Alignment errors from resist coating topography

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Abstract
The registration difficulty associated with a resist‐coated alignment mark depends on the type of optics used to illuminate and view the wafer alignment aid. The interaction of the light with the mark geometry is studied by calculating the outgoing wave front that results when an arbitrary collection of correlated or uncorrelated incident wave fronts is reflected from the nonplanar target structure. This solution is combined with a model of the partially coherent imaging system to predict the alignment signal. Bright‐field and dark‐field systems typical of those used in optical lithography are studied. Dark‐field viewing optics are generally less sensitive to offsets caused by resist coating asymmetries. The statistical alignment precision is related to the shape of the alignment signal and the signal‐to‐noise ratio. It is shown that the increased contrast in dark‐field systems can provide an alignment precision comparable to that of bright‐field systems despite a substantially lower photon collection efficiency.

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