DIFFUSION OF COPPER THROUGH DIELECTRIC FILMS UNDER BIAS TEMPERATURE STRESS

Copper diffusion in various dielectric films as a function of electric field and temperature is reported. In this study, we characterized the leakage current through various dielectric films as a function of electrical field and elevated temperature. Both electric field and temperature are observed to affect strongly the dielectric barrier lifetime. Nitride and oxynitride films are found to be much better barriers than thermal oxide while plasma TEOS had a much lower barrier lifetime. The activation energy of copper diffusion in thermal oxide is determined to be 1.2 eV. A three-step model is proposed to explain the observed current-time characteristics. In the first stage, the applied bias causes the injection of positively charged copper ions into the dielectric. The lack of a neutralizing electron current results in space-charge build up which sets up an opposing field and reduces the ionic current. The second stage represents primarily the thermal diffusion of copper ions and neutral atoms. Finally, in the third stage, enhanced electric fields in the dielectric lead to breakdown.