Ta-based liner CMP - 40nm

The chemical oxidation and repeated mechanical scratching lower the local yield threshold of the barrier material, allowing the abrasives to plastically deform and remove the Ta-oxide film under applied pad pressure .

After the preceding bulk Cu CMP step removes the thick copper overburden, the u

nderlying Ta-based liner (typically a Ta/TaN stack) remains exposed on the field dielectric . This Ta-based liner CMP step is required to selectively remove this residual conductive barrier, thereby electrically isolating the adjacent MET6 copper lines and preventing short circuits . As part of the MET6 module in a 40nm BSI CMOS Image Sensor flow, this step is distinct from the earlier liner CMP steps (such as those for MET1 through MET5) because MET6 frequently handles wider power routing architectures and serves as the topmost intra-metal layer before final passivation or wafer bonding . Consequently, it demands uniquely strict control over global planarity and minimal dishing across exceptionally large Cu pattern areas (Engineering Practice). The successful completion of this step provides a highly planar, defect-free surface that is necessary for the subsequent Post CMP Cleaning and ILD 6-1 Deposition processes . The fundamental operation of barrier CMP is governed by a tightly coupled "chemical passivation–mechanical removal" mechanism . In the presence of oxidizing agents within the slurry, the Ta-based liner undergoes surface electrochemical reactions to form a nanoscale metal-oxide passivation layer (e.g. , TaOx) . Because elemental Ta is chemically inert and highly resistant to spontaneous wet etching, its bulk removal cannot be achieved by chemical dissolution alone . Instead, the process relies on the continuous mechanical shearing of this oxidized passivation layer by abrasive particles, exposing fresh metal that is immediately re-oxidized . At the microscopic level, this mechanical removal functions as a nanoscale selective plastic deformation and micro-cutting process . The chemical oxidation and repeated mechanical scratching lower the local yield threshold of the barrier material, allowing the abrasives to plastically deform and remove the Ta-oxide film under applied pad pressure . The material and method selections for this step are driven by the need to achieve high removal selectivity of the Ta liner over the exposed Cu lines and the underlying dielectric . Standard Cu CMP slurries prioritize Cu oxidation and are generally ineffective at removing Ta liners, necessitating a specialized barrier slurry . These barrier slurries employ tailored pH levels and specific oxidizers to accelerate the passivation rate of Ta, while concurrently utilizing chemical inhibitors (such as benzotriazole) to form a protective chemisorbed layer on the exposed Cu . This chemical formulation suppresses further Cu dissolution while harder abrasive particles maintain a high mechanical removal rate for the tough Ta layer . By tuning the abrasive concentration and the mechanical downforce, process engineers can modulate the balance between the passivation rate and the mechanical removal rate, which dictates the ultimate CMP selectivity and minimizes surface defectivity . At the 40nm technology node, the extreme miniaturization of interconnect structures means that local contact mechanics and slurry mass transport become highly sensitive to pattern density . Material removal rates and selectivities determined on blanket wafers cannot be directly extrapolated to patterned device structures, as varying line-width geometries alter the localized mechanical stress distribution . During MET6 processing, attempting to clear Ta residues in dense array regions often requires over-polishing, which risks accelerating localized Cu dishing and dielectric erosion . Ensuring complete Ta barrier removal without inducing nanoscale contact recesses or excessive copper smearing is therefore critical to maintaining the structural integrity and long-term electromigration reliability of the device .

[High] Cu Dishing and Dielectric Erosion: Because the Ta-based liner is mechanically tougher than the Cu interconnects, the necessary over-polishing step can cause the softer Cu lines and adjacent inter-level dielectric to be abraded at a faster rate than the barrier itself . This pattern-dependent topography degradation compromises the global planarity required for subsequent dielectric deposition .
[High] Ta Liner Residues (Stringers): If the mechanical shear force from the abrasives is insufficient to overcome the passivation layer formation rate, incomplete removal of the Ta oxide film occurs . These conductive Ta residues remain on the field dielectric and can create direct short circuits or leakage paths between adjacent MET6 interconnects .
[Medium] Galvanic Corrosion at the Cu/Ta Interface: The differing electrochemical corrosion potentials between the exposed copper and the Ta-based liner can establish a localized galvanic cell in the conductive, oxidizing slurry . This effect accelerates the dissolution of copper at the boundary, forming interface trenches that degrade the electromigration resistance of the interconnect .
[Low] Nanoscale Scratching and Plastic Deformation: The requirement for high mechanical stress and concentrated abrasive particles to remove the resilient Ta liner lowers the localized Hertzian contact pressure threshold for plasticity . Non-uniform abrasive agglomeration or excessive downforce can cause severe micro-cutting, leaving deep scratches in both the soft Cu lines and the dielectric matrix .

After the preceding bulk Cu CMP step removes the thick copper overburden, the u

[High] Cu Dishing and Dielectric Erosion: Because the Ta-based liner is mechanically tougher than the Cu interconnects, the necessary over-polishing step can cause the softer Cu lines and adjacent inter-level dielectric to be abraded at a faster rate than the barrier itself . This pattern-dependent topography degradation compromises the global planarity required for subsequent dielectric deposition .
[High] Ta Liner Residues (Stringers): If the mechanical shear force from the abrasives is insufficient to overcome the passivation layer formation rate, incomplete removal of the Ta oxide film occurs . These conductive Ta residues remain on the field dielectric and can create direct short circuits or leakage paths between adjacent MET6 interconnects .
[Medium] Galvanic Corrosion at the Cu/Ta Interface: The differing electrochemical corrosion potentials between the exposed copper and the Ta-based liner can establish a localized galvanic cell in the conductive, oxidizing slurry . This effect accelerates the dissolution of copper at the boundary, forming interface trenches that degrade the electromigration resistance of the interconnect .
[Low] Nanoscale Scratching and Plastic Deformation: The requirement for high mechanical stress and concentrated abrasive particles to remove the resilient Ta liner lowers the localized Hertzian contact pressure threshold for plasticity . Non-uniform abrasive agglomeration or excessive downforce can cause severe micro-cutting, leaving deep scratches in both the soft Cu lines and the dielectric matrix .

40nm BSI CMOS Image SensorPreview

Process Cross-Section

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In depth

Risks & Challenges

40nm BSI CMOS Image SensorPreview

Process Cross-Section

Step highlight

In depth

Risks & Challenges