Moisture Effect on Adhesion Between Polybenzoxazole and Sputtering Titanium in WLCSP

The mechanism of moisture effect on the bumping process in Wafer Level Chip Scale Package (WLCSP) was investigated, to understand the influence of moisture on the interfacial adhesion between Polybenzoxazole (PBO) and Sputtering Titanium. Residue Gas Analyzer (RGA) tool was used for PBO outgassing measurement during de-gassing step, Inductively Coupled Plasma (ICP) step and Titanium (Ti) sputtering step. Focused Ion Beam (FIB) and Scanning Electron Microscope (SEM) were used to study interfacial adhesion performance. According to statistical analysis of the investigation result, it was found that PBO-Ti interfacial adhesion is correlated to the idling time of PBO plasma clean to oven baking. The longer the idling time, the more moisture will be absorbed, that will directly impact the adhesion of PBO and sputtering Ti. The investigation shows good adhesion when idling time is well controlled. Furthermore, it is also correlated to de-gas temperature of sputtering process. The higher the degas temperature, the more water (H2O) volatilization amount will be detected by RGA, which indicated less moisture remained in the PBO. Less moisture absorption will reduce delamination detected by FIB and SEM. Therefore, the optimized de-gas temperature and idling time must be well controlled in WLCSP bumping process flow. This investigation provided a breakthrough guideline of process control in order to achieve good PBO-Ti interfacial adhesion.