Exploring thermal surface chemistry inherent in focused electron beam induced deposition (FEBID) towards an optimal control of nanofabrication

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Focused electron beam induced deposition (FEBID) is a state-of-the-art tool for fabrication of nanoscale materials and devices. It relies on the electron-induced decomposition of volatile metal complexes when these precursors are dosed onto a surface that is scanned by a tightly focused electron beam. However, further improvement of FEBID is needed to achieve optimal control with respect to deposit growth and composition. In particular, the fundamental understanding of FEBID is so far based on the simplified view that an adsorbed precursor molecule remains intact until it encounters an impinging, backscattered, or secondary electron that will dissociate the precursor and thus immobilize material at the site of impact. The electron-driven reaction of the precursor is thus considered as decisive step in deposit formation. This neglects that thermal chemistry akin to chemical vapor deposition (CVD) can contribute to deposit growth and thus counteract the spatial control of deposition by the electron beam leading to uncontrolled variations of the deposit composition and shape. Therefore, the aim of this project is to provide a fundamental understanding of the transition between electron-driven and thermal surface chemistry of FEBID precursors. The novelty of the project is that the chemistry inherent in deposit formation is systematically studied using surface science tools under UHV conditions but on surfaces that are representative of FEBID deposits and thus relevant to deposit growth. The reactivity of the deposit surface as a result of electron irradiation is of particular interest. The use of heteroleptic precursors will allow us to study how the reactions can be controlled by replacing selected ligands within the complex. The perspectives of the proposed research are threefold: (1) Insight into the thermal surface chemistry of FEBID precursors on the growing deposit will allow to devise precursors and process conditions that suppress thermal surface reactions and thus enable exclusive control of deposit formation by the electron beam. The results will guide the development of novel FEBID processes with optimal control of deposit shape and composition. (2) Fundamental insight in how a reactive surface induces precursor dissociation and how this depends on the precursor architecture is relevant to nanofabrication processes that induce autocatalytic deposit growth by electron beam induced surface activation (EBISA). This will widen the range of precursors that can be used for thermal deposition with spatial selectivity defined by EBISA so that a larger variety of materials becomes accessible. (3) Knowledge of conditions under which specific FEBID precursors exhibit self-limiting dissociative adsorption on the surface of a growing deposit will provide guidance with respect to the integration of precise layer-by-layer deposition, akin to an electron-enhanced atomic layer deposition (EE-ALD) process, into spatially selective FEBID processes.