Electrochemical Plating of Nano-Twinned Cu for WLP Applications

We developed a direct current, electrochemical plating process to control the formation of nano-twinned copper (nt-Cu) to fulfill the keen market interest for copper-to-copper direct bonding, or hybrid-bonding, for wafer-level packaging applications. Unlike other studies that require either pulse plating or high agitation for nt-Cu formation, this newly designed process can produce nt-Cu without pulse plating, and under low agitation conditions. The microstructure transition between the seed layer and nt-Cu formation happens in less than 0.50 μm. In this paper, we will discuss three different formulations that enable nt-Cu creation. We will also address the effects of current density (CD), additive concentration, nucleation density, agitation, and their relevance to the nt-Cu formation. We achieve close to 100% columnar grains of nt-Cu with Twin Boundary (TB) parallel to the substrate surface when deposited on (111) texture dominated Cu substrate. The strong interaction of additive with the copper seed layer plays a crucial role in the fast nt-Cu initiation and growth. Current density and nucleation density also play an essential role in the nt-Cu formation. Some level of additive adsorption is necessary to enable the critical nucleation density for a fast nt-Cu initiation, which increases with increasing current density. Nano-twinned Cu grain size decreases with rising deposition rate. However, further increases in deposition rate result in a slightly larger nt-Cu grain size. We can produce a uniform nt-Cu with a grain size of a few hundreds of nanometers when the current density is in the range of 30 to 60 mA/cm2, and additive concentration is 2.0 ml/L to 6.0 ml/L. Either strong or weak convection has minor effects on nt-Cu formation. With this process, an nt-Cu configuration is possible when plating regular pillar structures but also enables nano-twinned copper deposition for RDL lines and vias with recess.