**7. Conclusion**

The multi-pad empirical diffraction-based overlay (eDBO) technique is capable of controlling the overlay in double patterning optical lithography processes (DPT). The usable range of LELE DPT eDBO is ±70nm. eDBO results agree well with traditional image-based overlay (IBO) results and with overlay calculated from CD-SEM data. While good correlation and linearity between DBO and IBO was observed, a significant systematic offset can occur that appears to originate in the IBO data. Reduction in the number of pads from 2x4 to 2x3 results in only a small deterioration in precision.

DBO measurements have near-zero TIS. TMU (including tool-to-tool matching) is less than 0.5nm for both the LELE and freeze process. The overlay errors determined by eDBO (4 pad measurement) agree well with modeled DBO (2 pad measurement) data. This ability to model the signal formation process allows model-generated spectra to be used to predict measurement precision with good success.

Simultaneous model-based measurement of overlay in X&Y (2D mDBO), is possible with good results and also allows reduction in the overall target size. mDBO requires knowledge of the film stack, material optical properties and target layout and consequently more effort in creating recipes than eDBO but provides significant value in reducing measurement time and target size. In addition to overlay data, mDBO provides CD measurements and profile data for the target, which is not possible with other methods. The multi-pad DBO approach is a good method of overlay process control, especially if combined with in-chip measurements using an alternative technique [3].
