Fine RDL Formation Using Alternative Patterning Solution for Advanced Packaging

Abstract

There is a continuing need for higher density routing and heterogeneous integration of different devices, but also for continuous cost reduction. While traditional organic flip-chip substrates using semi-additive processes (SAP) have not been able to scale to ultra-fine DL pitches and via opening below 10 $\mu$m, photo-sensitive spin-on dielectrics and RDL processes used for wafer level packaging do not sufficiently address the cost reduction need, and also face serious technical challenges. Therefore, the integrity and reliability of the pattern becomes necessary as feature sizes shrink to below 5 $\mu$m, demanding innovative thinking on the process integration to overcome these barriers. An innovative fabrication process has been developed for RDL and via formation using excimer laser ablation of an optimized dielectric material for each layer. Metallization is a key factor in making this process practical and cost effective. Yield and reliability is dependent on the adhesion between seed layer and dielectric film. Precise and optimized bottom-up plating is required to eliminate the need for a chemical mechanical planarization (CMP) process. This paper presents a novel excimer laser enabled dual damascene process for ultra-fine routing for BEOL. Fine RDL down to 2/2$\mu$m L/S and vias down to 5$\mu$m diameter are patterned using an excimer laser stepper with a reticle mask. We will report progress in demonstrating fabrication and technical characterization of this new embedded trace approach with and without planarization by CMP or fly-cutting methods. Capabilities and limitations of this bottom-up plating process will be discussed, its technical robustness and commercial advantages are demonstrated, and topics for further optimization are highlighted.