Characterization of complex inter-layer dielectric stack by spectroscopic ellipsometry: A simple method to reduce parameters correlations

The accurate and stable measurements of inter-layer dielectric (ILD) film thicknesses and optical properties in a multi-layer stack have been always a key factor in semiconductor development and manufacturing. Spectroscopic ellipsometry is one of the most adapted optical metrology techniques to perform such measurements but it requires the use of one of the multi-parameter non-linear optimization methods due to its indirect nature. It creates a big challenge for analysis of multi-layer structures since the number of simultaneously determined model parameters is restricted due to parameter cross-correlations. This paper describes a simple way to reduce the correlations for single-angle ellipsometric measurements when applied to monitor the thicknesses of the dielectric films (silicon oxide, ultra low-k dielectric and low-k dielectric barrier/etch stop films) in a multi-layer stack. The method is based on inclusion of a thin Ta metal layer (~ 160 Å thick) into the multi-layer structure in order to suppress the inter-layer correlations, thereby allowing an accurate determination of the thicknesses of individual films in the thin-film stack. The optical characterization of all layers in the spectral range of 200-800 nm (6.20-1.55 eV) has been performed using an “additive” thin-film stack approach. The method was employed for analyzing the multi-layer ILD test stack with up to five-layer film structures in which four of the films are dielectric. The final model uniqueness and accuracy of the obtained ellipsometric solution were also verified.