modified top atomic monolayer is removed, etching stops completely, regardless of extended step duration.
Figure 1
FIG. 1

03Directional vs. Thermal ALE

ALE processes fall into two main operational categories based on material requirements and feature geometries:

ParameterDirectional Plasma ALEThermal Isotropic ALE
Removal MechanismLow-energy directional ion impact ()Ligand exchange or fluorination/complexation
Profile ControlHighly Anisotropic (Vertical 90°)Isotropic (Conformal 360° removal)
Primary MaterialsSilicon, Silicon Nitride (), MetalsHigh-k oxides (, ), Metals, Nitrides
Target ApplicationSelf-Aligned Contacts (SAC), Nanosheet Spacer EtchGAA Nanosheet Sacrificial Silicon-Germanium Release
Figure 2
FIG. 2

04Etch Per Cycle (EPC) & Synergy

The defining metric of ALE is Etch Per Cycle (EPC), typically expressed in Ångströms per cycle ():

Ideal ALE processes achieve synergy, ensuring that material removal occurs exclusively through the combined sequential cycle rather than unconditioned spontaneous etching.

Figure 3
FIG. 3

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Prerequisites: ["p06-t6.1-t1-plasma-etching-rie-icp-ccp-fundamentals"]

Also in Topic 6.2, Atomic Layer Etching:

Tier 2 · ALE Tool Architecture: High-Speed Gas Pulsing, Fast-…Tier 3 · Directional Plasma vs. Thermal Isotropic ALE: Surfac…