Oxidation mechanism of ionic transport of copper in SiO2 dielectrics

Secondary ion mass spectroscopy (SIMS) and electrical measurements have been used to investigate the mechanism of copper transport in dielectric materials. Vacuum bias-temperature-stress (BTS) measurements on circular metal-oxide-semiconductor (MOS) capacitor structures show no sign of copper diffusion at temperatures up to 300 degreesC with electric fields up to 0.5 MV/cm. In contrast, measurements on samples oxidized via ambient gases or pure oxygen show significant copper transport through SiO2 based dielectric materials. The results suggest a mechanism where oxidized copper is a source of copper ions that are transported through the dielectric layer via diffusion and drift. After transport through the oxide, the copper crosses the SiO2/Si interface and diffuses into the silicon substrate, which acts as a sink for copper. An analytical model provides an apparent activation energy of 1.8 eV for the diffusion coefficient, and an exponential field dependence with a multiplier of 5.6 x 10(-7) cm. A more rigorous model based on a one-dimensional solution of the drift-diffusion equation and Poisson's equation is also developed to fit the flux data. The model reveals that a unique activation energy cannot be assigned due to uncertainty about the chemical-physics at the Cu/SiO2 interface. (C) 2004 Elsevier B.V. All rights reserved.