Experimental identification of thermal induced warpage in polymer–metal composite films

• PI exacerbates overall wafer curvature but mitigates the plastic deformation in Cu, which benefits device reliability.
• Nonlinearity due to mutual effect between PI and Cu is revealed by in situ measurement of warpage-temperature response.
• The stress relaxation of PI at high temperature can be suppressed by the Cu coating film.

Composite thick films consisting of multi-layered polymers and metals are widely used in integrated circuits(IC) and its packaging, and it arises intricate stress and warpage problems due to complex inner stress distribution and evolution. The wafer warpage origination and evolution of multi-layered polyimide (PI)/Cu composite film is measured in-situ by a Multi-beam Laser Optical Sensor (MOS) system. It's found that PI has an intricate influence on wafer warpage evolution and Cu plastic deformation due to viscoelasticity and glass-transition, and the influence differs in different structures and at different temperatures. Nonlinearity of the curvature–temperature curve of the composite occurs at much lower temperature than in single PI or Cu film, showing mutual effect of PI and Cu. Unlike bare or capping PI film that totally stress relaxed at high temperature, bottom PI coated by Cu film sustains a medium compressive stress, indicating that Cu coating film has restrained stress relaxation of PI. The warpage evolution during heating is different from that during cooling, perhaps due to different deformation mechanism.