In PEALD processes, precursor molecules undergo surface self-limiting adsorption and react with plasma-activated atomic hydrogen to form dense, highly conformal films at low temperatures, enabling atomic-scale thickness control .
In depth
Following the PMD oxide etch and subsequent clean processes, the Ta
-based liner deposition serves as the foundational barrier layer for the Metal 1 (MET1) copper interconnect . Copper is utilized as the primary conductor in modern interconnects due to its low resistivity, but its high diffusivity dictates that it will drift or diffuse into the surrounding dielectric without a conformal barrier, leading to catastrophic circuit shorting . To mitigate this, a Ta-based stack—typically consisting of a TaN barrier followed by a metallic Ta liner—is deposited into the etched trenches and vias prior to the copper seed deposition [P3, P4]. This specific MET1 deposition step is distinct from subsequent BEOL Ta-based liner steps (such as steps 171 and 187) because it often interfaces directly with the pre-metal dielectric (PMD) and lower-level contacts (e.g. , tungsten plugs), whereas higher-level metal liners interface with softer, highly porous low-k inter-metal dielectrics (IMD) . The physical mechanism of the Ta-based stack relies on microstructural and chemical properties to inhibit copper transport and ensure structural integrity . Tantalum nitride (TaN) is conventionally adopted due to its superior capability of blocking Cu diffusion, functioning primarily through a dense, often amorphous structure that minimizes continuous grain boundary diffusion pathways [P2, P3]. However, because the physical adhesion of Cu to TaN is not ideal, a thin metallic Ta layer is integrated directly between the TaN and Cu layers . This metallic Ta liner lowers the interface energy with the subsequent copper layer, maintaining good adhesion and establishing a stable mechanical boundary . Deposition at advanced nodes often relies on highly directional ionized Physical Vapor Deposition (PVD) or Plasma-Enhanced Atomic Layer Deposition (PEALD) . In PEALD processes, precursor molecules undergo surface self-limiting adsorption and react with plasma-activated atomic hydrogen to form dense, highly conformal films at low temperatures, enabling atomic-scale thickness control . The selection of the Ta/TaN material system is driven by its excellent thermal stability, chemical inertness, and proven manufacturability in standard BEOL environments . However, process parameter optimization is heavily constrained by the physical dimensions of the ultra-scaled interconnect trench . Because the TaN/Ta bilayer is substantially more resistive than Cu, any excess thickness occupies a large portion of the cross-sectional area, which tremendously increases the total line resistance [P3, P4]. Consequently, the deposition parameters must precisely balance the minimum thickness required to maintain an unbroken diffusion barrier against the necessity of maximizing the volume available for the highly conductive copper fill . Factors such as deposition temperature, plasma power, and cycle times directly dictate the film's step coverage, mass density, and the potential inclusion of resistivity-increasing impurities (such as oxygen or halogens), representing a critical physical trade-off for device performance .
Risks & Challenges
[High] Copper Diffusion Shorting: Copper ions or atoms drift through the barrier into the dielectric material, resulting in inter-wire leakage or circuit shorting . This failure mode is typically triggered if the TaN barrier is locally thinned below its continuity threshold (e.g. , ~4-nanoscale for standard PVD films) at the bottom corners of high-aspect-ratio vias .
[Medium] Interconnect Resistance Degradation: Because Ta and TaN possess significantly higher intrinsic resistivity than copper, an excessively thick deposited liner stack displaces too much of the copper volume within the ultra-scaled trench . This physical reduction in copper cross-sectional area tremendously increases the total effective line resistance, degrading the RC time constant of the interconnect [P3, P4].
[Medium] Adhesion Failure and Electromigration: High interface energy between the copper fill and an oxidized or improperly formed Ta liner causes poor physical adhesion . Weak interfacial bonding accelerates copper atom mobility along the liner-copper boundary under high current densities, leading to early electromigration (EM) voiding and failure .
[Low] Impurity Incorporation: If chemical deposition techniques (like PEALD or CVD) are utilized, incomplete precursor dissociation or insufficient purge cycles can leave residual impurities such as chlorine or oxygen within the barrier film . These impurities disrupt the atomic density of the barrier and increase its electrical resistivity, compromising both its blocking capability and the electrical performance of the via .