C4 is a batch process, coating more than 10 wafers at a time. Typically, the process begins with e-beam sublimation of the Cr. Once stabilized, and upon depositing the required underlayer thickness, Cu evaporation is begun and runs concurrently. Increasing the Cu deposition rate, while holding the Cr process stable, produces a phased interlayer structure. The final Cu layer is deposited by shutting off the Cr source.
Evaporation provides a seamless process, eliminating any interfaces among the three layers.
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.