Fabrication of Multi-RDL Layers with Polymeric IDL for RDL Interposer

The application of heterogeneous integration as a structural platform for next-generation semiconductors has led to extensive exploration of 2.x D package technologies, such as silicon interposers, bridges, and RDL interposers. RDL interposers consist of metal wiring layers (redistribution layer, RDL) and insulating layers (inter layer dielectric, ILD). One of the key challenges in fabricating RDL interposers is the planarization of the ILD. This paper presents the development of a planarization process for multilayer RDL interposers by the utilization of a pressing method with a plate. To determine the level of planarization, we employed the Degree of Planarization (DOP) and conducted measurements using a surface profiler. The specimens were fabricated to have 3.5μm thick Cu RDL covered by 7μm thick PI (HD-4100) ILD on Si wafers. The pressing method was applied during the soft bake process of the ILD. We used scanning electron microscope (SEM) to observe the cross-sectional profiles of the specimens with and without the pressing method. The pressing method improved the degree of planarization (DOP) of the ILD, as confirmed by both surface profiler measurements and SEM analysis. The results demonstrate that the pressing technique is an effective approach to enhance the flatness of RDL interposers, which is critical for the reliable fabrication of multilayer structures.