Optimizing Atomic Layer Etching for Sub-2nm Semiconductor Node Fabrication

The Precision Challenge in Next-Generation Chip Manufacturing

As semiconductor nodes shrink below 2nm, traditional etching techniques face insurmountable challenges. The industry's relentless pursuit of Moore's Law now demands atomic-scale precision—a realm where even single-atom defects can derail entire chip architectures.

The Physics of Atomic-Scale Removal

Atomic layer etching (ALE) operates through sequential self-limiting reactions, typically involving:

• Surface modification: A controlled chemical reaction creates an altered surface layer
• Desorption: Physical or chemical removal of the modified layer

Recent studies at IMEC have demonstrated ALE with sub-ångström precision, achieving removal rates of 0.4-0.6 Å/cycle for silicon.

Material-Specific ALE Approaches

Silicon ALE: Chlorine-Based Cyclic Processes

The dominant approach for silicon involves:

• Cl₂ adsorption at 300-400K
• Ar⁺ ion bombardment at energies below 20eV
• SiCl_x desorption at controlled temperatures

High-k Dielectric Etching: Fluorocarbon Chemistry

For HfO₂ and other high-k materials, researchers at TEL have developed:

• C₄F₈/O₂ plasma surface modification
• Low-energy Ar⁺ or He⁺ ion bombardment
• Selectivity >100:1 relative to silicon achieved at 50°C

The Defect Control Imperative

Surface Roughening Mechanisms

Even with ALE, several factors can introduce atomic-scale defects:

• Ion channeling: Crystalline orientation effects causing non-uniform removal
• Redeposition: Etch byproducts redepositing at feature edges
• Subsurface damage: Ion implantation beyond the modified layer

Mitigation Strategies

Recent breakthroughs from Applied Materials include:

• Pulsed plasma modulation: 10-100μs pulses reduce average ion energy spread
• Cryogenic ALE: -50°C operation minimizes surface diffusion of etch byproducts
• Real-time optical emission spectroscopy: Closed-loop control of radical concentrations

The Tooling Revolution

Next-Generation ALE Reactor Design

Leading equipment vendors are developing specialized architectures:

• Separated reaction zones: Physical isolation of modification and desorption steps
• Multi-beam ion sources: Independently controlled ion species and energies
• In-situ metrology: XPS and ellipsometry integrated into process chambers

The Vacuum Conundrum

At sub-2nm dimensions, base pressure requirements become extreme:

• Base pressure < 10^-9 Torr to prevent surface contamination
• Partial pressure of H₂O must be < 10^-11 Torr during processing
• Cryopumps with LN₂-cooled shrouds now standard

The Selectivity Challenge

Mask Materials Evolution

Traditional photoresists fail at sub-2nm critical dimensions. Current approaches include:

• HSQ (Hydrogen silsesquioxane): 1.5nm resolution demonstrated by IBM Research
• Metal-oxide resists: HfO_x-based materials with etch selectivity >50:1
• Self-assembled monolayers: Molecular precision but limited thermal stability

The Interface Problem

ALE must maintain selectivity across multiple material interfaces:

• Si/SiO₂: Standard processes achieve 100:1 selectivity
• Si/SiN_x: More challenging, typically 30:1 with optimized chemistry
• Metal/dielectric: Requires novel inhibitor chemistries like acetylacetone derivatives

The Future Landscape: Directed ALE

Beam-Guided Etching

Emerging techniques promise even greater control:

• Electron-stimulated ALE: Localized reactions using 1-5keV e-beams (Intel patents)
• Templated ALE: DNA origami masks for sub-nm pattern transfer (MIT research)
• Plasmon-enhanced ALE: Localized surface reactions using nanoantennas (Stanford work)

The Ultimate Limit: Single-Atom Removal

Theoretical and experimental work suggests fundamental boundaries:

• Tunneling effects: Electron spillover limits minimum feature spacing to ~0.5nm
• Thermodynamic constraints: Minimum activation energy of ~0.7eV/atom for silicon removal
• Spatial uncertainty: Heisenberg uncertainty principle imposes ~0.1nm positional blurring at room temperature

The Metrology Bottleneck

Measuring the Unmeasurable

Existing techniques struggle with atomic-scale process control:

• TEM limitations: Sample preparation artifacts and low throughput
• CD-SEM variability: 3σ > 0.3nm at best current performance
• X-ray techniques: Brilliant synchrotron sources required for sufficient resolution

The Promise of Quantum Metrology

Next-generation solutions may leverage quantum phenomena:

• NV center microscopy: Single-spin detection of surface defects (Delft University)
• SQUID-based profilometry: Sub-ångström height resolution demonstrated by NIST
• Terahertz CD mapping:

The Economic Reality of Atomic Precision

The Cost-Per-Transistor Inflection Point

The industry faces unprecedented cost challenges:

• Capex per wafer pass:
• Takt time impact:
• Yield learning curves:

The Materials Supply Chain Challenge

The purity requirements create new bottlenecks:

• Precursor purity:2 now requires <10ppt metallic impurities
• Gas delivery systems:
• Cryogenic argon: