Ultra Fine RDL Structure Fabrication Using Alternative Patterning and Bottom-Up Plating Processes

The continued adoption of fan-out structures for advanced packaging and the rise of heterogeneous integration of different devices require several layers of interconnects, with feature sizes shrinking below 5 microns. Traditional organic flip-chip substrates using the semi-additive process (SAP) have not been able to scale to ultra-fine RDL pitches and via openings below 10 microns. Photo-sensitive spin-on dielectrics and RDL processes used for wafer level packaging face serious technical challenges, causing reliability and pattern integrity concerns. Furthermore, both processes do not sufficiently address the need for cost reduction. An innovative dual damascene fabrication process has been developed to create embedded fan-out structures using excimer laser ablation. An optimized dielectric material for RDL layers is used that is capable of resolving features down to 2/2 microns line/space through a mask reticle with high throughput. Metallization is a key factor in making this process cost effective. Yield and reliability are dependent on excellent adhesion between seed layer and dielectric film. Precise and optimized bottom-up plating as well as good within-wafer uniformity is required to eliminate the need for a chemical mechanical planarization (CMP) process. In this paper, we will report progress in demonstrating fabrication of multi-layer RDL with 2-5 micron trenches and vias enabled by the new embedded trace approach without requiring planarization by CMP or fly-cutting methods. We will present electrical properties together with mechanical and thermal reliability data of via and RDL traces patterned by this excimer laser enabled dual damascene process. The cost advantage over traditional fabrication processes will be highlighted.