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Image contrast of line-cut / contact hole features in Complementary E-Beam Lithography (CEBL)

Image contrast of line-cut and contact hole features patterned using Complementary E-Beam Lithography (CEBL) at advanced technology nodes are analyzed. The study assumes one beam in each column is used to pattern features less than 20 nm (Full Width Half Maximum, FWHM), consistent with Multibeam’s multi-column vector-scan approach for CEBL patterning. When the feature size approaches the resolution of the e-beam column design, the dose intensity profile follows a Gaussian model. Using Gaussian profiles, the image contrast of line cut or contact hole features can be studied as a function of beam FWHM size, spacing between features, and proximity effect. As expected, the image contrast was dominated by contact hole stepping distance (i.e., spacing between neighboring contact holes) and proximity effect. The plot of image contrast versus contact position becomes very useful in studying the impact of contact spacing, proximity effect and process window in writing line-cut or contact features in CEBL applications. Based on a given design rule of contact hole size and spacing, we can determine the appropriate e-beam size and resist contrast to achieve good image contrast. The relationship between resist contrast and image contrast is discussed to estimate the process window in CEBL applications. Finally, the impact of electron forward scattering in resist is analyzed, including the effects of resist thickness and beam voltage selections. We determined that the influence of back scattered electrons is not a significant factor in CEBL applications when feature pattern density is less than 11%.