Development of Cu-Cu Side-by-Side Interconnection Using Controlled Electroless Cu Plating

In this research, traditional head-to-head interconnection is replaced by side-by side interconnection. Instead of head-to-head interconnection aligning pillars vertically, side- by-side interconnection means pillars from the top and bottom dies are adjacent each other. With side-by-side interconnection, the geometry advantage of domed-shaped pillars will remain due to the natural curvature of the cylinder’s sidewall. Therefore, side-by-side interconnection presents a joining section with a low void ratio, which contributes to outstanding mechanical and electrical properties. Furthermore, the distance between pillars will be the same, making the bonding time predictable through precise chip alignment. Besides, hydrogen bubbles, byproducts of electroless Cu reaction, will affect the electroless plating reaction seriously. Thus, it is important to dislodge the hydrogen bubbles by the flow of electroless plating solution in order to present a uniform bonding result. The key factor of bubble dislodgment will also be discussed in this research, which can predict the flow condition of the electroless plating solution. It is concluded that a side-by-side Cu pillar interconnection using controlled electroless Cu plating combined the advantages of flat-topped and dome-shaped pillars.