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 μm 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 μm in diameter and 50 μm in pitch. This article explores the feasibility of fabricating microvias of 5 μm or less in diameter with a commercially available picosecond UV laser system. The experimental results show that microvias of 5 μm or less in diameter in a 5-μm-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-μm microvias. The via pitch of 8-12 μm is demonstrated. UV laser ablation also addresses the issue of limited availability of photosensitive dielectric materials for photolithography-based microvia fabrication.