Simulation of the effect of a resist-surface bound air bubble on imaging in immersion lithography

Resist-surface bound air bubbles have been identified as a possible defect mechanism in immersion lithography. The general expectation is that the bubble will primarily cause local dose reductions, but no detailed simulations on this effect have been published. The work described in this paper is a first attempt to do so: we have simulated the effect of bubbles on 1:1 dense Line/Space patterning. Our results confirm that the major effect of the presence of a bubble is indeed underexposure - or in most cases even non-exposure - of the pattern in the area occupied by the bubble, but it also identifies a few more subtle characteristics of bubble-induced defects which can help identify defects observed on immersion wafers as being caused by a bubble. Apart from the simulation results, we also show a few experimentally observed immersion defects, which we believe are indeed generated by a bubble.