Modification of Electrokinetic Behavior of CeO2 Abrasive Particles in Chemical Mechanical Polishing for Shallow Trench Isolation

Abstract

Shallow trench isolation (STI) has become a new technique to substitute local oxidation of silicon (LOCOS). Key ingredients to successful STI process are the achievement of well-dispersed abrasive ceramic particles having high oxide to nitride selectivity and little microscrathes on wafer. In this work, oxide to nitride selectivity and stability were manipulated with the modification ceria surface potentials. The electrokinetic behaviors of silica, silicon nitride, and ceria suspended in aqueous medium as a function of pH is shown in Figure 1. The pHiep, isoelectric points of pH, of silica, silicon nitride, and ceria have 3.5, 6.5, and 9.5, respectively. There are strong attractive forces between silica and ceria due to electrostatic attractive forces in a neutral suspension pH, which resulted in low removal rate of oxide. Figure 2 shows that the electrokinetic behavior of ceria particles modified with organic additives. The electrophoretic mobility is strongly dependent on suspension pH as well as surface-active additives. The ceria particles were found to have a native pHiep near 9.5 shown in Figure 1. Below this pH, particles carry a net positive charge. However, with modification of surface potentials, the pHiep of ceria shifted toward acidic pH, resulting in an increasing negative surface potentials at a neutral pH around 5-7. This surface modification resulted in an increase of oxide removal rate with modified ceria particles due to mutually repulsive electrostatic forces between modified ceria and silica. However, modified ceria particles dispersed in a neutral pH where the surface potentials of silicon nitride is minimal as near a pHiep of silicon nitride, the removal rate of nitride is sharply reduced and thus increased oxide and nitride selectivity about 1:50 listed in Table 1.