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Influence of mask surface processing on CD-SEM imaging

Photomask processing during mask manufacturing can influence key-parameters of CD-SEM measurement such as measurement accuracy and repeatability. In particular, resist strip and mask clean processes seriously affect the surface properties of CoG-reticles and therefore increase the chrome-CD-measurement uncertainty. In extreme cases, the change of chemo-physical properties of the top Chromium oxide layer can have dramatically impact on the electrons emitted by the surface, utilized by the imaging process of CD SEMs. This change leads to a decreased secondary electron yield, and even more dramatic, induces charging near isolated chromium structures. Although the surface changes appear to be reversible with typical charge decay times in the order of days, the associated charging effect is in severe conflict to the common demand for low cycle times and high measurement accuracy experiments for the CD-measurement processes. In this work, we present fundamental experiments on Chromium oxide layers taking into account the effect of hydrophilicity on the optical as well as on the electrical behavior. We observe that the dependence of secondary electron yield on primary electron energy turns out to be the main issue effecting charging of electrically floating chromium structures. This charging effect thus can be understood in terms of shifting the so-called "iso-electrical point", ultimately resulting in contrast reversion. Furthermore, our data are compared to a simple model respecting electron induced charging during mask surface activation. Our model is supported by numerical calculations of the effective surface potentials near isolated chromium structures. Moreover, the model provides ways to find novel routes for improved surface preparation prior to CD-measurement that allow a good SEM imaging behavior.