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The phenomena of footing's variation caused by oxidation of nitrogen-containing substrates

With the substrate suffering from the contamination of nitrogen, the variation of after-developing-inspection (ADI) linewidth with time could be monitored by CDSEM. This phenomenon has been attributed to the footing produced by acid loss. Moreover, the oxidation of nitrogen-containing substrates can cause the variation of capability of trapping acid, so the profile of photoresist (PR) changes with time, which has been verified by top-down CDSEM picture and SEM cut. It is reported that plasma and chemical treatment for the substrate can passivate the trapping sites for acid to remove this instability. In this paper, simulation and contrast experiment will be applied to illustrate the mechanism of footing and to verify this explanation. First, reflectivity and standing wave have been calculated based on the mask pattern and substrate. Second, light intensity distribution in photoresist has been calculated through the simulation of image in resist. At last, we add photoacid diffusion to the image intensity distribution. Such depiction by diffusion equation also goes across the boundary between the photoresist and the substrate, where several effective diffusion constants are used. The variation caused by the oxidation of surface is explained and analyzed based on the model above. The method of monitoring the variation of footing only by top-down CDSEM picture is proposed.