Study on undercutting of electroplated micro-bumps with different etchants

With the miniaturization of electronic packages, fine pitch interconnect technology becomes increasingly important and new fabrication techniques have to be developed. For micro-bumps with diameter less than 50μm, undercutting can occur at the seed layer etching step and it will affect the bonding strength of micro-bumps. In this paper we explore a number of methods to reduce and avoid undercutting by selecting different etchants formulations and concentrations for the Ti adhesion layer and Cu seed layer. Through a series of experiments, we observed the following results. 50% solution of 1NH₄OH (25%): 1H₂O₂ (30%) can be used as a Cu seed layer etchant. However at high concentrations the Cu pillar would be attacked. 2CH₃COOH (50%): 1H₂O₂ can also be used to etch the Cu seed layer. And diluted 1HNO₃ (50%): 1HF (40%) can be used to etch the Ti adhesion layer. Etching results are very satisfactory. By careful adjustment of etch recipe we have obtained Cu/Sn micro-bumps having a 30μm in diameter and 50μm in pitch with minimal undercutting.