Optimization of High Aspect Ratio Copper Pillar Fabrication for Through Mold Interposer (TMI) Processing

This study focuses on key challenges in fabrication of high aspect ratio copper pillars in Through Mold interposer (TMI) processing. Critical Dimension (CD) ranging from 25 µm to 30 µm with a thickness of 100 µm, as well as 60 µm to and 100 µm CDs on a 200 µm thick negative photoresists have been used in this work. Key challenges were addressed through the optimization of surface pre-wetting conditions for plating and eliminating defects in 100 to 200um thick photoresist coating, such as bubble entrapment, inadequate photoresist cross-linking, and variations in photoresist thickness across the wafer during the lithography process. Additionally, this work demonstrated the process flow optimization through soft-bake conditions, plasma surface treatment prior to plating and applying a thinning solution to the wafer edges to minimize the photoresist thickness variations.