Figure 4

Comparison of standard electroless copper plating process (left) with the shadow plating process (right). In both processes the original laminate of copper (pink) and polyimide (yellow) has the vias drilled out. Left: A thin copper seed layer (green) is chemically attached to the entire laminate surface. Right: A thin carbon layer (black) is attached to only the polyimide layer, making it conductive. After these steps, in both processes a thicker copper layer (brown) is electroplated on to all conductive surfaces. The tables alongside each step show the measured ²³⁸U and ²³²Th contamination at that step, before and after the cleaning is applied