Plasma process considerations for copper wire bonding

Plasma cleaning has been employed to increase the yield and reliability of gold wire bonding for many years in the IC packaging industry. However with the rising cost of gold, increasing numbers of manufacturers are transitioning to copper wire for the economic benefits. Bonding with copper wire is not without challenges; special equipment and different process parameters are required as copper wire is harder and is easily oxidised. Due to these properties of copper, obtaining an intimate contact between copper wire and bond pad can be difficult. Hence it is important to ensure that the bonding surface is clean of contaminants which could inhibit the bonding process. Bond pads with different surface materials exhibit different characteristics which necessitates different approaches to plasma treatment. Aluminium has a self-passivating property that helps it to be less susceptible to severe oxidation. Hence, the main concern for aluminium pads will be the removal of surface contaminants. On the other, hand copper is more susceptible to oxide formation. Therefore extra care must be taken to ensure a surface clean of both contaminants and oxides is achieved prior to the copper wire bonding process. This paper looks at the effects of plasma treatment for copper wire bonding, taking into account the prior considerations. It also discusses results which show that proper optimization of plasma process parameters is necessary to improve the uniformity of the wire bonding process. This is especially critical when trying to obtain a higher bonding yield for surfaces which exhibit poor bondability.