Scalable Four-Layer Redistribution Layers via Fully Additive Build-Up: Demonstration of Defect-Free Multilayer Integration

The increasing complexity of heterogeneous integration and chiplet-based electronic systems is driving demand for multilayer redistribution layer (RDL) architectures with reliable lateral routing and vertical interconnection capability. However, increasing the number of RDL layers remains challenging due to cumulative surface topography, interfacial instability, layer-by-layer alignment complexity, and microvia integration challenges. In this work, we report the first four-layer RDL architecture fabricated using the fully additive Sequential Build-Up Covalent Bonded Metallization (SBU-CBM) process. The structure was fabricated on an FR-4 substrate and comprised four lateral copper redistribution layers interconnected by three staggered copper microvias embedded within five polyurethane dielectric layers. X-ray computed tomography (XCT) confirmed the successful formation of the four-layer architecture and the spatially distributed staggered microvia configuration. Optical and topographical analyses revealed progressive surface-topography accumulation with increasing RDL layer number, while the fabricated stack remained structurally stable with no visible delamination, edge separation, or large-scale deformation. This work represents the highest multilayer integration level achieved to date using the SBU-CBM process and establishes an important milestone toward fully additive multilayer RDL manufacturing for next-generation heterogeneous integration and chiplet-based electronic systems.