The switch from dry to immersion lithography has important consequences regarding wafer defectivity. It has been shown
that for successful and efficient defect reductions related to immersion lithography the capability to distinguish
immersion/patterning related defects from stack related defects is very useful during process control. These stack related
defects can be observed after careful partitioning of individual layer inspections and the analysis of this data through DSA in
Klarity. The optimisation of the dark field inspection SP2 tool, central in this paper, shows that improved sensitivity at
adequate signal to noise ratio can be obtained on the resist stacks by using the smaller wavelength as the UV-laser light
present in the SP2. For bare Si and BARC oblique incidence illumination gives the best sensitivity and captures the most
defects. However monitoring of the resist and stacks with resist requires normal incidence illumination since the nature of
defects and film result in a higher scattering intensity using normal illumination. The use of an optical filter and a 10% laser
power also contributed to establishing a lower and stable background signal for each inspection scan. As immersion tool
development is improved and immersion specific defectivity is reduced, the proportion of the stack related defects will
become a significant fraction of the overall target for further defect reduction. This includes point defects (embedded
particles) or flow defects (streaks) identified and classified using SURFimage. Finally this information is to be used to
identify the defect origin(s) for ultimate elimination of defects in the stacks.
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