Material Removal Mechanisms in Focused Ion Beam Nanopore Drilling

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The objective of this project is to better understand the means by which focused ion beam (FIB) instruments remove material from surfaces. FIB instruments, already common tools in the semiconductor industry, use extremely narrow beams of ions, as small as a few nanometers wide, to mill or etch surfaces. As FIB processing finds broader applications in other areas of nanotechnology, such as in the drilling of nanopores for DNA sequencing, which is an enabling technology in the multi-billion dollar molecular medical diagnostics industry, there is an emerging need for better understanding of FIB processing at the finest scales. This project will lead to more effective use of FIB instruments through the development of predictive models for the material response to the ion beam. The project will involve collaboration with a leading European research group working on improving the resolution of FIB nanopore fabrication, and regular consultation with the leading US producer of FIB instruments. Graduate and undergraduate students will be trained in both the computational and experimental science of ion interactions with materials, and an outreach program aimed at K-12 students will be developed.The hypothesis driving the project is that the fundamental mode of material removal in FIB processing is strongly flux-dependent at the smallest scale, changing from an erosion-driven process at low fluxes to a phase-change process at high fluxes. To test this hypothesis, FIB nanopore drilling will be investigated over a range of conditions, with particular emphasis on the effects of ion beam flux. Large-scale parallel molecular dynamics simulations will be carried out to simulate FIB nanopore drilling in a freestanding thin film target. The simulations will involve up to 10 million atoms over millions of time steps, and will be the first-ever full molecular dynamics simulations of FIB drilling. Material removal and rearrangement mechanisms, in particular explosive boiling and thermocapillary mass transport, will be fully characterized and explained. A multiscale statistical model will be developed to extend the molecular dynamics simulations to study long time-scale processes such as low-flux FIB processing. Finally, targeted experiments on FIB nanopore drilling will guide the development and validation of the computation-based model. FIB nanopores, with diameters of less than 10nm, will be fabricated using a high-resolution dual-beam FIB instrument and characterized for comparison to the simulation results.