Super-Resolution Nanolithography of Two-Dimensional Materials by Anisotropic Etching

Dorte R Danielsen; Anton Lyksborg-Andersen; Kirstine E S Nielsen; Bjarke S Jessen; Timothy J Booth; Manh-Ha Doan; Yingqiu Zhou; Peter Bøggild; Lene Gammelgaard

doi:10.1021/acsami.1c09923

Super-Resolution Nanolithography of Two-Dimensional Materials by Anisotropic Etching

ACS Appl Mater Interfaces. 2021 Sep 8;13(35):41886-41894. doi: 10.1021/acsami.1c09923. Epub 2021 Aug 25.

Authors

Dorte R Danielsen^{1

2}, Anton Lyksborg-Andersen^{2

3}, Kirstine E S Nielsen^{1

2}, Bjarke S Jessen⁴, Timothy J Booth^{1

2}, Manh-Ha Doan^{1

2}, Yingqiu Zhou^{1

2}, Peter Bøggild^{1

2}, Lene Gammelgaard^{1

2}

Affiliations

¹ Department of Physics, Technical University of Denmark (DTU), Kgs. Lyngby 2800, Denmark.
² Centre for Nanostructured Graphene (CNG), Technical University of Denmark, Ørsteds Plads 345C, Kgs. Lyngby DK-2800, Denmark.
³ DTU Nanolab - National Centre for Nano Fabrication and Characterization, Technical University of Denmark (DTU), Kgs. Lyngby 2800, Denmark.
⁴ Department of Physics, Columbia University, New York, New York 10027, United States.

PMID: 34431654
DOI: 10.1021/acsami.1c09923

Abstract

Nanostructuring allows altering of the electronic and photonic properties of two-dimensional (2D) materials. The efficiency, flexibility, and convenience of top-down lithography processes are, however, compromised by nanometer-scale edge roughness and resolution variability issues, which especially affect the performance of 2D materials. Here, we study how dry anisotropic etching of multilayer 2D materials with sulfur hexafluoride (SF₆) may overcome some of these issues, showing results for hexagonal boron nitride (hBN), tungsten disulfide (WS₂), tungsten diselenide (WSe₂), molybdenum disulfide (MoS₂), and molybdenum ditelluride (MoTe₂). Scanning electron microscopy and transmission electron microscopy reveal that etching leads to anisotropic hexagonal features in the studied transition metal dichalcogenides, with the relative degree of anisotropy ranked as: WS₂ > WSe₂ > MoTe₂ ∼ MoS₂. Etched holes are terminated by zigzag edges while etched dots (protrusions) are terminated by armchair edges. This can be explained by Wulff constructions, taking the relative stabilities of the edges and the AA' stacking order into account. Patterns in WS₂ are transferred to an underlying graphite layer, demonstrating a possible use for creating sub-10 nm features. In contrast, multilayer hBN exhibits no lateral anisotropy but shows consistent vertical etch angles, independent of crystal orientation. Using an hBN crystal as the base, ultrasharp corners can be created in lithographic patterns, which are then transferred to a graphite crystal underneath. We find that the anisotropic SF₆ reactive ion etching process makes it possible to downsize nanostructures and obtain smooth edges, sharp corners, and feature sizes significantly below the resolution limit of electron beam lithography. The nanostructured 2D materials can be used themselves or as etch masks to pattern other nanomaterials.

Keywords: 2D materials; TMDs; Wulff constructions; anisotropic etching; downsizing; electron beam lithography; hBN; nanostructuring; reactive ion etching.