Dispersion of Ceramic Powders for Chemical Mechanical Polishing in Interlayer Dielectric and Shallow Trench Isolation

Abstract

Chemical mechanical polishing (CMP) has become the most common technique used in wafer polishing for dynamic memory and microprocessor applications. This technique enables one to achieve a global planarization close to line width 0.25 mm or below. The CMP process is composed of a chemical effect from nano-sized ceramic particles and a physical effect from the pressed pad. CMP processes are conventionally carried out using abrasive ceramic particles dispersed in aqueous media. As a silica slurry for interlayer dielectric (ILD) CMP is prepared in the alkaline region to accelerate the chemical corrosion process of plasma enhanced tetraethylorthosilicate (PETEOS), Si is dissolved to the point where it will adversely affect the colloidal stability of the silica particles. Table 1 shows the amount of Si dissolution by ICP and converted to the ionic strength of silica suspension. The amount of Si 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. The mobility of silica particles decreases as increases of ionic strength modified by NaNO3. 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 Si. Shallow trench isolation (STI) is a relatively new technique that is replacing local oxidation of silicon (LOCOS) for the manufacture of 64MB semiconductor devices. Key ingredients to successful STI process are the achievement of well-dispersed abrasive ceramic particles having high oxide-to-nitride selectivity and producing few microscratches on the wafer. Oxide-to-nitride selectivity and stability were manipulated with the modification ceria surface potentials. The electrokinetic behavior of silica, silicon nitride, ceria and ceria with ionizable acrylic polymer added, as a function of pH, is shown in Figure 2. The isoelectric pH (pHiep) of silica, silicon nitride and ceria are 3.5, 6.5, and 9.5, respectively. This attraction results from the opposite electrostatic potential exhibited below pH 9.5 where silica is negatively charged and ceria is positively charged. This mutual attraction resulted in a low removal rate of oxide. The ceria particles were found to have a native pHiep near 9.5. However, with addition of the ionizable acrylic polymer, the pHiep of ceria shifted toward the acid side, resulting in an increasing negative surface potential at pH values between 5 and 7. This surface modification resulted in an increase of oxide removal rate due to the relative colloidal stability achieved relative to the ceria and silica particles.