tungsten cvd contact fill

**Tungsten CVD Contact and Via Fill** is the **chemical vapor deposition process that fills the narrow, high-aspect-ratio contact holes and vias with tungsten metal — providing the vertical electrical connections between the transistor silicide contacts and the first copper interconnect layer (M1), and between copper routing layers, where void-free fill in sub-20 nm diameter holes with aspect ratios exceeding 10:1 requires precise nucleation and growth control**. **Why Tungsten for Contacts/Vias** Tungsten offers several advantages for local interconnect fill: - **CVD Conformality**: WF6-based CVD deposits tungsten conformally in high-aspect-ratio features — unlike copper electroplating, which requires a seed layer and bottom-up chemistry. The conformal nature means W fills from all surfaces inward. - **Barrier Compatibility**: W does not diffuse through standard diffusion barriers (TiN) and does not require the thick TaN/Ta barriers that copper demands. - **Process Simplicity**: Tungsten fill uses a single CVD step followed by CMP, avoiding the multi-step seed/plate/anneal process of copper damascene. **Tungsten CVD Process** 1. **Barrier/Liner Deposition**: PVD or ALD Ti (adhesion layer) + CVD or ALD TiN (barrier, 2-5 nm). The TiN prevents WF6 from attacking the underlying silicon or oxide during W deposition. 2. **Nucleation Layer**: A thin (~5 nm) nucleation layer of W is deposited using SiH4 or B2H6 reduction of WF6 at low pressure. This nucleation chemistry produces a smooth, continuous W film on the TiN surface. Without proper nucleation, the subsequent bulk fill would be rough and contain voids. 3. **Bulk Fill**: WF6 + H2 → W + 6HF at 300-400°C, 40-80 Torr. The conformal deposition fills the contact/via from all surfaces simultaneously. For narrow features, the fill proceeds inward until the W film from opposite sidewalls meets at the center (pinch-off). Void-free fill requires the growth fronts to merge cleanly. 4. **CMP**: Excess W on the field surface is removed by CMP, leaving W plugs only inside the contact holes and vias. **Scaling Challenges** - **Resistance**: Tungsten's bulk resistivity (5.3 uOhm·cm) is 3x higher than copper. As contact diameters shrink below 20 nm, the total plug resistance increases (both from resistivity and from the disproportionate barrier thickness). This motivates exploration of alternative fill metals (Co, Ru, Mo) for the smallest contacts. - **Seam/Void Formation**: Conformal deposition in very narrow features can create a vertical seam (interface where the two growth fronts meet). If the seam is not fully healed, it acts as a high-resistance defect. ALD W nucleation and optimized fill chemistries minimize seam formation. - **Fluorine Attack**: WF6 is highly reactive. Fluorine byproducts can attack the TiN barrier and underlying silicon, creating voids at the W/TiN interface ("volcano" defects). Adequate nucleation layer thickness and barrier integrity prevent this. Tungsten CVD Contact Fill is **the reliable, conformal via-filling workhorse** — connecting the nanoscale transistor contacts to the copper wiring network above through high-aspect-ratio vertical plugs that must be perfectly void-free to carry current without failure.