Effect of Molecular Weight on the Feature Size in Organic Ice Resists

The feature size of patterns obtained by electron-beam lithography (EBL) depends critically on resist properties, beam parameters, development process, and instrument limitations. Frozen layers of simple organic molecules such as n-alkanes behave as negative-tone resists for EBL. With the unique advantage of an in situ thermal treatment replacing chemical development, the entire lithographic process can be performed within a single instrument, thus removing the influence of chemical developers on the feature size. By using an environmental transmission electron microscope, we can also minimize the influence of instrumental limitations and explore the fundamental link between resist characteristics and feature size. Our results reveal that the onset dose of organic ice resists correlates with the inverse molecular weight and that in the thermal development the role of change in solubility of polymers is mirrored in a shift in the solid/vapor critical temperature of organic ices. With a 0.4 pA beam current, we obtained 4.5, 5.5, and 8.5 nm lines with frozen octane, undecane, and tetradecane, respectively, consistent with the predictions of a model we developed that links beam profile and feature size. The knowledge acquired on the response of small organic molecules to electron irradiation, combined with the flexibility and operational advantages of using them as qualified EBL resists, provides us with new opportunities for the design and production of nanodevices and broadens the reach of EBL especially toward biological applications.