Advancements in Electroless Copper Plating for Via Shrinking Technology Trend

The pursuit of smaller blind micro via size in IC substrate technology development is driven by the need for higher density integration, increased functionality, and improved performance of electronic devices. As via size shrinks and aspect ratio increases, it is inevitable to face challenges to maintain good copper via filling and reliability at via interconnect interface. Herein we demonstrate an electroless copper deposition of semi-additive process (SAP) fulfills both good reliability and throwing power for via diameter size ranging from 20 to 50 µm and dielectric thickness from 20 to 30 µm. One of the keys relies on the “accelerator” bath before panels enter electroless copper bath for electroless copper plating. In such bath, cleaner, antioxidant, wetting agent are included. The cleaner is used to remove copper oxide from the via bottom, ensuring the best possible copper surface quality before starting electroless copper plating. Antioxidant resides in the via can delay the oxidation of copper and simultaneously facilitate subsequent copper ion deposition in the same area. Wetting agent, though, helps these functional components penetrate the small via. This electroless copper deposition design enables throwing power of approximately 80% to be achieved in vias with a diameter of 20 µm and a depth of 20 µm. The copper grain exhibits good continuity at the electroless copper interface, with minimal voids, as observed under 30kx SEM cross-sectional inspection of vias with the same diameter and depth. Quick via pull reliability tests were also performed on the same vias, and no failures were found among 4000 tested vias. Other typical performance evaluation items for electroless copper, such as peel strength, electroless copper thickness and coverage, will not be compromised. In summary, electroless copper process is being investigated to address the new challenges associated with the technology trend of shrinking via sizes. We demonstrated an electroless copper product SAP8000 with an accelerator bath consisting of cleaner, antioxidant and wetting agents, which enhances throwing power and reliability performance.