SummaryFocused electron beam induced deposition (FEBID) is a single-step, direct-write nanofabrication technique capable of writing three-dimensional metal-containing nanoscale structures on surfaces using electron-induced reactions of organometallic precursors. Currently FEBID is, however, limited in resolution due to deposition outside the area of the primary electron beam and in metal purity due to incomplete precursor decomposition. Both limitations are likely in part caused by reactions of precursor molecules with low-energy (<100 eV) secondary electrons generated by interactions of the primary beam with the substrate. These low-energy electrons are abundant both inside and outside the area of the primary electron beam and are associated with reactions causing incomplete ligand dissociation from FEBID precursors. As it is not possible to directly study the effects of secondary electrons in situ in FEBID, other means must be used to elucidate their role. In this context, gas phase studies can obtain well-resolved information on low-energy electron-induced reactions with FEBID precursors by studying isolated molecules interacting with single electrons of well-defined energy. In contrast, ultra-high vacuum surface studies on adsorbed precursor molecules can provide information on surface speciation and identify species desorbing from a substrate during electron irradiation under conditions more representative of FEBID. Comparing gas phase and surface science studies allows for insight into the primary deposition mechanisms for individual precursors; ideally, this information can be used to design future FEBID precursors and optimize deposition conditions. In this review, we give a summary of different low-energy electron-induced fragmentation processes that can be initiated by the secondary electrons generated in FEBID, specifically, dissociative electron attachment, dissociative ionization, neutral dissociation, and dipolar dissociation, emphasizing the different nature and energy dependence of each process. We then explore the value of studying these processes through comparative gas phase and surface studies for four commonly-used FEBID precursors: MeCpPtMe3, Pt(PF3)4, Co(CO)3NO, and W(CO)6. Through these case studies, it is evident that this combination of studies can provide valuable insight into potential mechanisms governing deposit formation in FEBID. Although further experiments and new approaches are needed, these studies are an important stepping-stone toward better understanding the fundamental physics behind the deposition process and establishing design criteria for optimized FEBID precursors.

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Motivated by the potentially adverse effects of dissociative electron attachment (DEA) in focused electron beam induced processing (FEBIP), we have conducted a gasphase DEA study on the common FEBIP precursor molecule, cobalt tricarbonyl nitrosyl. We have determined the absolute DEA cross-sections and the branching ratios for the individual fragmentation processes in the energy range from about 0-9 eV. We further report the adiabatic electron affinities (EAs) of the corresponding neutral radicals. Finally, we propose a fragmentation mechanism, which we believe is valid for DEA to metal-carbonyl compounds in general.Initially discovered as an unwelcome side effect in electron microscopy, [1] FEBIP has quickly been embraced as a clean and precise tool for manipulating and controlling matter on a small scale. A focused, high-energy electron beam is used to locally dissociate adsorbed precursor molecules. Ideally, a chemically and structurally well-defined deposit is left behind while volatile fragments are pumped away. Minimization of the spatial resolution and eliminating contamination of the deposited structures remain the two main challenges of FEBIP. [2,3] Albeit a highly focused primary electron beam, the width of the deposits is typically a multiple of the incident beam diameter.[4] Elastic and inelastic scattering events are unavoidable when irradiating samples with high-energy electron beams. Consequently, secondary (SE) and backscattered electrons (BSE) are emitted from the sample surface and the deposit, creating electron flux outside the focus point of the primary beam. Simulations as well as experiments show that the SE energy distribution typically peaks at low energies, that is, < 15 eV, and their intensities are far from negligible. [2,4,5] At these low energies, a new fragmentation pathway becomes available, that is, DEA. In contrast to fragmentation by direct electron impact, where excess energy of several electron volts is required, DEA can occur close to zero eV threshold. Furthermore, DEA reactions can exhibit fairly large cross-sections of 10 À18 to 10 À16 m 2 and are highly bond selective with regards to the electron energy. [6,7] Because of the abundance of low-energy SEs and BSEs generated in FEBIP and the potentially high cross-sections, DEA may play a significant role in FEBIP broadening. Additionally, the high bond selectivity in DEA may contribute to the deposition of incompletely decomposed precursor molecules and thus to an increased nonmetallic fraction of the deposit. Presently, no DEA cross-section data has been reported on relevant FEBIP precursor molecules and current simulations either neglect dissociation caused by low-energy electrons altogether or have to rely on an educated guess for the cross-sections.[4]Herein we report absolute DEA cross-sections and branching ratios for cobalt tricarbonyl nitrosyl. We report the electron affinities for the neutral radical fragments and we propose a general DEA mechanism for metal-carbonyl compounds.The present experiments were performed in...