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RDL and Pillar Fabrication from a Versatile Copper Plating Process

Advanced packaging suppliers are having two primary challenges during IC substrate fabrication. These challenges are not only in meeting the copper plating performance requirements but also in reducing manufacturing process costs. Copper plating processes must provide both high resolution and strict height uniformity within a unit/die (WIU) and within a wafer/panel. This is because the designs include features such as fine lines and pillars, whose top shape and coplanarity are critical to the product quality. To reduce manufacturing process costs, copper plating solution can be developed to plate multiple types of features in the same electrolyte, reducing the need for multiple plating tools. This flexibility allows fabricators to save on floor space, equipment and processing time. In this paper, an electroplating package, Systek ETS, is introduced to plate both RDL and pillars under different current densities in vertical continuous platers (VCP). The plating uniformity and coplanarity of both RDL fine lines and pillars were evaluated on a panel level. The Systek ETS package offered excellent coplanarity within a pattern unit for RDL plating. The variation in the plated height (or thickness) between fine lines of 5 µm widths and pads 50 µm widths respectively, was below 1.0 µm when using a current density of 1.5 ASD. The tops of the fine lines have defined, rectangular shapes and their resultant profile offers excellent conductivity. The same electrolyte package for pillar plating under higher current densities of around 5–10 ASD resulted in pillars with slightly domed profiles. These copper pillars had very good uniformity within unit and within panel. Physical properties of the plated copper deposit are essential for the reliability of the finished product. A few key physical properties are tensile strength, elongation %, and internal stress. Copper deposited with the electrolyte package had tensile strength above 40,000 psi, elongation % above 18%, and internal stress below 1.0 Kg/mm2. The physical properties of the deposited copper did not change considerably during the bath aging, showing that the package had a stable performance over bath life. The reliability of both pillars and RDL features were evaluated via solder dip at 288°C for 60 repetitions. The plated features did not show any crack or separation from the substrate.