Measurement of linewidths on silicon and iron-oxide photomasks is hampered by the dark banding which occurs along the edges. It is shown that this banding arises from the combination of low contrast and optical path difference introduced by the silicon or iron-oxide in conjunction with the partial coherence of the illumination. As previously shown in work with chromium-oxide photomasks, when the condenser numerical aperture is sufficiently less than that of the objective, an expression can be derived for the proper transmittance threshold for determining edge location. An expression is also derived for the linewidth error which would result from locating the edge at the center of the dark band. Theoretical and experimental results are compared.
"Optical Linewidth Measurements On Silicon And Iron-Oxide Photomasks", Proc. SPIE 0100, Developments in Semiconductor Microlithography II, (8 August 1977); doi: 10.1117/12.955363; https://doi.org/10.1117/12.955363