atomic layer etching

**Atomic Layer Etching (ALE)** is **a precision material removal technique that etches one atomic or molecular layer at a time through self-limiting sequential reaction steps, providing angstrom-level depth control and exceptional uniformity that conventional continuous plasma etching cannot achieve** — enabling the fabrication of nanoscale features with the tight dimensional tolerances required at the most advanced CMOS technology nodes. - **Self-Limiting Mechanism**: ALE operates in two alternating half-cycles: a modification step that chemically alters only the topmost atomic layer of the target material (through adsorption of a reactive species such as chlorine or fluorocarbon), and a removal step that selectively removes only the modified layer (through ion bombardment or thermal energy) without attacking the unmodified material beneath; this self-limiting behavior ensures that exactly one atomic layer is removed per cycle regardless of local flux variations. - **Directional (Anisotropic) ALE**: Low-energy ion bombardment (typically 10-30 eV argon ions) removes the modified surface layer preferentially from horizontal surfaces while leaving sidewalls intact, producing highly anisotropic etch profiles; the ion energy must be above the threshold for removing the modified layer but below the threshold for sputtering the unmodified material, creating a precise energy window of only a few electron-volts. - **Isotropic ALE**: Thermal ALE uses gas-phase chemistry without ion bombardment to isotropically remove the modified layer, enabling precise lateral etching for applications such as nanosheet channel release, gate recess, and spacer trimming; sequential exposure to fluorination agents and ligand-exchange reactants achieves self-limiting removal on all exposed surfaces simultaneously. - **Etch Per Cycle (EPC)**: Each ALE cycle typically removes 0.5-2.0 angstroms of material depending on the material system and chemistry; total etch depth is controlled by the number of cycles, not by time, providing digital depth control with repeatability better than plus or minus 1 angstrom. - **Selectivity Enhancement**: Because the modification chemistry can be tuned to react preferentially with specific materials, ALE achieves extreme selectivity (greater than 100:1) between target and non-target materials; this selectivity arises from differences in surface binding energies and reactant adsorption behavior rather than from etch rate ratios. - **Applications in Advanced CMOS**: ALE is used for fin recess etching, gate dielectric thickness trimming, self-aligned contact etch, spacer etch-back, and nanosheet channel release where sub-nanometer depth control and extreme selectivity are essential for device performance and yield. - **Throughput Considerations**: ALE is inherently slower than continuous etching due to its cyclic nature, with typical cycle times of 10-30 seconds; to maintain manufacturing throughput, ALE is applied selectively for the most critical process steps where its precision is indispensable, while continuous etch handles bulk material removal. Atomic layer etching has become an indispensable capability in the advanced semiconductor process toolkit because it provides the precision and control needed to fabricate device structures where dimensional tolerances are measured in individual atomic layers.