CMP of polyimide for low-k dielectric application in ULSI

Polyimide CMP is investigated for its feasibility in IMD planarization applications. The polish rates of polyimide are found to be heavily dependent upon the degree of imidization and hydroxyl activity in silica-based alkaline slurry. TMAH, tetra-methyl-ammonium hydroxide, added into the slurry enhances the removal rate of polyimide due to the improved wettability on the hydrophobic polyimide surface. Surface planarity is degraded during CMP, but can be significantly improved by a curing after CMP. By means of bias-temperature-stress analysis, it is found that mobile ions, like K⁺ and Na⁺ , do not diffuse into the bulk of the polished film. Dielectric constant and leakage current density of polyimide being polished do not deteriorate, indicating polyimide directly capped with an oxide layer is promising for use as IMDs