Conventional overlay control minimizes the registration error between aligned level pairs. Post alignment corrections are applied to the adjustable parameters on the exposure tool. These can be grouped into grid terms relating to stepper stage control and field terms related to mask, stage and lens control. To minimize alignment time and alignment mark real estate on the wafer, common practice is to align at a fixed field location over a sufficient number of fields to represent the grid. A problem with conventional overlay control is that it places no constraint on the magnitude of field terms at first-level or any subsequent level; it requires only that they be matched between the alignment pairs. Incorporation of field-stitching overlay control within each level establishes the constraint that field terms remain consistent with the wafer stage stepping at each process level. A novel target layout and analysis method enables the simultaneous determination of both conventional overlay and neighboring-field error. Application of our method at first level and subsequent alignment levels precludes the random walk of field terms, thereby improving the consistency of field matching. The net result is an improvement in overlay control, particularly in cases where 'single-point' alignment or mixed tool set matching is required.

Date Published: 16 July 2002
PDF: 13 pages
Proc. SPIE 4689, Metrology, Inspection, and Process Control for Microlithography XVI, (16 July 2002); doi: 10.1117/12.473463

Published in SPIE Proceedings Vol. 4689:
Metrology, Inspection, and Process Control for Microlithography XVI
Daniel J. C. Herr, Editor(s)