Deep Understanding the role of Cu in RDL to Warpage by Exploring the Warpage Evolution with Microstructural Changes

Wafer level chip-scale package (WL-CSP) becomes more and more popular in IC packaging industry due to its small form factor and low cost. Redistribution layer (RDL) which is widely used in WL-CSP usually causes serious warpage due to the plastic deformation in the thermal processes, and the Cu layer in RDL contributes 1/3 of the total warpage in 2P1M configuration (2 PI layers and 1 metal layer) which cannot be neglected. In this paper, 3 sets of 5 μm thick copper films using different plating parameters and passivation layers are prepared, and the wafer warpage evolution between ambient temperature and 400°C is measured. The wafer warpage evolution showed a hysteretic behavior during the thermal process, and the thermomechanical responses of these 3 sets are different from each other. Three parameters are defined from this thermomechanical response to evaluate the degree of stress relaxation and the dominant stress relaxation mechanism of these three sets are discussed in detail. The result indicates that microstructure and surface passivation of the copper film have a significant influence on the thermal behavior, and the wafer warpage at ambient temperature can be reduced by depositing a passivation layer or controlling the plating parameters.