CHARACTERIZING LOW ENERGY ELECTRON-INDUCED REACTIONS WITH THIN FILMS, ADSORBATES, AND GAS PHASE MOLECULES

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Date
2019-01-22
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Johns Hopkins University
Abstract
Low energy electrons play an important role in the creation and modification of nanoscale thin films. The ability to characterize the lateral effects of these reactions is a necessary technique in the nanoscale fields of tribology and lithography. Of particular interest is the ability to discern the difference between sp2 and sp3-hybridized carbon. In nanoscale thin films, differences in the ratios between these two allotropes has been shown to drastically change the physical properties of the films they inhabit. However, due to their similarities, both allotropes can be somewhat difficult to distinguish, especially in nanoscale thin films. To this end, low energy electrons were used to deposit and modify sp3-like carbon into a more graphitic sp2-like carbon species. X-ray photoelectron spectroscopy (XPS) and Spectroscopic Ellipsometry (SE), were then used to interrogate and spatially map the resulting changes. The electron-induced transformations produced measurable changes to the C 1s peak profiles (XPS) and the dielectric functions (SE) of the carbon films. The films were then successfully characterized as a function of electron dose using (i), a linear combination of the individual components obtained in XPS, and (ii), a Bruggeman effective medium approximation of the SE film response for both types of carbon species. Both techniques were able to show similar results regarding both lateral film composition and thickness. Further changes to the films’ dimensions and structure were induced using atomic hydrogen etching. Both characterization techniques were able to track the conversion of sp2-like carbon back into sp3-like carbon species in addition to providing etching rates for the films. The holy grail of nanoscale film fabrication is the ability to control the chemical composition of the resulting film. While low energy electrons have been shown to induce a variety of reactions, dissociation of a molecule through electron attachment (DEA) is of considerable interest. The resonant nature of the electron attachment process allows for DEA to have high cross sections and be bond specific. This opens a wide range of possibilities regarding the control of nanoscale thin films. To this end, the efficacy of this approach was studied with regard to carbon nanomembrane fabrication. Due to biphenyl’s success as a CNM precursor, 2-iodobiphenyl (2-I-BPh), 2-bromobiphenyl (2-Br-BPh), and 2-chlorobiphenyl (2-Cl-BPh) were subjected to gas phase studies to probe their sensitivity to DEA interactions. High DEA cross sections were observed at 0 eV for 2-I-BPh as compared to the other halogenated biphenyl compounds. The efficacy of these findings was then tested by monitoring the effects of electrons on thiolated versions of these compounds. The results showed that the iodized compound formed cross-linkages at a faster rate than the other halogenated species and non-halogenated biphenyl thiol control. Further gas phase studies on 2-I-BPh, 2-Br-BPh, and 2-Cl-BPh focused on the DI pathways of all three molecules. These results showed that all three molecules showed similar fragmentation patterns. The appearance energies of the dominant fragments were identified from the ion-yields of all three compounds. By comparing hessian thermochemical calculations to the observed appearance energies of these dominant fragments, several DI fragmentation pathways were identified and shown to be consistent for all three compounds. Lastly, electron beam induced deposition is a fabrication technique capable of creating complex three-dimensional nano-structures through the decomposition and deposition of transiently adsorbed precursors. In general, the composition of these resulting structures is strongly dependent on the precursors used in the deposition process. As a result, understanding the kinetics behind this process is essential to designing an ideal precursor. To this end the effects of 500 eV electrons on several iron carbonyl precursors (Fe(CO)5, Fe2(CO)9, and Fe3(CO)12) were studied. In general, their results showed that the electron induced deposition of these species is a two-step mechanism that begins with the creation of a partially decarbonylated species through CO emission and ends with the electron stimulated decomposition of the remaining CO species to produce a metal (in this case iron) oxide and graphitic carbon. A kinetic model was fit to data obtained for Fe(CO)5 showing that the production of partially decarbonylated iron species proceeds at a much faster rate than the electron decomposition, k1 = 0.004147 s-1 and k2 = 0.000068 s-1 respectively. Comparing the different iron carbonyl species showed that while the rates of decomposition are precursor dependent (about an order of magnitude faster for Fe2(CO)9 and Fe3(CO)12), the overall CO loss is effectively similar for all three compounds. Annealing experiments, performed on Fe(CO)5 and Fe3(CO)12 showed that the partially decarbonylated species of both compounds were particularly sensitive to thermal reactions at room temperature. The implications of these results for EBID conditions were postulated, indicating that the susceptibility of partially decarbonylated species to thermal reactions at room temperature is suspected to allow the creation of high purity deposits in situations where the electron dose is sufficiently small. Advisor: D. Howard Fairbrother
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Keywords
Low Energy Electrons, Thin Films, X-ray Photoelectron Spectroscopy, Spectroscopic Ellipsometry, Dissociative Electron Attachment, Electron Beam Induced Deposition, Carbon Nanomembranes, Atomic Hydrogen
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