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.