Fabrication of nanopore with different parameters in depth and diameter on silicon substrate using one-step focused ion beam milling

Abstract

Focused ion beam (FIB) technique uses a focused beam of ions to scan the surface of a specimen, analogous to the way scanning electron microscope utilizes electrons. Recent developments of FIB technology have emerged as one of the most advanced multifunctional platforms for nanofabrication. It is a versatile tool for material removal with high accuracy at nanoscale. As a consequence, FIB has become increasingly popular among researchers for fabrication of nanopore structures. Nanopores are generally categorized as biological nanopores and solid-state nanopores. Biological nanopores are highly reproducible but suffer from shortcomings such as fixed pore size, mechanical instability and operate within minimal ranges of pH and temperature. Solid-state nanopores, in contrast, are favorable due to their robustness, controllable morphology, and high stability in various environmental conditions. Hence, solid-state nanopores fabricated using FIB are the focus of this research. Fabricating nanopore on silicon substrate using FIB is doubtless challenging in order to achieve fine structure and tip diameter less than 50 nm. These problems can be rectified by utilizing high-quality nanopores with relatively small tip diameter, an appropriate pore shape and proper materials. The ability to control the diameter of nanopores across a range of dimensions is considered crucial for this research. A smooth and fine surface of nanopore is challenging to obtain as it requires the proper selection of FIB parameters. Therefore, operating the milling process with appropriate FIB parameter selection is essential to achieve successful FIB milling. An acceleration voltage of 30 kV and beam current of 18 pA were used throughout this experiment. It was found that milling from the outer to the inner direction produced a better nanopore structure with less redeposition and a smaller tip. The nanopores milled using one-step FIB milling on silicon substrate resembled a conical-shaped structure. The maximum width and depth were measured at the base and tip of the nanopores, respectively. The milling diameter of 800 nm and depth of 1500 nm were found to be successfully employed to fabricate nanopores with tip diameter of 49.5 nm on thick silicon substrate. A smaller tip diameter was obtained when the aspect ratio was more than 1. These findings suggest that lowering the upper base diameter and increasing the depth may reduce the bottom tip diameter. The overall trend demonstrated that as the aspect ratio increases or milling depth increases, the angle of the nanopore sidewall gets more gradual and the conical shape becomes more defined. Finally, a nanopore with tip diameter of 49.2 nm was also demonstrated on thin silicon substrate using optimal parameters. In conclusion, this study provides greater insight into the nanopore fabrication process using FIB. It may serve as an important study for application of DNA sequencing and more applications involving solid-state nanopores.