Thin Film Under Bump Metallization for Wafer Level Packaging

Inhalt:
A new thin film under bump metallization (UBM) for complete wafer level packaging processes is introduced. The thin film UBM following glass frit wafer bonding and TSV (Through Silicon Vias) integration results in a monolithic MEMSCMOS integrated and hermetic sealed sensor solution that can be directly soldered on a printed circuit board (PCB). The application of thin film UBM, based on sputtering processes, is easier and more economical to implement than the commonly used solution based on µm-thick nickel and flash gold. However, when using the thin film UBM, the metals microstructures and diffusion behavior must be understood in the context of the overall thermal budget (e.g., baking steps during wafer probing, prior to soldering). By means of thorough optimization and detailed characterization by transmission electron microscopy, an optimal sputtering process based UBM system of 600 nm nickel and 150 nm gold was developed and found to be sufficient for soldering.