Metrology-Design Co-Optimization for BEOL dimensional characterization using scatterometry

Abstract

A systematic study of the co-optimization of target design and metrology technique is presented to accurately measure the critical dimensions of BEOL metal lines. Rigorous coupled-wave analysis calculations and measurement data are combined to discuss various design scenarios with and without patterned underlayers in conjunction with either traditional scatterometry or vertical traveling scatterometry (VTS) using spectral interferometry. It was found that for traditional OCD techniques employing polarized reflectometry or ellipsometry, two buried and crisscrossed metal lines below the level of interest are sufficient to suppress the majority of the optical contributions from any underlayer stack beneath. Alternatively, VTS utilizing spectral interferometry and signal filtering can suppress the contribution from thickness interferences in the transparent underlayer stack or from front-end patterning thus only the top layer of interest needs to be considered in the model analysis (without the need for extra buried metal lines). Analysis of measurement data obtained on a BEOL wafer demonstrates the benefits and challenges of each approach.