A study of the chemical and physical interaction between copper and polyimide

The interaction of pyromellitic dianhydride–oxydianiline based polyamic acid precursor with copper was investigated under various curing conditions. Two reactions were found to occur at the interphase boundary during the curing cycle: the interaction of Cu with polyamic acid, and Cu oxidation. It was found that the rates of both reactions are controlled by the oxygen level and supply of oxygen to the interface. Thus, the thickness of the polymer coating and the oxygen level in the curing ambient are found to be the rate determining factors. The presence of a high level of oxygen during curing of the polyimide film, particularly of a thin coating, was found to severely degrade the polyimide and to create an oxidized Cu film underneath. This resulted in a polyimide film having higher dielectric constant, reduced dielectric breakdown strength, a high metal-to-metal interface contact resistance, and a degraded adhesion between polyimide and the underlying Cu. The interface structure and the properties of the individual polyimide and Cu films can be improved significantly by inhibiting the interactions between Cu and polyimide and Cu oxidation during curing. This was achieved by either minimizing the oxygen content in the curing ambient or passivating the copper with a continuous chromium film. The passivation metal approach offers better tolerance to various curing conditions while still maintaining good-quality polyimide and Cu films. In addition to the Cu sheet resistance and parallel-plate capacitance measurements, the Fourier transform infrared reflection–absorption and electron spectroscopy for chemical analysis/Auger electron spectroscopy analytical techniques are used to study the imidization reactions and degradation.