Application of Acoustic Metrology for In-Line Microbump Process Monitoring in Advanced Packaging

Solder-based fine pitch microbumps along with through silicon vias (TSV) enable high density interconnects for die-to-die and die-to-wafer stacking for different applications. With reduction in bump dimensions, several challenges arise that need to be addressed. For example, selection of under bump metallization (UBM), solder and barrier between them can determine the consumption of the UBM by solder and transformation to intermetallic compound (IMC), during thermocompression bonding. In this study, we systematically investigate the effectiveness of PULSE™ metrology on microbumps with a range of diameters and a variety of metal stacks. On wafers with a Ti/Cu seed layer, a test pattern of microbump features with diameters down to 20μm was imaged in 50 μm photoresist and plated with a matrix of bi-layer (Cu/Ni), tri-layer (Cu/Ni/SnAg) and multilayer Cu/Ni/Cu/SnAg stacks to form the microbumps. We present results from the study to demonstrate the capability of the technique for in-line measurements of individual layer thickness.