Fully Encapsulated Fine Pitch Dual Damascene Organic RDL with Low Dk Df Photosensitive Polyimide and Its Reliability

In this paper we report fabrication of two metal level fine pitch organic redistribution layers by a dual damascene process using low $D_k$ & $D_f$ photo sensitive polyimide as the interlayer dielectric. Three micron pitch Cu wires and $3 \mu m$ circular vias were achieved photolithographically on $2\mu m$ thick polyimide. Electrical testing of 250 mm long fine pitch double serpentine structures showed a tight distribution in resistance and extremely low leakage currents. To reduce the risk of Cu oxidation and Cu migration into polymer, an encapsulation process for polished Cu surfaces using inorganic dielectric $SiN_x$ or a metal liner has been developed. Wafers with Cu damascene lines built with different capping methods are compared with PSPI coated damascene and semi-additive process (SAP) M1 wafers. The results show that although Cu oxidation in three-side protected Cu damascene RDL is much suppressed compared to SAP RDL, the exposed Cu surface in direct contact with PSPI is still prone to reliability issues related to Cu migration. Cracks or pin holes in $SiN_x$ capping layer can lead to Cu corrosion. Fully encapsulated Cu wires with metal liners is the most effective way to prevent Cu corrosion.