Optimizing Atomic Layer Etching for 2nm Semiconductor Nodes with Plasma Precision Control

The Critical Need for Atomic-Level Precision in 2nm Fabrication

As semiconductor technology advances toward the 2nm node, traditional etching techniques face significant challenges in maintaining precision, uniformity, and material selectivity. Atomic layer etching (ALE) has emerged as a key enabler for next-generation fabrication, offering monolayer-level control over material removal. However, achieving consistent atomic-scale precision requires advanced plasma control techniques that minimize damage while maximizing selectivity.

Fundamentals of Atomic Layer Etching

ALE operates through self-limiting surface reactions, typically consisting of two alternating half-cycles:

• Modification phase: A reactive species (often plasma-generated) modifies the surface layer of the substrate.
• Removal phase: A separate chemical or physical process selectively removes the modified layer.

This cyclic approach theoretically enables atomic-level control over etch depth, but practical implementation at 2nm scales demands unprecedented process stability.

Plasma Control Challenges at 2nm Nodes

Ion Energy Distribution Control

Precise control of ion energy distribution (IED) becomes critical when dealing with sub-5nm feature sizes. Traditional capacitively coupled plasmas generate broad IEDs that can cause subsurface damage and profile distortion. Recent advancements in:

• Pulsed plasma techniques
• Tailored bias waveforms
• Electron beam-generated plasmas

have demonstrated the ability to narrow IEDs to <1eV spreads, enabling damage-free etching of sensitive materials.

Radical Flux Management

The ratio of ions to radicals in the plasma significantly impacts ALE mechanisms. Excessive radical flux leads to spontaneous chemical etching that disrupts the self-limiting nature of ALE. Advanced plasma diagnostics including:

• Cavity ring-down spectroscopy
• Two-photon absorption laser-induced fluorescence
• Mass spectrometry sampling

enable real-time monitoring and closed-loop control of radical populations.

Advanced Plasma Source Configurations

Inductively Coupled Plasma with Bias Control

Modern ICP systems combine high-density plasma generation with independent bias control. The decoupling of plasma generation from substrate bias allows:

• Independent optimization of radical generation and ion energy
• Precise tuning of ion/radical flux ratios
• Reduction of plasma-induced damage through lower operating pressures

Remote Plasma Sources for Surface Modification

Separating the plasma generation region from the process chamber minimizes unwanted ion bombardment during the modification phase. This approach:

• Enables pure chemical surface modification
• Reduces particle generation
• Improves across-wafer uniformity

Process Monitoring and Control Systems

In-Situ Metrology Integration

Real-time process monitoring has become essential for maintaining atomic-scale precision. Leading-edge systems incorporate:

• Ellipsometry for angstrom-level thickness monitoring
• Optical emission spectroscopy for plasma chemistry analysis
• Residual gas analysis for byproduct detection

Machine Learning for Process Optimization

The complex multivariate nature of plasma ALE makes it particularly suitable for AI/ML optimization. Recent implementations have demonstrated:

• 30% reduction in process variation through neural network control
• Faster recipe development via digital twin simulations
• Predictive maintenance through plasma signature analysis

Material-Specific ALE Development

Silicon and Silicon-Germanium Etching

The transition to gate-all-around (GAA) nanosheet transistors at 2nm nodes requires precise Si/SiGe superlattice etching. Advanced chlorine-based ALE processes have achieved:

• <0.3nm/cycle removal rates with high selectivity to SiO₂
• Near-vertical sidewall profiles with sub-nm roughness
• Minimal germanium segregation during SiGe etching

High-k Dielectric Etching

The scaling of gate dielectrics demands ALE processes for materials like HfO₂. Fluorine-based ALE chemistries combined with low-energy Ar⁺ bombardment have shown:

• Self-limiting behavior at 0.4nm/cycle
• >100:1 selectivity to underlying Si
• Minimal electrical property degradation

Overcoming Selectivity Challenges

The increasing use of heterogeneous material stacks at 2nm nodes creates demanding selectivity requirements. Emerging solutions include:

• Area-selective ALE: Using inhibitor chemistry to protect non-target materials
• Thermodynamic process design: Exploiting reaction energy differences between materials
• Plasma pulsing strategies: Temporal control of surface reactions

The Path to High-Volume Manufacturing

Transitioning lab-scale ALE processes to production requires addressing several key challenges:

Challenge	Potential Solution	Current Status
Process speed	Spatial ALE configurations	Early development phase
Equipment cost	Modular plasma source designs	Prototype evaluation
Across-wafer uniformity	Intelligent electrode shaping	Pilot line testing
Defect control	In-situ cleaning cycles	Production implementation

The Future of Plasma ALE Technology

Looking beyond 2nm nodes, several promising directions are emerging:

• Cryogenic ALE: Combining low-temperature operation with plasma processes for enhanced selectivity
• Synchronized pulsed plasmas: Precise temporal control over reaction kinetics
• Plasma-ALE hybrids: Integrating conventional plasma etching with ALE for complex structures