Abstract
Copper wire bonding offers several mechanical and electrical advantages as well as cost saving compared to its gold wire predecessor. In the recent years, driven by the increasing price of gold, copper wire material and copper wire bonding process are widely researched for the high end IC packaging in the IC assembly area. In this paper, characterization work has been done in order to understand how the copper wire bond process is impacted by the thickness of bond pad metallization. The experiment design is: bond copper wire (99.99%, 23um wire diameter) with same bonding recipe on the different thickness of bond pad metallization ranged from ultra thin (less than 0.8um) to ultra thick (greater than 4um). Most of the wafers that are tested are with BOA (Bond Over Active) structures. The bonding recipe is pre-optimized on the 90nm low-k wafer with moderate thickness of bond pad metallization. Key responses data were collected including pad crater, pad metal peeling as bonded, Aluminum push-out, and pad metal peeling in wire pull test after bonding. The result suggests there is an optimum range of pad metallization thickness to obtain satisfying Cu wire bond responses without additional parameter adjustment. In general, for typical Al/Cu metallization with similar BEOL process (sputtering, annealing, and etc), thinner bond pad showed higher risk of Si structure damage (crater, peeling) and thicker bond pad is able to withstand Cu wire bond to underlying structures but has higher risk of aluminum pushout caused failure (shorting or reliability failure). Adding tungsten to common Al/Cu metallization is able to improve the hardness of the pad, thus improve the copper wire bondability by reducing aluminum push-out. Aluminum pushout correlation with hardness of bonding pad metallization is studied also in this paper.
Published Version
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