High resolution TEM becomes more powerful when coupled with a Gatan imaging filter (GIF). It gives both structural and compositional information simultaneously with subnanometer and nanometer resolution, respectively. By means of these combined techniques, the failure mechanism of diffusion barrier layers in IC devices, having the structure of Al-1%Si-0.5%Cu/TiN/Ti/Si substrate, was investigated. In non-oxygen stuffed samples thermally stressed at 550°C for 1 h, serious Al spikes below contacts were observed where metal layer Ti also reacted with Si substrate to form C49-TiSi2 and was consumed completely. TiN and Al become unstable due to high temperature thermal stress and reacted into TiAl3 and AlN. Decomposition of TiN degraded the function of the barrier layer, and so a diffusion path at the weakest point of the contact corner was opened for metal Al alloy to diffuse through the barrier layer and to form a spike below the TiN layer. We also showed that the oxygen-stuffing processes after each TiN/Ti deposition enhances the barrier capability against Al/Si diffusion in thermal stress. It was found that Ti and TiN were oxidized to form titanium oxide which reacted with the Al alloy. As a result, Al2O3 and TiAl3 were found above the TiN layer. Therefore, it is concluded that oxidation during oxygen-stuffing processes provides a means to inhibit diffusion in thermal stress processes, and maintains integrity of the stack structure.