CVD tungsten is universally selected over physical vapor deposition (PVD) metals for contact plugs because surface-reaction-controlled CVD provides the necessary step coverage for high-aspect-ratio vias .
The W deposition step serves as the primary gap-fill process to form conductive contact plug
s, bridging the underlying silicon source/drain or gate regions with the subsequent first metal layer (MET0) . Prior to this step, a Ti/TiN stack is deposited to provide an ohmic contact interface and act as a diffusion barrier . Without this conformal TiN barrier, the highly reactive tungsten hexafluoride (WF6) precursor used in W deposition would aggressively attack the underlying silicon or silicide, leading to severe device degradation [P1, P3]. Following the complete filling of the contact holes, W chemical mechanical planarization (CMP) is performed to remove the excess metal overburden, isolating individual contact plugs . Optimized contact structures and low-resistance plug materials like tungsten are strictly required in advanced nodes to prevent unacceptable increases in contact resistance [A1, P2]. The physical deposition of tungsten relies on chemical vapor deposition (CVD), which is generally divided into a nucleation phase and a bulk fill phase . During nucleation, silane (SiH4) is commonly used as the reducing agent, following the overall reaction: WF6 + 3/2 SiH4 -> W + 3/2 SiF4 + 3H2 . This SiH4-based reduction provides fast deposition with minimal incubation time on the TiN barrier . However, to improve conformality in deep submicron contacts and prevent top-overgrowth, pulsed CVD can be employed . This process introduces an ALD-like time-sequenced reaction mechanism that alternates WF6/SiH4 exposures with inert gas purges, which suppresses undesirable gas-phase reactions and enhances surface mobility . The bulk fill stage subsequently relies on the H2 reduction of WF6, providing high throughput and excellent conformality for the remaining plug volume . CVD tungsten is universally selected over physical vapor deposition (PVD) metals for contact plugs because surface-reaction-controlled CVD provides the necessary step coverage for high-aspect-ratio vias . The choice of carrier gas fundamentally alters the deposition dynamics and mass transport . Carrier gases with lower thermal diffusivity, such as argon, favor reactant accumulation near the substrate, thereby increasing the deposition rate and yielding high-density films . Conversely, highly diffusive carrier gases like helium increase the formation of gas-phase intermediates that are swept out of the reaction zone, which can result in powdery, poorly adherent films . Process temperature and pressure act as the primary control levers, modulating the competition between these gas-phase fluid dynamics and heterogeneous surface reactions . At the 40nm node, structural dimensions restrict the planar footprint of contacts, sharply increasing localized contact resistance . To mitigate this limitation, advanced architectures occasionally employ non-planar, sloped, or V-shaped contact bottom profiles to increase the effective electrical contact area within the same lateral footprint [A1, A2]. Furthermore, as MOSFET channel lengths shrink, fundamental thermodynamic limits and subthreshold leakage impose strict constraints on device drive current . Consequently, minimizing parasitic contact resistance via an optimized, void-free W plug process is essential to maintaining the desired on-current to off-current (Ion/Ioff) ratio in nanoscale logic and image sensor devices [P2, T2].
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