Selective Deposition
Selective Deposition (Area-Selective Deposition, ASD) is the technique of depositing material only on specific surfaces while avoiding growth on adjacent surfaces — eliminating the need for lithography and etch steps to pattern certain films, reducing process complexity and enabling self-aligned structures at advanced nodes where overlay tolerances are approaching physical limits.
Why Selective Deposition?
- Traditional approach: Deposit everywhere → Lithography → Etch to remove unwanted areas → 3 steps.
- Selective deposition: Deposit only where needed → 1 step.
- At sub-5nm nodes: Overlay accuracy (< 2 nm) makes traditional pattern-and-etch increasingly difficult.
- Self-aligned selective deposition eliminates overlay concerns entirely.
How ASD Works
Inherent Selectivity:
- ALD precursors naturally nucleate on some surfaces but not others.
- Example: TiO2 ALD nucleates readily on -OH terminated SiO2 but poorly on H-terminated Si.
- Limited selectivity window: After ~2-5 nm, defect nucleation occurs on non-growth surface.
Enhanced Selectivity Methods:
| Method | Mechanism | Selectivity Window |
|---|---|---|
| SAM (Self-Assembled Monolayer) | Block precursor adsorption on non-growth surface | 5-20 nm |
| Small-Molecule Inhibitor | Reversible passivation of non-growth surface | 3-10 nm |
| Super-Cycle ASD | Alternating ALD deposition + selective etch correction | > 20 nm |
| Plasma-Enhanced Selectivity | Substrate-dependent plasma activation | 5-15 nm |
Super-Cycle Approach (most practical for production):
1. Deposit ~2-3 nm by ALD (nucleates everywhere, more on target surface).
2. Selective etch removes nucleation defects from non-growth surface.
3. Repeat deposit-etch cycles until target thickness reached.
4. Achieves > 20 nm selective films with < 1 nm defect density.
Applications in Advanced CMOS
- Selective metal cap: Deposit Co cap only on Cu lines (not on dielectric) — prevents electromigration without extra litho/etch.
- Selective dielectric: SiN deposition only on spacer sidewalls — self-aligned structure.
- Selective contact fill: Metal nucleation only at bottom of contact (not on sidewalls) — improved bottom-up fill.
- Selective barrier: Barrier deposition only where Cu contacts dielectric — maximizes conductor volume.
Industry Status
- Active R&D at imec, Lam Research, ASM International, TEL.
- Limited production insertion — selectivity window and defect density still challenging.
- Most promising near-term: Super-cycle ASD for metal capping and dielectric patterning.
Selective deposition is the next frontier in self-aligned semiconductor processing — by eliminating lithography steps through chemistry-driven spatial selectivity, ASD promises to simplify integration, improve pattern fidelity, and enable transistor architectures that would be impossible to fabricate with conventional deposit-litho-etch sequences.