Observation of passivated Al-1%Cu lines using environmental scanning electron microscopy (ESEM)

Abstract

Characterization of failure phenomena in passivated interconnects requires the ability to observe dynamic changes in buried lines (e.g. formation of electromigration voids or microstructural changes during an anneal). Conventional SEM cannot image passivated structures due to surface charging. HVSEM has been used with some success to image voids in metal lines under passivation, but it requires intensive specimen preparation and is not commercially available. The present study is intended to determine whether ESEM is suitable for such dynamic experiments on passivated structures.The lines were composed of Al-l%Cu and had a uniform thickness of 1.4μm. All lines also had both an underlayer and an anfi-reflective coating of Ti/TiN. The thickness of these layers varied slightly, but was typically around 600 Å. Observations were performed in an Electroscan model 2020 environmental scanning microscope with a LaB6 thermionic source using the gaseous secondary electron detector (GSED) and water vapor as an imaging gas.