Reverse selectivity – High silicon nitride and low silicon dioxide removal rates using ceria abrasive-based dispersions

We show that by adding poly(acrylicacid-co-diallyldimethylammonium chloride), a cationic polymer with a weight average molecular weight of about 4200 g/mole, to ceria-based dispersions, it is possible to achieve a silicon nitride removal rate (RR) of >100 nm/min and a silicon dioxide RR of <2 nm/min at pH 4 and 4 psi down pressure during chemical mechanical polishing. Furthermore, the RRs of the silicon dioxide films can be tuned by varying the polymer to abrasive weight ratio in the dispersion while the nitride RR is unaffected. We also characterized the role of adsorption of this polymer additive on ceria, silica and silicon nitride powders using zeta potential, adsorption isotherms, UV–Vis spectroscopy, contact angle, thermo-gravimetric analysis and friction coefficient measurements. Our results show that the polymer film formed on the ceria particle surface is strongly bound to it, survives use in polishing and appears to control its reactivity with the silicon dioxide surface in conjunction with electrostatic interactions.

Preferential removal of silicon nitride over silicon dioxide using 0.25 wt.% ceria abrasives and 0.32 wt.% poly(acrylicacid-co-diallyldimethylammonium chloride). At this concentration, the polymer adsorbs strongly on ceria, blocking the interaction of surface Ce3+ species with the oxide film and suppresses its removal but has no effect the nitride removal rate. At lower values of X, the ratio of the amount of polymer to ceria, the blockage is not complete.Figure optionsDownload high-quality image (273 K)Download as PowerPoint slide