Smaller Microvias for Packaging Interconnects by Picosecond UV Laser with a Nanometer Metal Barrier Layer: A Feasibility Study

This study, to the best of authors' knowledge, is the first to investigate the feasibility of using picosecond UV laser ablation to fabricate ultrasmall microvias scaled down to 3 \mu \text{m} and less with a pitch of 8 \mu \text{m} in a 5- \mu \text{m} Ajinomoto buildup film (ABF). The state-of-the-art microvias are 20 \mu \text{m} in diameter by a nanosecond UV laser and 5 \mu \text{m} in diameter by a picosecond UV laser reported in our previous study, but microvias of less than 2 \mu \text{m} in diameter are needed to meet IO density requirements for today and future's high-bandwidth packaging and heterogeneous system integration. In this study, we have investigated the impact of laser power, beam sizes, and materials on the via size and explored the feasibility of picosecond UV laser ablation for ultrasmall microvias with an additional layer of 80-nm-thick copper on top of the dielectric as a barrier layer. The power used to fabricate the 5- \mu \text{m} microvias in a 5- \mu \text{m} -thick ABF could open 2- \mu \text{m} holes in the copper barrier layer due to the higher ablation threshold of copper. The drilled copper layer then serves as a mask to produce smaller microvias in the dielectric layer beneath the copper. In this article, fully opened microvias of 3 \mu \text{m} diameter in ABF will be demonstrated. Submicrometer openings in copper are also achieved, which suggests the feasibility to scale down via diameter to submicrometer level.