In recent years, numerous authors have reported the advantages of Diffraction Based Overlay (DBO) over Image
Based Overlay (IBO), mainly by comparison of metrology figures of merit such as TIS and TMU. Some have even gone
as far as to say that DBO is the only viable overlay metrology technique for advanced technology nodes; 22nm and
beyond. Typically the only reported drawback of DBO is the size of the required targets. This severely limits its effective
use, when all critical layers of a product, including double patterned layers need to be measured, and in-die overlay
measurements are required.
In this paper we ask whether target size is the only limitation to the adoption of DBO for overlay characterization and
control, or are there other metrics, which need to be considered. For example, overlay accuracy with respect to scanner
baseline or on-product process overlay control? In this work, we critically re-assess the strengths and weaknesses of
DBO for the applications of scanner baseline and on-product process layer overlay control. A comprehensive comparison
is made to IBO. For on product process layer control we compare the performance on critical process layers; Gate,
Contact and Metal. In particularly we focus on the response of the scanner to the corrections determined by each
metrology technique for each process layer, as a measure of the accuracy. Our results show that to characterize an
overlay metrology technique that is suitable for use in advanced technology nodes requires much more than just
evaluating the conventional metrology metrics of TIS and TMU.