Multiscale simulation of the focused electron beam induced deposition process

www.nature.com/scientificreports OPEN Multiscale simulation of the focused electron beam induced deposition process Pablo de Vera1,4*, Martina Azzolini2, Gennady Sushko1, Isabel Abril3, Rafael Garcia‑Molina4, Maurizio Dapor2, Ilia A. Solov’yov5 & Andrey V. Solov’yov1 Focused electron beam induced deposition (FEBID) is a powerful technique for 3D-printing of complex nanodevices. However, for resolutions below 10 nm, it struggles to control size, morphology and composition of the structures, due to a lack of molecular-level understanding of the underlying irradiation-driven chemistry (IDC). Computational modeling is a tool to comprehend and further optimize FEBID-related technologies. Here we utilize a novel multiscale methodology which couples Monte Carlo simulations for radiation transport with irradiation-driven molecular dynamics for simulating IDC with atomistic resolution. Through an in depth analysis of W(CO)6 deposition on SiO2 and its subsequent irradiation with electrons, we provide a comprehensive description of the FEBID process and its intrinsic operation. Our analysis reveals that simulations deliver unprecedented results in modeling the FEBID process, demonstrating an excellent agreement with available experimental data of the simulated nanomaterial composition, microstructure and growth rate as a function of the primary beam parameters. The generality of the methodology provides a powerful tool to study versatile problems where IDC and multiscale phenomena play an essential role. Interaction of photon, neutron and charged particle beams with matter finds plenty of technological applications, particularly in materials science and n anotechnology1–4. Improvements in beam focusing and control are yielding cutting-edge methodologies for the fabrication of nanometer-size devices featuring unique electronic, magnetic, superconducting, mechanical and optical p roperties2, 3, 5–9. Among them, focused electron beam induced deposition (FEBID) is especially promising, as it enables reliable direct-write fabrication of complex, free-standing 3D nano-architectures3, 10. Still, as the intended resolution falls below 10 nm, even FEBID struggles to yield the desired size, shape and chemical c omposition10–13, which primarily originates from the lack of molecular-level understanding of the irradiation-driven chemistry (IDC) underlying nanostructure formation and growth10, 14. Further progress requires to learn how to finely control IDC, a goal which will require important experimental and theoretical efforts. Multiscale simulations15–17 can become a powerful tool to help in this endeavour, provided that a model sufficiently accurate can be developed. This investigation aims to explore this possibility. FEBID operates through successive cycles of organometallic precursor molecules replenishment on a substrate and irradiation by a tightly-focused electron beam, which induces the release of organic ligands and the growth of metal-enriched nanodeposits. It involves a complex interplay of phenomena, each of them requiring dedicated computational approaches: (a) deposition, diffusion and desorption of precursor molecules on the substrate; (b) multiple scattering of the primary electrons (PE) through the substrate, with a fraction of them being reflected (backscattered electrons, BSE) and the generation of additional secondary electrons (SE) by ionization; (c) electron-induced dissociation of the deposited molecules; and (d) the subsequent chemical reactions, along with potential thermo-mechanical e ffects18. While processes (b) and (c) typically happen on the femtosecondpicosecond timescale, (a) and (d) may require up to microseconds or even longer. Monte Carlo (MC) simulations have become a tool of choice for studying electron transport in condensed matter, and can also account for diffusion-reaction of m olecules19–23, although without offering atomistic details. At the atomic/molecular level, ab initio methods permit the precise simulation of electronic transitions or chemical bond r eorganization24, 25, although their applicability is typically limited to the femtosecond–picosecond timescales and to relatively small molecular sizes. In between these approaches, classical molecular dynamics (MD)17 and particularly reactive 1 MBN Research Center, Altenhöferallee 3, 60438 Frankfurt am Main, Germany. 2European Centre for Theoretical Studies in Nuclear Physics and Related Areas (ECT*), 38123 Trento, Italy. 3Departament de Física Aplicada, Universitat d’Alacant, 03080 Alacant, Spain. 4Departamento de Física – Centro de Investigación en Óptica y Nanofísica (CIOyN), Universidad de Murcia, 30100 Murcia, Spain. 5Department of Physics, Carl von Ossietzky University, Carl‑von‑Ossietzky Straße 9‑11, 26129 Oldenburg, Germany. *email: Scientific Reports | (2020) 10:20827 | https://doi.org/10.1038/s41598-020-77120-z 1 Vol.:(0123456789) www.nature.com/scientificreports/ MD26 have proved to be very useful in the atomistic-scale analysis of molecular fragmentation and chemical reactions up to nanoseconds and m icroseconds26, 27. Still, a comprehensive and predictive multiscale simulation including all the FEBID-related processes has been, up to now, an elusive task. A breakthrough into the atomistic description of FEBID was recently achieved16 by means of the new method that permitted simulations of irradiation-driven MD (IDMD) with the use of the software packages MBN Explorer28 and MBN S tudio29. IDMD superimposes probabilities of various quantum processes (e.g., ionization, dissociative electron attachment) occurring in large and complex irradiated systems, stochastically introducing chemically reactive sites in the course of affordable reactive MD simulations. In the present investigation we utilize a combination of the aforementioned MC and IDMD methodologies and perform the first inclusive simulation of radiation transport and effects in a complex system where all the FEBID-related processes (deposition, irradiation, replenishment) are accounted for. Here specifically, detailed space-energy distributions of electrons, obtained from MC23, 30, 31 at different irradiation conditions, were used as an input for IDMD simulations16, 17 on experimentally-relevant timescales, where a direct comparison could be performed. The coupled MC-IDMD approach was employed, for the first time, to analyze IDC at the atomistic level of detail for W(CO)6 molecules deposited on hydroxylated SiO2 . In particular, the dependence on the primary beam energy and current of the surface morphology, composition and growth rate of the created nanostructures was analyzed and was shown to be in an excellent agreement with results of available e xperiments32. This new methodology provides the necessary molecular-level insights into the key processes behind FEBID for its further development. Furthermore, the approach being general and readily applicable to any combination of radiation type and material, opens (...truncated)