27 March 2007 Best focus determination: bridging the gap between optical and physical topography
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Abstract
With decreasing critical dimension (CD) budgets and smaller k1 values the need for perfect focus control becomes paramount. Among the individual contributors to the overall focus budget, the accuracy of the leveling system on a process wafer and the focus setting accuracy for the individual layers are two major contributors. In our study we discuss the usage of a new non-optical leveling system and its measurement capability of wafer topography. By exposing focus-exposure matrices (FEMs) and measuring them on multiple points in the field, we demonstrate the systematic and random focus variation across the scanner exposure field for several layers. Critical back end of line (BEoL) layers in particular show considerable impact of topography, thus resulting in the across field focus variations shown. By using the newly developed AGILE leveling system which uses an air-gauge focus sensor we demonstrate a more accurate best focus determination across field, resulting in better overall focus performance. This AGILE system is expected to be independent on any process variation, since there is no (optical) interaction between the measurement device and the process layer stack. By the use of multi-point FEMs we show that the intrafield focus range can be reduced by as much as 50%, depending on certain layer and layout characteristics. We discuss the impact of the new sensor in conjunction with the extended FEM scheme on the overall focus budget for critical layers. Finally, we briefly show a possible integration scenario into the overall exposure strategy.
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Frank Kahlenberg, Rolf Seltmann, Bruno M. La Fontaine, René Wirtz, Aernout Kisteman, Roel N. M. Vanneer, Marco Pieters, "Best focus determination: bridging the gap between optical and physical topography", Proc. SPIE 6520, Optical Microlithography XX, 65200Z (27 March 2007); doi: 10.1117/12.712169; https://doi.org/10.1117/12.712169
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