Study on the stabilization of alumina slurry for chemical mechanical polishing of copper

Abstract

In this thesis, stabilization of alumina slurries for chemical-mechanical polishing of copper(Cu CMP) in weak acidic media was investigated. The alumina particles of acidic slurry bring serious contamination, and recently organic acid has been employed in order to reduce contamination. Addition of citric acid leads to reduce alumina particulate contamination on copper films during Cu CMP, but it deteriorates stability of alumina slurry. To gain the required stability, alumina particles were milled untill the size was reduced up to a certain level. Alumina slurry was prepared by mixing alumina suspension with additives solution, which contained BTA, hydrogen peroxide and citric acid. The alumina particulate contamination on copper surface was more serious at pH4 than pH5. Likewise, the stability of alumina slurry in the presence of citric acid was deteriorated as the pH decreased. Then other experimental tests were conducted at pH5. ζ- potential was measured to elucidate the observed effects. In order to enhance steric stabilization, nonionic surfactant poly (ethylene glycol) was used. PEG does not generate bubbles during mixing $vis-\grave{a}-vis$ other surfactants. In case of alumina suspension at pH4 in the absence of citric acid, the concentration and molecular weight of PEG did not affect on the size of alumina particles in alumina suspension. For the alumina slurry containing citric acid, however, PEG had an influence on the particle size and the rheological behavior. The adequate amount of PEG improved the stabilization of slurry during polishing.