By precisely tuning the abrasive concentration and the slurry pH, engineers can balance the passivation rate against the mechanical removal rate to optimize planarization efficiency and selectivity .
In the manufacturing of nanoscale back-end-of-line (BEOL) interconnects, the chemical mechanical
planarization (CMP) process is typically divided into two major steps: bulk copper removal and barrier planarization [P1, P2]. Following the first step where the copper overburden is cleared and polishing selectively stops on the liner, the Ta-based liner CMP step is executed to remove the remaining barrier metal layer across the field regions . This process electrically isolates adjacent copper interconnects by thoroughly exposing the underlying inter-layer dielectric . Because this specific step resides within the Direct Bond Interconnect (DBI) module for a CMOS Image Sensor, it holds distinct importance compared to standard lower-level metal CMP steps (Engineering Practice). Specifically, this barrier removal must yield an exceptionally planar and defect-free surface to establish the foundation for the subsequent Final Oxide CMP, which fundamentally dictates the success of the hybrid CIS/ISP wafer bond pairing . The physical removal of the Ta-based liner relies on a highly coupled chemical passivation and mechanical abrasion mechanism . When exposed to the oxidizer-rich slurry, the Ta/TaN material undergoes surface electrochemical reactions, rapidly forming a nanoscale metal-oxide passivation layer . The formation rate of this passivation layer is governed by the corrosion potential of the barrier material, the oxidation kinetics, and the specific slurry chemistry . Simultaneously, mechanical stress applied through the polishing pad and abrasive particles removes this chemically softened oxide layer . The overall material removal rate adheres to Preston's law, scaling directly with the applied contact pressure and relative rotational velocity . By precisely tuning the abrasive concentration and the slurry pH, engineers can balance the passivation rate against the mechanical removal rate to optimize planarization efficiency and selectivity . The selection of a dedicated Ta-based liner CMP step, distinct from the main Cu CMP, is driven by the fundamentally different electrochemical properties of copper and tantalum . Tantalum and tantalum nitride are employed because they serve as excellent diffusion barriers, preventing copper ions from migrating into the adjacent low-k dielectrics and causing transistor malfunction . However, because Ta is significantly harder and more chemically inert than Cu, it requires a specialized slurry formulation with different abrasives and chemical additives to achieve the necessary removal rate without destroying the copper . Slurry chemistry parameters, such as the concentration of oxidizers like hydrogen peroxide, must be carefully controlled; increasing the oxidizer accelerates the barrier passivation rate, thereby increasing the overall CMP removal rate . Conversely, if the chemical dissolution of the neighboring copper is not actively suppressed by appropriate inhibitors during this step, severe galvanic corrosion or metal recess will occur at the Cu/Ta interface . At the 40nm technology node, the reduced line widths and tight spacing introduce profound pattern density challenges for the barrier CMP process . As interconnect dimensions shrink, the local contact area and stress distribution become highly non-uniform, causing the planarization behavior to heavily depend on pattern geometry rather than blanket material properties . This geometric sensitivity amplifies the risks of dishing in wide copper lines and erosion in dense interconnect arrays . Minimizing these topographical defects during the Ta-based liner CMP is absolutely critical in the DBI flow, as any localized step-height variations will prevent the intimate atomic-level contact required during the final face-to-face or face-to-back hybrid bonding process .
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