Electrodeposition of indium for bump bonding

Indium bump bonding has been employed to realize very high density interconnection used for hybrid pixel detector systems. The connection pitch sizes to achieve this may be below 50 μm for these assembly applications, such that the packaging density, i.e. I/Os, may exceed 40,000/cm². Electrodeposition is a promising approach to enable a lowcost and high yield bump bonding process, compared with conventional sputtering or evaporation which is currently utilized for small-scale production. This paper reports an initial investigation of the alternative electroplating indium bumping process focusing on the challenge of this process in terms of uniformity and consistency across the wafer at ultra-fine pitches to achieve the highest yield. Fundamentally, it has been concerned that the current distribution and associated mass transport during the electroplating process are the main factors for determining the indium deposit growth. Therefore, in this study, the effects of pulsed current and ultrasonic agitation on the deposit growth are investigated through an initial plating trial. Indium bumps with finer grain size have been achieved by using pulsed current plating, which demonstrated the potential improvement of uniformity. It is known that ultrasonic agitation is beneficial to enhance the mass transport, and is expected to be able to penetrate the solution into the patterned apertures formed by the photo-resist at a microscopic scale. The experimental study has shown a positive effect of ultrasonic agitation in terms of an increase of pre-wetting of the ultra-fine apertures with electrolyte. However, caution must be taken since there may exist a danger of damaging the photoresist layer due to severe vibration induced by the ultrasonic waves. Future work will consider the possible optimization of the waveform of the pulsed current and utilization of higher frequency ultrasonic agitation with a robust photoresist mask.