UBM Formation on Single Die/Dice for Flip Chip Applications

This thesis presents the low cost process for UBM formation on aluminum pads of single die/dice for Flip Chip applications. The UBM (Under Bump Metallurgy) is required in solder bump structure to provide adhesion/diffusion barrier layer, solder wettable layer, and oxidation barrier layer between the bonding pads of the die and the bumps. Typically, UBM is deposited on the whole wafers by sputtering, evaporation, or electroless plating. These deposition techniques are not practical for UBM formation on single die/dice, thus preventing Flip Chip technology to be applied in applications where the whole wafers are not available. The process presented in this thesis has been developed to overcome this problem. The developed UBM formation process allows the UBM layer to be deposited on a single die, thus eliminating the requirement to have the whole wafer in the deposition process. With the combination of the UBM formation process developed in this work and a suitable bump formation technique, solder bumping on a single die can be achieved, thus making Flip Chip technology available for use in low volume applications and prototyping stages.

The developed UBM formation process consists of two major steps; temporary die attach process and UBM deposition process. The first process is developed using thermoplastic adhesive film. The second process is developed using electroless nickel plating, followed by gold immersion. It has been demonstrated in this thesis that the developed process can be used to form the UBM layer on the die successfully regardless of the die size and the complexity of the die pattern, and that this process does not damage nor affect electrically the sensitive die.