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.

Surface reactions of electrons and ions with physisorbed organometallic precursors are fundamental processes in focused electron and ion beam-induced deposition (FEBID and FIBID, respectively) of metal-containing nanostructures. Markedly different surface reactions occur upon exposure of nanometer-scale films of (η5-Cp)­Fe­(CO)2Re­(CO)5 to low-energy electrons (500 eV) compared to argon ions (860 eV). Electron-induced surface reactions are initiated by electronic excitation and fragmentation of (η5-Cp)­Fe­(CO)2Re­(CO)5, causing half of the CO ligands to desorb. Residual CO ligands decompose under further electron irradiation. In contrast, Ar+-induced surface reactions proceed by an ion–molecule momentum/energy transfer process, causing the desorption of all CO ligands without significant ion-induced precursor desorption. This initial decomposition step is followed by ion-induced sputtering of the deposited atoms. The fundamental insights derived from this study can be used not only to rationalize the composition of deposits made by FEBID and FIBID but also to inform the choice of a charged particle deposition strategy and the design of new precursors for these emerging nanofabrication tools.

Using in situ X-ray photoelectron spectroscopy (XPS), the effects of low energy (500 eV) electrons and low energy (1200 eV) Ar+ ions on thin films of Fe­(CO)5, a prototypical organometallic precursor, have been investigated. These studies were motivated by the important role that these surface reactions play in the charged-particle-induced deposition of nanostructures. XPS data from the C­(1s) and O­(1s) regions were used to construct kinetic models to describe the effects of electron and ion irradiation, both of which occurred through a sequence of two sequential surface reactions, although the details of each step differ. During electron irradiation, precursor molecules initially decompose as a result of electronic excitation, resulting in desorption of approximately 50% of the CO ligands and partial decarbonylation within the Fe­(CO)5 film. In the second step, the partially decarbonylated intermediates undergo a much slower electron-stimulated CO decomposition process to produce iron oxides encased in a graphitic film. Fe2(CO)9 and Fe3(CO)12 reacted similarly to Fe­(CO)5, but the initial rate of decomposition was an order of magnitude higher. During Ar+ bombardment, Fe­(CO)5 molecules decompose as a consequence of energy transfer from the incident ions, causing complete fragmentation of the precursor and desorption of ≈80% of the CO molecules. The remaining 20% undergo CO bond cleavage, forming adsorbed carbon and volatile oxygen species. In the second step of the reaction, the residual iron and carbon atoms are subject to Ar+ ion sputtering. The implications of these reactions for focused ion beam-induced deposition (FIBID) and focused electron beam-induced deposition (FEBID) from Fe­(CO)5 are also discussed.

Electron-induced surface reactions of (η-CH)Fe(CO)Mn(CO) were explored in situ under ultra-high vacuum conditions using X-ray photoelectron spectroscopy and mass spectrometry. The initial step involves electron-stimulated decomposition of adsorbed (η-CH)Fe(CO)Mn(CO) molecules, accompanied by the desorption of an average of five CO ligands. A comparison with recent gas phase studies suggests that this precursor decomposition step occurs by a dissociative ionization (DI) process. Further electron irradiation decomposes the residual CO groups and (η-CH, Cp) ligand, in the absence of any ligand desorption. The decomposition of CO ligands leads to Mn oxidation, while electron stimulated Cp decomposition causes all of the associated carbon atoms to be retained in the deposit. The lack of any Fe oxidation is ascribed to either the presence of a protective carbonaceous matrix around the Fe atoms created by the decomposition of the Cp ligand, or to desorption of both CO ligands bound to Fe in the initial decomposition step. The selective oxidation of Mn in the absence of any Fe oxidation suggests that the fate of metal atoms in mixed-metal precursors for focused electron beam induced deposition (FEBID) will be sensitive to the nature and number of ligands in the immediate coordination sphere. In related studies, the composition of deposits created from (η-CH)Fe(CO)Mn(CO) under steady state deposition conditions, representative of those used to create nanostructures in electron microscopes, were measured and found to be qualitatively consistent with predictions from the UHV surface science studies.