Relevance of Zeta Potential for Wafer-Particle Interaction during the CMP Process
The electrostatic repulsion of slurry particles by a wafer surface is driving the success of a CMP process, as it directly prevents particle adhesion. Electrostatic interactions on the particle-wafer interface can be determined via zeta potential measurements. By exceeding a certain magnitude of the zeta potential values of wafer and particle, electrostatic repulsion and less particle adhesion on the wafer surface are expected. Electrophoretic light scattering and the measurement of the streaming current are used to measure the zeta potential of particles and wafer surface. Zeta potential analyses with Litesizer 500 and SurPASS 3 thus help to optimize slurry conditions and reduce the efforts in post-CMP cleaning.
The consideration of electrostatic interactions between slurry particles and wafer surfaces is crucial for the optimization of Chemical Mechanical Polishing or Planarization (CMP) processes. Already 25 years ago, the group of Prof. Srini Raghavan at the University of Arizona, Tucson, demonstrated the affinity of particles towards their adhesion on wafer surfaces. The zeta potential of the PSL particles is negative in the complete pH range, which was studied in these adhesion experiments while the wafer surface exhibits an isoelectric point (IEP, i.e., pH where ζ = 0 mV) at pH 3.7. The number of PSL particles that remained on the wafer surface after dipping it into a dispersion of 5000 particles/ml at various pH followed by spin-drying was counted and correlates well with the zeta potential of the wafer surface. At high pH both PSL particles and LPCVD nitride wafer are strongly negatively charged such that electrostatic repulsion prevents the deposition of particles. At a pH of the particle dispersion in the proximity of the IEP of the wafer surface, a significant increase in the number of attached particles is observed.
In this report we apply a similar analytical approach to a silicon oxide wafer and a commercial CMP slurry and discuss the relevance of the zeta potential for both slurry particles and the wafer surface for the success of a CMP process.
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