Facile fabrication of nanogap electrodes for suspended graphene characterization using direct ion beam patterning

Abstract

Graphene is a two-dimensional sheet of carbon atoms with exceptional electronic and mechanical properties, giving it tremendous potential in nanoelectromechanical system devices. Here, we present a method to easily and reproducibly fabricate suspended graphene nanoribbons across nanogap electrodes of various separation lengths, demonstrating a technique with aggressive gap scalability and device geometry control. Fabrication is based on using a focused gallium ion beam to create a slit between joined electrodes prepatterened on a 100 nm thick silicon nitride membrane. The transparency of the nitride membrane provides reduced ion backscattering and adds milling resolution. Large-area monolayer graphene grown by atmospheric pressure chemical vapor deposition was transferred onto the silicon nitride chip and patterned into a free-standing ribbon geometry via electron beam lithography on organic ebeam resist followed by an O2 plasma etch. We find that commonly used inorganic negative tone resist that requires a buffered oxide etch for resist removal will attack the adhesion layer (Cr2O3) between the electrode and nitride membrane, which is exposed immediately after milling, so an organic resist was selected to avoid this. Using this technique, we fabricate freestanding graphene devices contacted by electrodes of sub-100 nm separation length and preform a comparative study on the effects of current annealing on device resistance. The gap resolution of this technique is limited by the gallium ion beam, which allows for sub-100 nm gaps. Sub-10 nm gaps are feasible with He ion beams, proving direct applications in probing the high field transport properties of graphene nanoribbons at post-CMOS length scales.