However, there are several key drawbacks to evaporation. As wafer size increases, the value of a fully processed wafer is very high by the time it reaches the UBM stage of processing. The monetary risks of a UBM process failure, resulting in the loss of a complete batch, leads engineers to seek a single-wafer process. Batch or in-line sputter processes are not ideal solutions either. They are also throughput limited and suffer from similar product loss risks.
Cluster systems provide an ideal solution.
They can also produce the interface-free UBM structure that evaporation provides, if it is done in a single module. A phased transition from the metal adhering to the top level device metal, aluminum, to a solderable or plateable material, typically Cu, is desired for excellent mechanical properties such as bump shear and bump pull strength. Sputtered Films has developed a sputter source with two concentric dual ring cathodes that are independently powered. Deposition rates for the cathodes are linear with power when operated in the required power range. Calibrating the deposition rates of the individual cathodes for the phasing process can be done with simple automatic control algorithms.