Overlay target design and evaluation for SADP process

Overlay performance has been a critical factor for advanced semiconductor manufacturing for years. Over time these requirements become more stringent as design rules shrink. Overlay mark design and selection are the first two steps of overlay control, and it is known that different overlay mark designs will have different responses to process conditions. An overlay mark optimized for traditional process might not be suitable for SADP (self-aligned double patterning) technology due to changes in lithography and etching process conditions. For instance, the traditional BIB (box-in-box) target defined by the core mask becomes a template structure in SADP flow, the pitch and cycle of the overlay mark is further changed after spacer formation and core film removal hence the mark recognition and robustness have been challenging for the subsequent process layers. The comprehensive study on the methodology of overlay mark design and selection is still not available for SADP process. In this paper, various types of overlay marks were designed to comply with the SADP process to get rid of the weaknesses of traditional targets. TMU (total measurement uncertainty) performance was adopted to determine the optimal overlay marks for meeting production overlay control requirements in SADP process flow. The results have suggested the segmented marks outperform to non-segmented marks on image contrast as well as TMU.