Direct writing immersion laser lithography on graphene monolayers using two-photon absorption

Abstract

Direct writing laser lithography has become increasingly attractive due to its mask-free nature, offering significant design flexibility and minimizing additional costs associated with new exposure masks. Among the various direct laser writing techniques, two-photon absorption direct laser writing stands out for its ability to fabricate very small features through nonlinear optical effects. Operating this technique in immersion, where the air gap between the lens and photoresist or probe is liquid-filled, has been shown to enable reduced feature sizes [D. P. Sanders, Chem. Rev. 110, 321 (2010).]. Previous studies have typically employed an immersion oil for this purpose [Jakkinapalli et al., J. Micromech. Microeng. 30, 125014 (2020).]. However, our experiments revealed that prolonged immersion times can lead to oil interacting with the photoresist, resulting in the formation of new compounds that are challenging to remove. Consequently, the altered photoresist becomes unresponsive to both exposure and development. In light of these findings, we conducted tests using alternative immersion liquids and propose a combination of different immersion liquids and air direct writing as a means to achieve optimal writing results.