We report on the use of mixed abrasive slurries (MAS) containing alumina and ceria abrasives for chemical mechanical planarization (CMP) of silicon dioxide and silicon nitride films for shallow trench isolation applications, extending an earlier investigation of alumina/silica MAS for the CMP of copper and tantalum films. These slurries show a polish rate selectivity between oxide and nitride films that is as high as 65 and show an excellent surface quality even without additives. Analysis of dried slurry particles using transmission electron microscopy indicates formation of a sheath of smaller ceria particles around larger alumina particles. Possible explanations and supportive arguments for the improved performance of MAS during CMP are presented based on the particle-particle and particle-film interactions. (C) 2003 The Electrochemical Society.