| The stability of colloidal dispersions is a critical parameter in many industries such as paints and pigments, minerals processing and electronics. Particle settling is often caused by the shielding of surface charges on the particles which otherwise would prevent coagulation and subsequent settling. This is particularly a problem in high ionic strength dispersions, where large amounts of ions serve to enhance the charge shielding and compression of the electrical double layer around the particles.; This phenomenon has been investigated for industrially significant slurries used for tungsten and copper chemical mechanical polishing (W-CMP and Cu-CMP). It has been found that the effects of addition of conventional stabilizing agents (e.g., ionic surfactants, polymers) to these high ionic strength slurries are neutralized by the electrolytes in solution. However, the synergistic combination of a properly chosen ionic and nonionic surfactant has been found to be a suitable stabilizing agent for this type of system.; The factors influencing this stabilization mechanism have been examined, yielding a robust model for stabilization of chemically complex slurries. The use of a relatively hydrophobic nonionic surfactant in the mixture yields optimal stability, with increasing hydrophobicity originating from either an increase in the hydrocarbon chain length or a decrease in the length of the ethoxylated chain. The increased stability with hydrophobicity of nonionic surfactant suggests that the partitioning of nonionic surfactant out of aqueous solution is a more important factor than the enhanced steric stabilization brought about by increasing the length of the polymeric polar group.; The influence of dispersion stability on polishing performance has been correlated. First, the use of stable dispersions is found to prevent particle agglomeration. Next, the use of stable dispersions is found to have little effect on the polishing rate of blanket tungsten wafers. The surface quality (or planarization) is found to increase by adding surfactant, although it, does not necessarily correlate with dispersion stability. The polishing performance is explained as due to a lubricating layer of surfactant film on the particles. (Abstract shortened by UMI.)... |
