Abstract
Focused 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.
Highlights
Focused electron beam induced deposition (FEBID) [1,2,3] is a direct-write method capable of creating nanostructures with potential scientific and industrial applications
This notion that the low energy secondary electrons (SEs) produced in FEBID may play a significant and even a determining role in the deposit formation has been verified both by simulations [11] and by experiments [12]. It has motivated a number of gas phase studies focusing on the energy dependence of the branching ratios and cross sections for various low energy (0–100 eV) electron-induced reactions with organometallic precursors such as Pt(PF3)4 [13,14], MeCpPtMe3 [15], W(CO)6 [16,17], Cu(hfac)2 and Pd(hfac)2 [18], Co(CO)3NO [10] and Fe(CO)5 [19]. These processes, which are comprised of dissociative electron attachment (DEA), dissociative ionization (DI), neutral dissociation (ND), and dipolar dissociation (DD), cannot be distinguished in FEBID or surface experiments with high-energy primary electrons (PEs) beams, where the precursor molecules are simultaneously exposed to a distribution of low energy SEs in addition to the PEs
To study low energy electron-induced processes in the gas phase, a low energy electron beam with a resolution of about 100 meV is crossed with an effusive beam of FEBID precursor molecules and the electron energy dependence for the formation of charged fragments is monitored by mass spectrometry (MS) with sufficient resolution and dynamic range to unambiguously detect all fragments formed
Summary
Focused electron beam induced deposition (FEBID) [1,2,3] is a direct-write method capable of creating nanostructures with potential scientific and industrial applications. It has motivated a number of gas phase studies focusing on the energy dependence of the branching ratios and cross sections for various low energy (0–100 eV) electron-induced reactions with organometallic precursors such as Pt(PF3)4 [13,14], MeCpPtMe3 [15], W(CO)6 [16,17], Cu(hfac) and Pd(hfac)2 [18], Co(CO)3NO [10] and Fe(CO)5 [19] These processes, which are comprised of DEA, DI, ND, and dipolar dissociation (DD), cannot be distinguished in FEBID or surface experiments with high-energy PE beams, where the precursor molecules are simultaneously exposed to a distribution of low energy SEs in addition to the PEs. in gas phase experiments, where these precursor molecules interact with well-defined low energy electron beams, the energy dependence and extent of individual fragmentation processes may be unambiguously determined. Future perspectives and the relevance of these studies to establishing design criteria for precursor molecules tailored for FEBID will be discussed
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