tungsten plug

Tungsten Plug Overview Tungsten (W) plugs are metal fills used for contacts and vias connecting transistors to the first metal layer and between lower metal layers. Tungsten was the original contact/via fill metal and remains in use for specific applications. Why Tungsten? - Excellent CVD Fill: W fills high-aspect-ratio contact holes void-free using WF₆ + H₂ CVD chemistry. - No Diffusion Risk: W does not diffuse into silicon or dielectrics (unlike Cu). - Good Adhesion: TiN/Ti liner provides excellent adhesion and contact resistance to silicon. - Established Process: Decades of manufacturing experience with high reliability. Process Flow 1. Etch contact holes through ILD to expose S/D or gate. 2. Deposit Ti adhesion/contact layer (~5-10nm) by PVD. Forms TiSi₂ at silicon interface for low contact resistance. 3. Deposit TiN barrier (~5-10nm) by CVD or PVD. Prevents WF₆ attack on Ti and silicon. 4. CVD Tungsten: WF₆ + H₂ → W + 6HF. Nucleation layer deposited first, then bulk fill. 5. CMP or etchback to remove W overburden, leaving W only in the contact holes (plugs). W vs. Co vs. Cu for Contacts - Tungsten: Reliable fill, high resistivity (5.3 μΩ·cm), thick TiN liner consumes contact volume. Standard through ~10nm node. - Cobalt: Lower contact resistance at < 20nm dimensions (thinner/no barrier). Used at 10nm+ (Intel, TSMC). - Cu: Lowest bulk resistivity but requires barrier, not used at contact level. Applications - Contact plugs to source/drain and gate (legacy and mature nodes). - Via plugs between metal layers (M1-M2 at older nodes). - Wordline and bitline in DRAM and 3D NAND flash memory.