Three-dimensional interconnect technology on a flexible polyimide film

We present polyimide (PI) film wet-etching technology, and a cost-effective fabrication process of three-dimensional interconnects on the thin flexible PI film. Densely arrayed micro through-holes (diameter < 100 µm) on the flexible PI film were fabricated by wet-etching technology, and three-dimensional metal interconnects were formed by the electroless Cu plating process. We have also proposed and fabricated a vertically interconnected high-density micro thermal sensor array directly on the 50 µm thick Kapton PI film, and we have performed some experiments. Sensor elements (0.1 µm thick nickel film; upper side of the PI film formed by vacuum evaporating) and electrical feed-through (1 µm thick Cu film; back side of the PI film formed by electroless Cu plating) were vertically interconnected by electroless Cu plating via the wet-etched micro through-holes. The evaporated nickel film showed good adhesion to the PI film without any pre-treatment, and its temperature coefficient of resistance was about 2600 ppm. The poor adhesion of plated Cu to the PI film surface could be improved by PI film wet treatment with a strong alkaline solution TPE3000. In the proposed sensor device, the sensing elements showed very fast thermal response and good thermal isolation from the substrate, due to its small thermal capacity and Kapton PI film inherent characteristics of excellent thermal isolation (λ = 0.12 W/(m °C)).