Cobalt stripping process integration for cobalt salicide residue improvement

In 0.11 um and larger technology node non-volatile memory process integration, undesired cobalt salicide residue formation is found to degrade ohmic contact resistant and cause severe yield loss. TiN/Ti/Co stack is applied in the process to get good CoSi₂ formation. The abnormal salicide residue formation is detected after cobalt stripping process which is applying a two step selective wet etching. First, the wafer is etched in H₂SO₄/H₂O₂ (SPM), rinsed in de-ionized water and then etched in NH₄OH/H₂O₂/H₂O (APM). It was found that the higher concentration of H₂O₂ in the SPM when the SPM life time is getting older caused this abnormal salicide residue formation. This paper presents the detail studies of this residue formation mechanism and works that have been carried out to optimize the selective wet etching process including the SPM & APM process sequence and the etching time. By implementing the optimum selective wet etching condition, we are able to suppress the cobalt salicide residue formation.