Tribological, Thermal and Kinetic Characterization of SiO₂ and Si₃N₄ Polishing for STI CMP on Blanket and Patterned Wafers

We investigated the tribological, thermal and kinetic aspects of SiO₂ and Si₃N₄ polishing on blanket and patterned wafers for STI CMP. Results showed the absence of anomalous tribological vibrational behaviors thanks to synergies between the colloidal CeO₂-based slurry and application-specific conditioner. Removal rates for the two processes showed non-Prestonian behavior as both mechanical and chemical factors were at work. However, Si₃N₄ was much more non-Prestonian than SiO₂. As expected, Si₃N₄ polishing resulted in COFvalues that were approximately one-half of their SiO₂ counterparts resulting in high SiO₂-Si₃N₄ removal rate selectivity. A modified Langmuir-Hinshelwood model was used to simulate removal rates allowing us to conclude that the process was mechanically-limited for SiO₂ and highly chemically-limited for Si₃N_4. Patterned wafer polishing time traces showed that COFcould be utilized as a real-time indicator for end-point detection and that, after 6 min of polishing, we observed the total removal of SiO₂ with a hard stop on Si₃N₄. End-points reached were also consistent with our blanket wafer polishing data. Regardless of pattern density and pitch, SiO₂ removed was not proportional to polish time. This was a result of the low colloidal ceria nano-particle content in the slurry which was explained via a phenomenological model.