Update on next generation metrology tool for DPL reticles

Abstract

Double Patterning Lithography (DPL) techniques for next generation wafer exposures are placing greater demand on the requirements for pattern placement accuracy on photomasks for three reasons. First, a new source of wafer overlay error results from interactions between the two masks, so the specification for each individual mask must be tightened to compensate. Second, specifications have become so tight that the distortion caused by the pellicle bending the mask has become a significant contributor to the wafer overlay error budget. Pellicle-induced distortions are particularly insidious because they are not repeatable from substrate to substrate. Third, the tightening of overlay specifications demands tighter e-beam pattern placement control throughout the die, regardless of pattern density. This makes measuring actual features in-die instead of registration test structures important.The combination of increased demand for greater pattern placement accuracy, a need to characterize the influence of pellicle distortions, and the requirement to measure actual device features drives the need for a pattern placement metrology system capable of high resolution through-pellicle in-die measurements. Key enablers of this capability include high measurement resolution, a low noise platform and a long working distance objective. This paper reports experimental results on mask features of various sizes using a next-generation pattern placement metrology system designed to meet the strict DPL requirements outlined here.