There are studies in which Si3N4 is used as a passivation layer on Cu to simulate the vertical stack structure in interconnect systems. However, these studies are only applicable to barrier studies involving metal lines. In this study, a passivated “sandwich” film stack is used to simulate the lateral structure found in dual damascene vias and trenches. X-ray diffraction analysis shows that an ultrathin Ta barrier remains robust up to 850 °C, at which the Ta barrier fails upon a β-to-α phase transformation. Time-of-flight secondary-ion mass spectrometry depth profiling and scanning electron microscopy analysis provide supporting evidence in the assessment of the thermal stability of the lateral film stack. The passivation provided by the lateral structure improves the thermal stability. However, the stability of the film stack was found to be dependent on the crystallography of the Ta barrier.