Innovative Sub-5- $\mu$ m Microvias by Picosecond UV Laser for Post-Moore Packaging Interconnects

This article presents for the first time microvias scaled down to sub- $5~\mu \text{m}$ in diameter fabricated using picosecond UV laser ablation in a nonphotoimageable dielectric film. The motivation of this article is to address post-Moore and More-than-Moore packaging interconnect needs. Microvias play a critical role in package interconnections in IO density and the IC bump pitch for 2.5-D interposers and fan-out packages. UV laser ablation has been the key technology for fabricating small microvias in high density interconnect (HDI) packaging for more than two decades. The state-of-the-art microvia fabricated by UV laser ablation is still at $20~\mu \text{m}$ in diameter and 50 $\mu \text{m}$ in pitch. This article explores the feasibility of fabricating microvias of $5~\mu \text{m}$ or less in diameter with a commercially available picosecond UV laser system. The experimental results show that microvias of $5~\mu \text{m}$ or less in diameter in a 5- $\mu \text{m}$ -thick Ajinomoto buildup dielectric film (ABF) are achieved. This article also addresses the fundamentals of picosecond pulsed laser ablation on polymer dielectric materials and processes optimization to generate sub-5- $\mu \text{m}$ microvias. The via pitch of 8– $12~\mu \text{m}$ is demonstrated. UV laser ablation also addresses the issue of limited availability of photosensitive dielectric materials for photolithography-based microvia fabrication.