A robust nucleation layer prevents WF6 from diffusing through the TiN and reacting with the underlying silicon or titanium, which would otherwise cause severe structural damage .
In the CONTACT module, tungsten (W) deposition follows the formation of the Ti and TiN layers to complete the metal pl
ug that connects the silicon substrate to the first metal routing layer . Tungsten is selected for contact plugs due to its excellent conformality when deposited via chemical vapor deposition (CVD), allowing it to fill high-aspect-ratio holes without voids . The preceding Ti layer acts to lower the Schottky barrier and form an Ohmic contact at the silicon interface . Meanwhile, the intermediate TiN layer serves as a diffusion barrier to protect the underlying materials from reactive gases during the W deposition . After the contact holes are completely filled with bulk tungsten, the subsequent W CMP step is required to remove the overburden metal on the field dielectric, thereby isolating the individual contact plugs . Tungsten deposition relies on the thermal chemical reduction of tungsten hexafluoride (WF6) and is typically divided into two distinct phases: nucleation and bulk fill . Initially, a thin W nucleation layer is grown using silane (SiH4) or diborane (B2H6) as a reducing agent, which provides fast deposition with minimal incubation time on the TiN barrier . This nucleation layer acts as a seed for the subsequent bulk fill process, which primarily utilizes the hydrogen (H2) reduction of WF6 . The H2-based reduction provides excellent step coverage and continuous bulk filling, but the reaction must be carefully separated from the underlying layers . A robust nucleation layer prevents WF6 from diffusing through the TiN and reacting with the underlying silicon or titanium, which would otherwise cause severe structural damage . Chemical vapor deposition is the standard method for W fill because traditional physical vapor deposition cannot achieve the required step coverage in narrow, deep contacts . To balance throughput and conformality, modern processes often employ pulsed CVD or atomic layer deposition (ALD) techniques for the nucleation layer . By cyclically separating the WF6 and SiH4 exposures with inert gas purges, pulsed CVD suppresses gas-phase reactions and promotes surface-controlled kinetics, significantly improving sidewall coverage and preventing top-overgrowth . The choice of precursor gases, deposition temperature, and pressure directly determines the crystalline phase (such as low-resistivity alpha-W versus metastable beta-W), grain size, and ultimate contact resistance . Additionally, controlling the W bulk fill parameters is critical to managing intrinsic film stress, as excessive tensile stress from the bulk W can induce structural delamination during the subsequent CMP process . At the 40nm node, contact aspect ratios are quite high due to the thick interlayer dielectrics required for isolation, making void-free W fill particularly challenging . Ensuring complete fill is paramount, as any seams or voids can trap CMP slurry in the next step, leading to high contact resistance or reliability failures . Furthermore, careful tuning of the nucleation layer thickness is necessary: it must be thick enough to uniformly protect the TiN/Ti stack from WF6 attack, yet thin enough to maximize the volume available for the lower-resistivity bulk W fill .
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