The issue regarding wafer alignment is arisen due to flattening of the alignment mark topography by oxide- and W- CMP process. This results in degradation in alignment signal intensity which is a crucial factor affecting overlay accuracy. Computer simulation of alignment signal intensity for the oxide- and W-CMP processes has been successfully performed. Result indicates alignment signal intensity swings with depth of the phase grating alignment mark. A critical range of depth has to be maintained for achieving alignment signal intensity high enough for overlay accuracy. The W-CMP process is thus utilized for obtaining depth of alignment mark within this range. Experiment has successfully demonstrated improvement in overlay.
"Improvement of overlay in the oxide- and W-chemical-mechanical polish processes", Proc. SPIE 3334, Optical Microlithography XI, (29 June 1998); doi: 10.1117/12.310740; https://doi.org/10.1117/12.310740