Polymer-based fine pitch Cu RDL to enable cost-effective re-routing for 2.5D interposer and 3D-IC

In this paper, 2 metal layers, very fine pitch Cu RDL process using spin-on photo-pattern-able polymer based material was demonstrated, with Cu wiring of a minimum of 2-μm/2-μm line/space (L/S) and dielectric via diameter of 2 μm. Electrical test structures were designed and fabricated to assess the performance of the Cu RDL process. Meander fork structures of 2-μm/2-μm L/S was used to measure the leakage current. The leakage current ranged from 1 to 6 pA for the different line lengths evaluated, indicating good insulation property of the polymer material. Daisy chain structure was used to measure the electrical resistance with different via numbers pitches. From the measurement, the resistance of the daisy chain structure increases with the number of vias. Single via resistance was also measured with Cross Bridge Kelvin Resistor (CBKR) structure. The measured contact resistance of 2-μm diameter via is ~20 mΩΣ The reliability of the Cu RDL process was also evaluated by subjecting the wafers to Unbiased Highly Accelerated Stress Test (uHAST). The daisy chain structures measurement shows no significant change in electrical resistance after the reliability test, indicating good reliability of the Cu RDL process.