Mechanism of dissolution and planarization of copper/tantalum films during chemical-mechanical polishing

Copper CMP under highly acidic conditions leads to severe corrosion problems while under alkaline conditions Cu polish rate selectivity with respect to SiO₂ is unfavorable leading to ILD erosion. Thus an intermediate pH condition (3–7) is better for Cu CMP. This work investigates the mechanism of Cu removal during CMP in H₂O₂-glycine based slurries, the pH of which lies in the intermediate (3–7) region. The kinetics of hydroxyl radical trapping using UV/Visible absorption spectrophotometry with PNDA as the *OH trapping agent was investigated. The catalytic role of Cu2+-amino acid complex in the decomposition of hydrogen peroxide to yield *OH in the presence of several amino acids was demonstrated. The steady state *OH concentration was determined and shown to be high for a large number of hydrogen peroxide solutions containing different amino acids and added copper ions. The effect of BTA as an inhibitor for Cu dissolution without significantly sacrificing polish rate during CMP using H₂O ₂-glycine based slurries has also been demonstrated.; Copper cannot be directly integrated as the mainstream interconnect material owing to certain problems and an adhesion promoter/diffusion barrier (such as Ta) should be incorporated along with copper to realize copper interconnects in ULSI circuits. Hence chemical-mechanical polishing of tantalum was performed using alumina particles dispersed in chemical environments that are relevant to Cu CMP. The highest tantalum polish rate was obtained with the abrasives dispersed in DI water and, in all cases investigated here, the polish rate decreased with the addition of the oxidant. Rotating disc electrode studies showed an increase in the open circuit potential with time, saturating eventually, in the absence of abrasion, indicating the formation of a passive film. Nevertheless, in situ electrochemical measurements (with abrasion) show that the open circuit potential is lower during polishing. This indicates that the passive film is being removed, at least partially, during polishing exposing a less noble surface. The effect of pH on tantalum polish rate was also studied. Finally, the effect of slurry chemicals on copper dishing was also evaluated using patterned wafers. Dishing was found to be higher in cases where the chemical dissolution is higher.