Atomic Layer Etching: Enabling Precision at the 2nm Node

The Evolution of Etching in Semiconductor Manufacturing

As semiconductor fabrication approaches the 2nm process node, traditional etching techniques face fundamental limitations. The industry's relentless pursuit of Moore's Law now demands atomic-scale precision in material removal, where conventional reactive ion etching (RIE) methods struggle with selectivity and damage control.

Limitations of Conventional Etching at Advanced Nodes

• Isotropic etching effects: Chemical-dominated processes cause unwanted lateral material removal
• Plasma-induced damage: High-energy ions create defects in sensitive structures
• Poor selectivity: Difficulty in stopping precisely at interface layers
• Line edge roughness: Statistical variations become significant at atomic scales

Atomic Layer Etching (ALE) Fundamentals

Atomic layer etching represents a paradigm shift from continuous etching processes to a sequential, self-limiting approach inspired by atomic layer deposition (ALD). The technique enables removal of material with single-atomic-layer precision through discrete reaction cycles.

The ALE Process Cycle

Each ALE cycle consists of two fundamental phases:

1. Surface Modification: A chemically reactive species forms a modified surface layer
2. Volatile Removal: A separate excitation step removes only the modified material

Implementing ALE at 2nm Nodes

The transition to 2nm manufacturing introduces several challenges that ALE specifically addresses:

Gate-All-Around (GAA) Nanosheet Fabrication

ALE enables precise thickness control of silicon nanosheets with minimal surface roughness. For 2nm nodes, the typical nanosheet thickness target of 5-8nm requires atomic-level precision in channel release etching.

Contact and Via Processing

ALE's selectivity allows for damage-free etching of ultra-thin barrier layers (often <1nm) while maintaining critical dimension control in high-aspect-ratio features exceeding 10:1.

Material Systems and ALE Chemistries

Different materials require tailored ALE approaches at the 2nm scale:

Material	Modification Chemistry	Removal Mechanism
Si/SiGe	Cl2 plasma	Ar+ ion bombardment
SiO2	HF precursor	Thermal desorption
Low-k dielectrics	Hydrogen plasma	O2 plasma

Process Control and Metrology Challenges

The atomic-scale precision of ALE introduces new requirements for process monitoring:

In-Situ Monitoring Techniques

• Optical emission spectroscopy: Real-time endpoint detection for cycle control
• Mass spectrometry: Detection of volatile byproducts
• Ellipsometry: Thickness measurement with sub-Ångström resolution

Equipment Considerations for High-Volume Manufacturing

Transitioning ALE from research to production requires addressing several practical challenges:

Throughput Optimization

The sequential nature of ALE typically results in lower material removal rates compared to continuous processes. Advanced reactor designs employ:

• Spatial separation of process steps
• Multi-wafer processing chambers
• Optimized gas delivery systems

The Future of ALE Beyond 2nm

As the industry looks toward sub-2nm nodes and complementary FET (CFET) architectures, ALE will play increasingly critical roles:

Monolithic 3D Integration

ALE enables precise interlayer via formation with minimal damage to underlying devices, essential for vertical transistor stacking.

2D Material Processing

The transition to materials like transition metal dichalcogenides (TMDs) will require ALE techniques capable of single-layer removal without damaging adjacent atomic planes.

Comparative Analysis: ALE vs. Conventional Etching at 2nm

Parameter	ALE	Conventional RIE
Etch rate control	Atomic layer precision	Statistical distribution
Selectivity	>100:1 achievable	Typically 10-20:1
Surface damage	<0.5nm affected layer	2-5nm damaged layer
Aspect ratio capability	>50:1 demonstrated	~20:1 practical limit

The Economic Perspective

The adoption of ALE at 2nm nodes presents both challenges and opportunities for semiconductor manufacturers:

Cost Considerations

• Tool investment: ALE equipment carries significant capital costs
• Process complexity: Additional steps increase cycle time
• Yield benefits: Improved uniformity reduces defect density

Environmental Impact and Sustainability

The environmental footprint of ALE processes differs from conventional etching:

Chemical Usage Patterns

• Reduced waste: Self-limiting reactions minimize chemical consumption
• Alternative chemistries: Potential for less hazardous precursors
• Energy consumption: Trade-offs between plasma power and process time

The Path to Manufacturing Readiness

The semiconductor industry's roadmap for ALE implementation involves several critical milestones:

1. Process standardization: Development of reference ALE sequences for key materials
2. Equipment qualification: Demonstration of >90% uptime in production environments
3. Metrology integration: Implementation of in-line process control solutions
4. Cost optimization: Reduction of cost-per-wafer to competitive levels