As the dimensions of integrated circuit continue to shrink, diffraction based overlay (DBO) technologies have
been developed to address the tighter overlay control challenges. Previously data of high accuracy and high precision
were reported for litho-etch-litho-etch double patterning (DP) process using normal incidence spectroscopic
reflectometry on specially designed targets composed of 1D gratings in x and y directions. Two measurement methods,
empirical algorithm (eDBO) using four pads per direction (2x4 target) and modeling based algorithm (mDBO) using two
pads per direction (2x2 target) were performed. In this work, we apply DBO techniques to measure overlay errors for a
different DP process, litho-freeze-litho-etch process. We explore the possibility of further reducing number of pads in a
DBO target using mDBO. For standard targets composed of 1D gratings, we reported results for eDBO 2x4 targets,
mDBO 2x2 targets, and mDBO 2x1 target. The results of all three types of targets are comparable in terms of accuracy,
dynamic precision, and TIS. TMU (not including tool matching) is less than 0.1nm. In addition, we investigated the
possibility of measuring overlay with one single pad that contains 2D gratings. We achieved good correlation to blossom
measurements. TMU (not including tool matching) is ~ 0.2nm. To our best knowledge, this is the first time that DBO
results are reported on a single pad. eDBO allows quick recipe setup but takes more space and measurement time.
Although mDBO needs details of optical properties and modeling, it offers smaller total target size and much faster
throughput, which is important in high volume manufacturing environment.