Electrochemical deposition and characterization of Copper Pillar Bumps — Application towards Flip chip bonding

Abstract

The pre-processes targeting Flip Chip Assembly of copper pillars consists of spin coating of photo resists, direct writing, development of the photo resist, electroplating. The Flip chip assembly is targeted to verify the fine pitch bonding that is practically possible. Hence a unique design consisting of different pitch dimensions and pillar diameters are designed. In order to achieve the minimum possible pitch in the flip chip bonding, various experiments with the pre-processes viz., spin coating with different resists, direct pattern writing with different laser powers and other parameters, electroplating of copper (Cu) with varying parameters, are conducted to see the effect of each of these processes in practically achieving successful flip chip bonding/assembly with minimum pitch and minimum diameter design. This paper describes Copper pillar deposition using acid copper electrolyte by electro chemical deposition. A positive photo-resist AZ9260 up to thickness of $16\ \mu\mathrm{m}$ and smallest feature of diameter $10\mu\mathrm{m}$ were fabricated by multi step spin coating. Spin coating and lithography parameters were optimized. Electroplating of copper pillars was carried out by pulse plating and pulse reverse technique. The deposited copper pillars were characterized by Con-focal microscopy, X-Ray Diffraction and Scanning Electron Microscopy. Electrical characterization of Copper Pillars was done by four probe method. The targeted gap (between two pillars) dimensions are $10\mu\mathrm{m},\ 20\mu\mathrm{m},\ 30\mu\mathrm{m}$ and $40\mu\mathrm{m}$ with the diameter dimensions of $10\mu\mathrm{m},\ 15\mu\mathrm{m},\ 20\mu\mathrm{m}$ and $25\mu\mathrm{m}$.