Thermomechanical effects in metal lines on integrated circuits analysed with a differential polarimetric interferometer

We present a new laser probe, a differential polarimetric interferometer, which is dedicated to the study of common failure mechanisms in microelectronic interconnects. Our investigation is mainly concentrated on the study of thermomechanical stress build-up and electromigration in metal lines. In the differential interferometer, two laser beams, separated by a few microns, are reflected from the surface of the device under test. Reflectance, phase and polarization changes between the two beams can be observed, this allows surface temperature and surface bending measurements together with the observation of stress induced in the silicon dioxide layer by Joule thermal expansion. The laser probe provides unique and useful information about local thermal effects due to electromigration. It also shows, for the first time to our knowledge, induced local stress effects due to the mismatch of thermal expansion coefficients of the metal, Si and SiO 2 involved in running interconnect lines.