Dispersion Property Evaluation of Chemical mechanical Polishing Slurry for Interlayer Dielectric and shallow Trench Isolation

Abstract

Chemical mechanical polishing (CMP) has become the most popular technique in dynamic memory and microprocessor, which enables us to achieve a global planarization close to 0.25mm or below. CMP processes are conventionally carried out using abrasive ceramic particles dispersed in aqueous media. As silica slurry for Interlayer Dielectric (ILD) CMP is prepared in alkaline region to accelerate the chemical corrosion process of Plasma enhanced tetraethyorthosilicate (PETEOS), the amount of Si ions from slurry are dissoluted at the point where it will adversely affect on the stability of silica. Table 1 shows the amount of Si4+ dissolution by ICP and converted to the ionic strength of silica suspension. The amount of Si4+ dissolution is equivalent to 0.07 M NaNO3, which reduces the surface potentials of silica particle from -9 to -7 (m2/V×s ×10-8) shown in Figure 1. Surface potentials of silica particles without the existence of passivated layer shows that silica suspension exhibited negative surface potentials at pH values above 3.5, and reached a maximum potential by pH 9. However, beyond pH 9, the surface potential decreases with increasing suspension pH. This effect is attributed to compressing of electrical double layer due to the dissolution of Si4+. Shallow trench isolation (STI) has also 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. 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 2. 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 3 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.