Pattern evolution studies in STI CMP via real-time shear and down force spectral analyses

This study explores the transition of force spectral fingerprints of STT CMP during early evolution of wafer topography and layer transition from silicon dioxide to silicon nitride. Polishing was done with a novel polisher and tribometer capable of measuring shear force and down force in real-time. Fast Fourier Transformation is performed to convert the force data from time to frequency domain and to illustrate the amplitude distribution of the force. Such frequency spectra provide in-depth insights into the interactions among abrasive particles, pad and wafer. STI patterned wafers are over-polished using cerium oxide slurry. Results show that shear force increases during polishing when the HDP oxide layer is removed thus exposing the Si₃N ₄ layer. Unique and consistent spectral fingerprints are generated showing significant changes in several fundamental peaks during the early evolution of wafer topography and subsequent layer transition to silicon nitride polishing. Variance of force is also plotted to show the progression of pattern evolution. Results show that a combination of unique spectral fingerprinting and analysis of force variance (based on shear and down force) can be used to monitor in real-time the polishing progress during STI CMP.