Selective Tungsten Deposition
Selective Tungsten Deposition is the chemical vapor deposition technique where tungsten metal grows preferentially on metallic or conductive surfaces while inhibiting growth on dielectric surfaces — enabling bottom-up fill of contact vias and trenches without the seam voids and pinholes that occur with conventional conformal deposition, and reducing the need for barrier/liner layers that consume an increasing fraction of the via cross-section at advanced nodes.
Why Selective Deposition
- Conventional CVD W: Grows conformally on all surfaces → seam void when sidewall films merge before via bottom fills.
- At sub-20nm via diameter: TiN barrier (2nm) + W nucleation layer (2nm) = 4nm total → consumes 40% of 10nm radius.
- Selective W: Grows from bottom (metal) up → no seam → more W cross-section → lower resistance.
- Area-selective: Grows only on metal → no barrier needed on sidewalls → even more volume for W.
Conventional vs. Selective Fill
Selectivity Mechanism
| Surface | W Nucleation | Growth | Reason |
|---|---|---|---|
| TiN (metal) | Immediate | Fast | WF₆ reacts with TiN → reduces to W |
| W (metal) | Immediate | Fast | WF₆ + H₂ → W (catalytic on W surface) |
| SiO₂ (dielectric) | Delayed/slow | Inhibited | No reduction pathway, weak adsorption |
| SiN (dielectric) | Delayed | Moderate | Some N-H sites promote nucleation |
Enhancing Selectivity
- Inhibitor approach: Expose wafer to inhibiting molecule (e.g., small organic) that binds to dielectric but not metal → blocks nucleation on dielectric.
- Plasma treatment: H₂ plasma activates metal surface → accelerates nucleation on metal only.
- Temperature tuning: Lower temperature → WF₆ requires catalytic surface (metal) → selectivity improves.
- Super-cycle ALD: Alternate W ALD cycles with inhibitor doses → extend selectivity window.
Selectivity Window
- Typical: 10-30nm of selective growth before loss of selectivity.
- After selectivity loss: Random nuclei on dielectric → conformal growth resumes.
- For 40nm deep via: 10-20nm selective growth from bottom → significantly reduces seam.
- Perfect selectivity (full via fill): Requires highly optimized inhibitor chemistry.
Applications
| Application | Via Size | Benefit |
|---|---|---|
| Contact (MOL) | 10-20nm | Void-free fill, lower resistance |
| Via0/Via1 | 15-25nm | Seam elimination |
| Wordline fill (DRAM) | 10-15nm | Uniform fill in high-AR structure |
| 3D NAND | 5-10nm (in stack) | Fill within multi-layer stack |
Resistance Reduction
| Method | Via Diameter | W Cross-Section | Resistance |
|---|---|---|---|
| Conformal (barrier + seed + W) | 14nm | ~7nm effective diameter | ~1000 Ω |
| Selective (minimal barrier + bottom-up W) | 14nm | ~11nm effective diameter | ~400 Ω |
| Improvement | — | +60% cross-section | 60% lower R |
Selective tungsten deposition is the metallization paradigm shift for advanced contact and via technology — by exploiting surface chemistry differences between metals and dielectrics to achieve bottom-up fill and area-selective growth, selective W processes overcome the fundamental scaling limitation of conformal deposition in narrow features, potentially delivering 2× lower via resistance while eliminating seam-related reliability failures.