Aluminum pad cleaning with diluted organic acids for CMOS image sensors

The aluminum-copper alloy reactivity in different acid solutions was studied by Scaning Electron Microscopy (SEM) observations and electrochemical methods using open-circuit monitoring and polarization curve measurements. In the mineral acid, the dissolution rate is significant due to the Al oxide and substrate dissolution. On contrary, in diluted oxalic acid, the dissolution rate is very low, and almost negligible in diluted glycolic acid. While in the oxalic solution, the dissolution of aluminum occurs, a passive behavior of the Al-glycolic acid interface is observed. This is confirmed by the OCP evolution and by the shape of the potentiodynamic curve. Indeed, an anodic plateau appears, which reflects the metal passivation . In addition, the cathodic shift of the corrosion potential reflects a nobler behaviour of Al in glycolic acid. Thus, according to thermodynamic considerations, the reaction of proton reduction cannot occur at these conditions of pH and potential. This can explain the very low dissolution of aluminum in glycolic acid solution.