Mechanics Framework of Pad Scratching in Chemical-Mechanical Planarization

Abstract

Chemical-Mechanical Planarization (CMP) is a crucial intermediate process in integrated circuit (IC) fabrication. As the average IC feature size decreases each year, scratches produced on wafers from polishing pads during CMP have become a prominent issue. These scratches can be much larger than features on the circuits, which results in an increase of damaged and discarded wafers after CMP. To determine the mechanisms of CMP pad scratching, an analytical model based in thermomechanics was constructed. This model accounted for potential sources of energy dissipation, which are important in properly understanding and accounting for processes which produce damage on the wafer. Multiple forms of energy dissipation were investigated through experimental analyses that include strain energies, energy flux through material removal, and internal energy dissipation. Mechanical characterization of CMP pad material and planarization experiments were performed. Porous polyurethane polishing pad material was mechanically characterized in compression and in tension to properly model its constitutive behavior during CMP. Stress relaxation experiments of the pad material in compression were performed to construct a stress relaxation model. The results showed that viscoelastic strain energy is significant in CMP processes. Additionally, the polishing pad relaxation behavior varied depending on whether the pad was loaded in tension or compression. The viscoelastic relaxation rate of the pad material increased after being soaked in distilled water and CMP slurry. This result suggests pad scratching can be reduced with proper soaking of the pad material. Measurements of pad compressibility were also performed by optically tracking pad samples which were under compressive stresses. This analysis quantified the significance of the pad material’s compressibility with regard to its internal stresses. Planarization experiments were performed to gather experimental information on mechanical and topographical changes of the pad and wafer materials through the CMP process. Generally, the coefficient of friction between the pad and wafer material decreased with increasing polishing time. From the constructed contact stress analysis, a smaller coefficient of friction resulted in the reduction of von Mises stress in the wafer material. The surface of a polishing pad became smoother with increasing polishing time. Experimental planarization results showed that roughness of the pad surface may have an influence on the coefficient of friction, while both of these variables relate to the scratching ability of the pad material. Plastic smoothing of the pad surface also indicated that the plastic strain energy dissipated into the pad material is important for the CMP thermomechanical model.