inhibitors like triazole compounds are utilized to form a passivating protective film on the Ta-based liner, ensuring that the polishing process abruptly slows down once the bulk copper is cleared .
The Cu CMP step in the MET6 module serves to remove the electroplated bulk copper overburden and a
chieve global planarization, strictly stopping on the underlying Ta-based liner . As the upper metallization layer in a 40nm BSI CMOS Image Sensor flow, MET6 typically features wider global routing lines and optical shielding structures compared to the finer M1-M3 interconnects . Consequently, this specific CMP step must handle larger localized volumes of copper removal while heavily mitigating layout-dependent topography variations over wide trenches . This step isolates the Cu interconnects within the dielectric trenches and prepares the wafer for the subsequent dedicated Ta-based liner CMP step, which will ultimately expose the underlying ILD . The fundamental mechanism of Cu CMP is a highly coupled process of surface electrochemical oxidation and nanoscale mechanical abrasion . Oxidizers in the slurry chemically convert the metallic copper surface into a softer, removable oxidized state or soluble complex . Macroscopically, the material removal rate is often modeled by the Preston equation, scaling proportionally with applied pad pressure and relative sliding velocity . Microscopically, material removal is driven by selective plastic deformation; the sliding friction of the abrasives significantly reduces the Hertzian contact pressure required to induce plasticity in the copper substrate . Furthermore, repeated abrasive scratching and chemical corrosion introduce surface roughening and crystal defects that lower the local yield threshold, allowing the abrasives to micro-cut the weakened copper efficiently . Slurry formulation is the most critical control vector for this step, as it must provide extremely high Cu-to-barrier selectivity to prevent premature barrier breakthrough . Complexing agents, such as amino acids, are selected to form stable, soluble complexes with the oxidized copper to enhance the chemical removal rate at a neutral or slightly acidic pH . Simultaneously, inhibitors like triazole compounds are utilized to form a passivating protective film on the Ta-based liner, ensuring that the polishing process abruptly slows down once the bulk copper is cleared . To prevent mechanical defect generation, nonionic dispersants may be added to the slurry to improve the surface wettability of the silica abrasives and suppress large particle agglomeration via steric hindrance . At the 40nm node, interconnect structures exhibit pronounced pattern density effects, making the CMP process highly sensitive to local contact mechanics and non-uniform mass transport . Because copper is physically softer and chemically more reactive than the surrounding barrier and dielectric materials, wide structural lines are particularly vulnerable to metal dishing . This layout-dependent dishing effect directly reduces the conductive cross-sectional area, which subsequently increases line resistance and degrades the electrical RC performance of the global routing network .
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