WLCSP Descum Process Criticality and its Impact on Leakage Performance

Wafer Level Chip Scale Package (WLCSP) is fast growing package choice for many mobile applications owing to its small footprint, high electrical performance and relatively lower cost compared to conventional Flip Chip or Wire Bond package. In WLCSP process, it involves multi-layer of repassivation material coating, develop, exposure and metal deposition, plating and etching. In the beginning, descum process is being introduced to remove repassivation residue (i.e. scum) at opening. Typical descum process applies oxygen (O2) into vacuum chamber which is then being ionized and becomes plasma. The O2 plasma gas is able to bind with the scum and remove it by chemical reduction. This will ensure consistent repassivation opening dimension. In latter stage, descum process is also being developed for surface treatment and surface cleaning purpose. To achieve that, other gas like nitrogen (N2) is found to be more effective than O2 gas. N2 gas after ionized will not react with organic or metal surface, but it will bombard the surface and remove a few hundred angstroms thickness of repassivation thickness and caused both repassivation and copper surface to be rougher. Hence by applying descum process, it is able to remove some metal residues that are bonded into repassivation surface, which is difficult to be removed by wet etch process. With rougher repassivation and metal surface, it will also improve their interfacial adhesion. In this paper, we will discuss the importance of descum process in reducing leakage current to achieve low variability in device performance.