The Effect of Si Dissolution on the Stability of Silica Particles and its Influence on Chemical Mechanical Polishing for Interlayer Dielectric

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. Under alkaline conditions above pH 9, silica solubility sharply increases and the silica layer is dissolved with the formation of silicic acid. 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. 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 Si4+. As silica surface modified with surface-active additives, the decrease of surface potential above pH 9 disappeared and resulted in an increase the stability of silica particles dispersed in alkaline region. Our experiment results also showed that microscratches on the wafers with modified slurry were reduced and thus the removal rate in CMP process was increased.