Focused Ion Beam Milling of Perylene Microcrystals into Photonic Modules: Effect of Substrate and the Ion Beam Current

The rapid development of organic photonic devices is possible with an industrially viable focused ion beam (FIB) milling technique. In this aspect, understanding and unveiling the optimized FIB milling parameters to attain nano-/microscale shape and size precision of molecular crystals are essential to achieve all-organic photonic modules. Herein, we demonstrate the FIB milling on perylene single crystals self-assembled on conductive (indium tin oxide coated polyethylene terephthalate: ITO-PET) and nonconductive (borosilicate glass coverslip) substrates. The beam parameters of 1 nA and 30 kV accelerating voltage are desirable for effectively milling organic crystal photonic modules like bent or curved waveguides and disc and ring resonators. The microcrystals on the glass substrate required a thin conductive gold layer for electron microscopy imaging to carry out FIB milling. Removal of this gold layer by post milling chemical processing helps to retain crystal fluorescence on a glass substrate. On the contrary, the milling of microcrystals on ITO-PET does not require any additional conductive layers but shows poor fluorescence as they are directly exposed to the milling ion beam. These results demonstrate the importance of optimal milling parameters, choice of substrate (conductive or nonconductive), and a protective gold layer for effectively milling organic crystal into useful photonic modules. Thus, this nascent and promising technology can be transferred toward fabricating industrially relevant organic-crystal-based microelectronic and nanophotonic devices.