Electron beam lithography for direct patterning of MoS2 on PDMS substrates

Abstract

Precise patterning of 2D materials into micro- and nanostructures presents a considerable challenge and many efforts are dedicated to the development of processes alternative to the standard lithography. In this work we show a fabrication technique based on direct electron beam lithography (EBL) on MoS₂ on polydimethylsiloxane (PDMS) substrates. This easy and fast method takes advantage of the interaction of the electron beam with the PDMS, which at high enough doses leads to cross-linking and shrinking of the polymer. At the same time, the adhesion of MoS₂ to PDMS is enhanced in the exposed regions. The EBL acceleration voltages and doses are optimized in order to fabricate well-defined microstructures, which can be subsequently transferred to either a flexible or a rigid substrate, to obtain the negative of the exposed image. The reported procedure greatly simplifies the fabrication process and reduces the number of steps compared to standard lithography and etching. As no additional polymer, such as polymethyl methacrylate (PMMA) or photoresists, are used during the whole process the resulting samples are free of residues.

Fig. 1. Schematic representation of the fabrication process. (a) Electron beam lithography on MoS₂ on PDMS substrate followed by dry transfer to (b) the final substrate where the negative of the exposed pattern is obtained.

Fig. 2. (a) Schematic of a section of PDMS polymer chain. (b) Real width of the (nominal) 1 μm wide rectangle exposed at 10, 20 and 30 kV and varying doses. Depth of the of the exposed features in MoS₂/PDMS as a function of exposure dose for (c) 20 kV and (d) 30 kV acceleration voltage. 3D representations of the AFM images of rectangles exposed at (e) 20 kV and 600 μC cm⁻² dose (f) 30 kV and 300 μC cm⁻² dose.

Fig. 3. (a) Optical image of the set of 1 mm long lines with varying widths exposed at 20 kV. Scale bar 100 μm. (b) Detail of the sample. The well-defined lines are only visible where the MoS₂ film is present. Scale bar 30 μm. (c) The sample transferred to PET substrate. Scale bar 100 μm.

Fig. 4. (a) Example of an optical image of MoS₂ layer on PDMS after EBL, exposed at 20 kV and 300 μC cm⁻² dose. (b) Optical image of MoS₂ layer after EBL transferred to Si/SiO₂ substrate. Scale bars 20 μm. (c) SEM image of the detail of the transferred pattern. Scale bar 5 μm. (d) Processed image of (c). Red curve indicates the segmentation of the pattern. (e) Distribution of roughness calculated from the segmented pattern. (f) Raman spectra of the MoS₂ layer on original growth substrate and transferred to Si/SiO₂ after EBL.