High current densities in copper microcoils: influence of substrate on failure mode

Copper planar microcoils were processed by U.V. lithography on SiO₂/Si and Kapton^®. They were packaged on different types of support in order to study the influence of thermal exchange conditions on device functioning. An electric current was injected in the coil and step by step increased until the electric connection broke, while the temperature of the microcoils remained free. This latter was estimated from the copper resistivity. It allowed demonstrating that the thermal exchange mode of the wire-bonded microcoils is conductive. The current density was calculated taking into account the deterioration of the coils by oxidation. Its maximum value is linearly decreasing with the thermal exchange ability of the support. The failure modes of the microcoils are related to track melting and oxidation, the current density remaining one order too weak to induce electromigration.