In line measurements of the side wall angles for litho process tool monitoring using profile reconstruction capabilities of a CD SEM

In our work the Applied Materials NanoSEM 3D CD-SEM is used for in-line monitoring of profile height and side wall angle on product wafers. The system allows the fully automatic reconstruction of profiles using the built in beam tilting capability. From these measurements an immediate status of the focus of the litho tool is obtained. The side wall information in combination with the CD allows to distinguish between focus and exposure excursions, respectively. The side wall angle of an isolated line on a focus sensitive backend product layer within the Infineon 140 nm technology has been monitored over a period of 3 months in a full volume DRAM manufacturing environment. The selected structure has been identified being much more sensitive to focus variations than dense lines. The changes in the side wall angle have been found to cover a side wall angle range from 80° up to 93° allowing a successful monitoring of the focus of the litho tool. This sensitivity of the side wall angle to focus changes is expected to enable an earlier warning than from top-down CD measurements only, if the focus drift of the litho tool passes a predefined limit. During the evaluation period, one minor focus excursion could be identified which was not detected by top-down CD-SEM data only.