Ta-based liner CMP removes exposed Ta/TaN barrier film from inter-level dielectric field regions, electrically isolating adjacent Cu interconnect lines in the MET5 layer .
In depth
This step follows the primary Cu CMP step, which clears the bulk Cu overburden and selectively stops on the Ta-based liner [P
1]. The purpose of the Ta-based liner CMP is to completely remove the exposed Ta/TaN barrier film from the inter-level dielectric (ILD) field regions, thereby electrically isolating the adjacent Cu interconnect lines in the MET5 layer . Because Cu readily diffuses into surrounding dielectrics and causes device failure, the Ta-based liner must remain intact within the trenches and vias, acting as an essential diffusion barrier . Completing this planarization step is a prerequisite for the subsequent post-CMP cleaning and the deposition of the ILD 5-1 and 5-2 layers, ensuring a defect-free and flat surface for reliable multi-level interconnect stacking . Barrier CMP operates on a synergistic "chemical passivation–mechanical removal" mechanism . Unlike the bulk Cu CMP which relies heavily on the rapid chemical oxidation of copper, the Ta-based liner is chemically inert and its removal requires a specific balance of mechanical abrasion and surface modification . During polishing, the slurry chemistry induces surface electrochemical reactions, forming a nanoscale metal-oxide passivation layer on the liner surface . The formation rate of this passivation layer is governed by the oxidation kinetics and slurry chemistry, while its removal is controlled by abrasive concentration and mechanical stress applied by the polishing pad . Because Ta is highly resistant to chemical dissolution, its removal often leans more heavily on the mechanical component compared to Cu CMP, necessitating optimized pad pressures and abrasive interactions to shear off the passivated layer . Slurry selection for the Ta-liner CMP step must prioritize high selectivity between the Ta barrier, the exposed Cu lines, and the underlying dielectric . If the chemical removal rate of Cu significantly exceeds that of the Ta liner, severe dishing of the Cu lines and erosion of the dielectric can occur, degrading interconnect planarity . To mitigate this, barrier CMP slurries often incorporate specific inhibitors that selectively adsorb onto the exposed Cu surfaces, suppressing active copper dissolution while the Ta liner is mechanically cleared . Additionally, polishing pad design, such as using controlled segment structures or porous materials, can help distribute downforce evenly and suppress lateral deformation propagation . This mechanical optimization reduces local pressure concentrations, thereby minimizing over-polishing defects and ensuring uniform planarization across the wafer . At the 40nm technology node, the scaling of interconnect dimensions fundamentally alters the local contact mechanics and slurry transport dynamics during CMP . Removal rates and selectivities measured on unpatterned blanket wafers cannot be directly extrapolated to these high-density patterned structures . As the line width and spacing shrink, the process becomes increasingly sensitive to pattern density, requiring stringent control of the slurry's chemical formulation and the pad's mechanical properties to prevent integration failures . Although advanced sub-14nm nodes may explore alternative liners like ultra-thin Co to entirely replace Ta, Ta/TaN remains the robust standard for nanoscale BEOL integration due to its proven thermal stability and barrier efficacy against Cu migration .
Risks & Challenges
[High] Cu Dishing and Dielectric Erosion: Over-polishing or insufficient slurry selectivity causes the softer Cu and dielectric materials to be removed faster than the harder Ta liner . This leads to non-planar surfaces that complicate subsequent ILD deposition and increase sheet resistance variation (Engineering Practice).
[High] Incomplete Liner Removal (Stringers): Inadequate mechanical abrasion or insufficient oxidation kinetics prevents the complete clearing of the Ta-based liner in densely patterned or topographically recessed regions . Remaining conductive Ta residues can create electrical short circuits between adjacent MET5 lines (Engineering Practice).
[Medium] Galvanic Corrosion at the Cu/Ta Interface: During the CMP process, the electrochemical potential difference between the dissimilar metals (Cu and Ta) in the presence of an electrolytic slurry can drive localized corrosion . This results in contact recess or interfacial voiding, which degrades electromigration reliability and increases via resistance .
[Low] Polishing Pad Deformation-Induced Non-Uniformity: Under applied downforce, excessive lateral deformation of the polishing pad can cause uneven stress distribution across the wafer surface . This long-range deformation can propagate pressure gradients that exacerbate local over-polishing or under-polishing defects .