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.

Electronic resonances commonly decay via internal conversion to vibrationally hot anions and subsequent statistical electron emission. We observed vibrational structure in such an emission from the nitrobenzene anion, in both the 2D electron energy loss and 2D photoelectron spectroscopy of the neutral and anion, respectively. The emission peaks could be correlated with calculated nonadiabatic coupling elements for vibrational modes to the electronic continuum from a nonvalence dipole-bound state. This autodetachment mechanism via a dipole-bound state is likely to be a common feature in both electron and photoelectron spectroscopies.

We probe the transient anion states (resonances) in the dielectric gas C4F7N by the electron energy loss spectroscopy and the dissociative electron attachment spectroscopy. The vibrationally inelastic electron scattering leads to two excitation types. The first is the excitation of specific vibrational modes that are assigned with the help of an infrared spectrum of this molecule and quantum chemistry calculations. In the second type of vibrational excitation, the excess energy is randomized via internal vibrational redistribution in the temporary anion, and the electrons are emitted statistically. The electron attachment proceeds in three different regimes. The first is the formation of the parent C4F7N− anion at energies close to 0 eV. The second is a statistical evaporation of the F-atom, leading to the defluorinated anion C4F6N−. Finally, the third is dissociative electron attachment proceeding via the formation of several resonances and leading to a number of fragments. The present data explain the puzzling recent results of the pulsed-Townsend experiments with this gas.

The use of bimetallic precursors in focused electron beam induced deposition (FEBID) allows mixed metal nanostructures with well-defined metal ratios to be generated in a single step process. HFeCo 3 (CO) 12 is an example of one such bimetallic precursor that has previously been shown to form deposits with unusually high metal content (>80%) as compared to that of typical FEBID deposits (<30% metal content). To better understand the elementary bond breaking steps involved in FEBID of HFeCo 3 (CO) 12 , we have employed a UHV surface science approach to study the effect of electron irradiation on nanometer thick films of HFeCo 3 (CO) 12 molecules. Using a combination of in situ Xray photoelectron spectroscopy and mass spectrometry, we observed that the initial step of electron induced HFeCo 3 (CO) 12 dissociation is accompanied by desorption of ∼75% of the CO ligands from the precursor. A comparison with recent gas phase studies of HFeCo 3 (CO) 12 indicates that this process is consistent with a dissociative ionization process, mediated by the secondary electrons produced by interaction of the primary beam with the substrate. The loss of CO ligands from HFeCo 3 (CO) 12 in the initial dissociation step creates partially decarbonylated intermediates, HFeCo 3 (CO) x (x avg. ≈ 3). During a typical FEBID process, further electron exposure or thermal reactions can further transform these intermediates. In our UHV surface science approach, the effect of these two processes can be studied in isolation and identified. Under the influence of further electron irradiation, XPS data reveals that the remaining CO ligands in the partially decarbonylated intermediates decompose to form residual carbon and iron oxides, suggesting that those CO ligands that desorbed in the initial step are lost predominantly from the Co atoms. However, annealing experiments demonstrate that CO ligands in the partially decarbonylated intermediates desorb under vacuum conditions at room temperature, leaving behind films that are free of almost any carbon or oxygen contaminants. This combination of efficient CO desorption during the initial dissociation step, followed by thermal CO desorption from the partially decarbonylated HFeCo 3 (CO) x (x avg. ≈ 3) intermediates provide a rationale for the high metal contents observed in FEBID nanostructures created from HFeCo 3 (CO) 12 .

Dissociative electron attachment, 11 eV above the ionization energy of the focused electron beam induced deposition (FEBID) precursor HFeCo3(CO)12. A unique observation with potential significance for FEBID precursor design.