Effect of ionic surfactants on shallow trench isolation for chemical mechanical polishing using ceria-based slurries

As the development of technology nodes proceeds to 7 nm node, chemical mechanical polishing (CMP) slurries for shallow trench isolation (STI) cannot fully meet the technical requirements. Higher goals are put forward for the polished surface qualities and the removal selectivity control. The polishing liquid exhibits issues such as easy agglomeration, removal rate of Si3N4 exceeding 50 angstrom/min, removal selectivity ratio of SiO2/Si3N4 below 20, increased surface scratches and roughness of SiO2 and Si3N4 after polishing exceeding 1 nm. Here, attention is given to studying the STI CMP process by introducing various ionic surfactants in ceria slurries, aiming to control removal rates, selectivity, as well as surface qualities. The findings of ball milling and settling tests were used as a starting point for choosing the effective surfactants, searching the minimal delamination phenomenon. Then the impact of surfactants on removal rates, selectivity and surface characteristics were next investigated in polishing trials at various pH levels. Depth of scratches on polished wafers and corresponding surface roughness, as well as morphology of ceria abrasive particles were characterized. Action mechanisms of selected ionic surfactants in ceria slurries have been revealed by solid-liquid interface adsorption characterization, thermogravimetric analysis and zeta potential tests. Through research finding the addition of piperazine and 2-methylpiperazine surfactant can reduce the number and depth of scratches on the surfaces of SiO2 and Si3N4 after polishing, and it exhibits better dispersibility in alkaline environment compared to acidic environment.