Via Directed Self-Assembly of Block Copolymers: Achieving Sub-5nm Semiconductor Patterning
Via Directed Self-Assembly of Block Copolymers: Achieving Sub-5nm Semiconductor Patterning
The Challenge of Sub-5nm Patterning in Semiconductor Fabrication
As semiconductor technology advances toward ever-smaller nodes, traditional photolithography techniques face significant challenges. Extreme ultraviolet (EUV) lithography, while revolutionary, still struggles with the fundamental limitations of light diffraction and resist chemistry at sub-5nm dimensions. Industry leaders have turned to alternative approaches, with directed self-assembly (DSA) of block copolymers emerging as a promising candidate.
Understanding Block Copolymer Self-Assembly
Block copolymers are macromolecules composed of two or more chemically distinct polymer chains (blocks) covalently bonded together. These materials spontaneously self-assemble into periodic nanostructures when the constituent blocks are immiscible. The characteristic dimensions of these nanostructures are determined by:
The overall degree of polymerization (N)
The Flory-Huggins interaction parameter (χ)
The volume fraction of each block (f)
Thermodynamics of Microphase Separation
The driving force behind block copolymer self-assembly lies in thermodynamics. When χN > 10.5 (for symmetric diblock copolymers), the system undergoes microphase separation to minimize free energy. The resulting morphologies include:
Lamellae (alternating sheets)
Hexagonally packed cylinders
Bicontinuous gyroids
Body-centered cubic spheres
Directed Self-Assembly: From Random to Controlled Patterns
While block copolymers naturally form periodic structures, semiconductor manufacturing requires precise pattern registration and defect control. DSA achieves this through various directing methods:
Graphoepitaxy: Physical Confinement
Graphoepitaxy uses topographic features (e.g., trenches or posts) to guide block copolymer assembly. Research has demonstrated:
20nm pitch patterns in PS-b-PMMA (polystyrene-block-poly(methyl methacrylate))
9nm line/space patterns using high-χ copolymers
Defect densities below 0.1 defects/μm² in optimized systems
Chemoepitaxy: Chemical Patterning
Chemoepitaxy employs chemical contrast patterns to direct assembly. Key developments include:
Neutral brush layers with varying surface affinities
Pre-patterns created by EUV or e-beam lithography
Pattern multiplication factors up to 4× demonstrated
Materials Innovation for Sub-5nm Patterning
Achieving sub-5nm features requires block copolymers with:
High χ parameters (>0.1 at room temperature)
Sufficient etch selectivity between blocks
Thermal and chemical stability during processing
High-χ Block Copolymer Systems
Recent material developments include:
PS-b-PDMS (polystyrene-block-polydimethylsiloxane): χ ≈ 0.26 at 25°C
PS-b-PEO (polystyrene-block-polyethylene oxide): χ ≈ 0.08 at 25°C
Organic-inorganic hybrids like PS-b-PFS (polystyrene-block-polyferrocenylsilane)
Block Copolymer System
χ Parameter
Minimum Demonstrated Pitch (nm)
PS-b-PMMA
≈0.04
22
PS-b-PDMS
≈0.26
9
PS-b-PEO
≈0.08
12
The Path to Sub-5nm: Current Research Frontiers
Several cutting-edge approaches are being explored to push DSA beyond current limits:
Sequential Infiltration Synthesis (SIS)
SIS enhances pattern transfer by selectively infiltrating one block with inorganic precursors. Recent results show:
Pattern transfer fidelity improvement by 30-50%
Etch selectivity enhancement up to 10:1
Successful application to 7nm half-pitch patterns
Ternary Block Copolymer Blends
The addition of homopolymers or other copolymers can modify self-assembly behavior:
Reduced defect densities through entropy control
Pattern perfection at higher throughputs
Demonstrated 6nm line/space patterns with blended systems
Three-Dimensional Patterning
Moving beyond 2D patterns, researchers are exploring:
Vertical lamellae for finFET applications
Bicontinuous structures for interconnect applications
Multi-layer DSA integration schemes
Metrology and Defect Control Challenges
The semiconductor industry requires defect densities below 0.01 defects/cm² for volume manufacturing. Current DSA systems face several metrology challenges:
Characterization Techniques
Advanced metrology methods for sub-5nm DSA include: