Secondary ion mass spectrometry backside analysis of barrier layers for copper diffusion

Secondary ion mass spectrometry (SIMS) backside analyses have been performed on a $Cu/TaN/Ta/SiO2/Si$ structure to determine barrier effectiveness for Cu diffusion. Sample backside access to the barrier layers was obtained by removal of the Si substrate using a polishing method that maintains parallelism between the sample surface and the polished back side by monitoring changes in facets at the four corners of the specimen. Determination of the Si thickness remaining during the polishing process was improved through the use of optical interference measurements using a narrow band pass optical filter. Samples having slopes with respect to the original surface less than 6 nm over 60 μm have been obtained. A difference in polishing rate between $SiO2$ and Si was exploited to obtain this parallelism. For SIMS analyses, the presence of a $SiO2$ layer required electron gun charge neutralization for the $O2+$ 0.5 keV impact energy analysis. SIMS analyses show the ability to distinguish all layers and to monitor copper through the barrier material. With the high depth resolution conditions used, SIMS analyses were able to provide detailed elemental distribution information such as the presence of nitrogen at specific interfaces.