Topography induced defocus with a scanning exposure system

Our case study experimentally gauges the defocus component induced by a step in the exposure field substrate, with the edge of the step aligned parallel to the scanning slit. Such steps frequently occur at the border of different chiplets or process monitors within one exposure field. A common assumption is that a step-and-scan imaging system can correct for the majority of such topography, since the wafer is dynamically leveled under the static image plane as it is scanned. Our results show that the range of defocus approaches about 85% of the actual step height and thus contributes significantly to the overall focusing variance. This area on the wafer in which defocus can be observed extends by more than 3mm to both sides of the step. In the same area a degradation of imaging fidelity can be observed in the form of exaggerated proximity effects.