Two CD control issues are discussed in this paper. One is on the subject of defocus caused by the local flatness variations of a silicon wafer. The second deals with the optical proximity effects. The use of high NA lenses to achieve submicron resolution has resulted in much diminished available depth of focus. The local wafer flatness variation, which was considered to be a negligible source of defocus for older technologies, now accounts for a significant part of the total available error budget. A method of characterizing wafer flatness using the auto-focus system of a stepper is described. Data from a comparison of wafer samples from two vendors are presented. The implication of edge exclusion and wafer leveling is discussed, and the impact on CD control is demonstrated. In order to address this issue, the participation of the stepper and wafer manufacturers is absolutely essential. The optical proximity effects, which were also negligible for Minimum Feature Size of greater than 1 micron, are becoming an important consideration for achieving CD control, as the lithographic tools are being operated at closer to the resolution limit. Both simulation and experimental data are presented, and their impact on devices is discussed. A co-ordinated effort between several engineering disciplines is required to effectively resolve this issue and achieve optimum device yield.

Date Published: 17 September 1987
PDF: 6 pages
Proc. SPIE 0811, Optical Microlithographic Technology for Integrated Circuit Fabrication and Inspection, (17 September 1987); doi: 10.1117/12.975592

Published in SPIE Proceedings Vol. 0811:
Optical Microlithographic Technology for Integrated Circuit Fabrication and Inspection
Harry L. Stover; Stefan Wittekoek, Editor(s)