The Role of Dissociative Electron Attachment in Focused Electron Beam Induced Processing: A Case Study on Cobalt Tricarbonyl Nitrosyl

Abstract

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...

Absolute cross sections for dissociative electron attachment and dissociative ionization of cobalt tricarbonyl nitrosyl in the energy range from 0 eV to 140 eV

“…Its formation is attributed to a higher-lying core-excited (one-hole two-particle) resonance, that also leads to the formation of [CoCO] − , but does not contribute to the ion yield of other fragments. 21 The parent anion is not observed in the negative ion mass spectra, which is presumably due to its lifetime, with regards to dissociation, being too short for it to be detected in our instrumental configuration.…”

“…Exceptions are the CoCO − and the Co − formation through the higher lying resonance that peaks around 6 eV in the respective ion yields. 21 In these contributions, the KER might thus be higher. Due to the high mass of the negative ions detected and the multiple ligand loss in the formation of the lighter fragments observed, we expect the KER in the DEA processes to be even lower than that for the positive ion fragments.…”

“…The dominant fragment formed through dissociative electron attachment to Co(CO) 3 NO is [Co(CO) 2 − (m/z = 143), formed through the loss of the nitrogen oxide group, is about an order of magnitude less intense. This is readily understandable as the thermochemical threshold for NO loss is about 0.5 eV 21,41 higher than the threshold for CO loss, making the CO loss the only accessible DEA channel close to threshold (0 eV) where the electron attachment cross sections are very high (at low energies these cross sections generally increase as a function of 1/ε, where ε is the incident electron energy). At sufficiently high internal energy, metastable decay and further loss of carbonyl ligands from [Co(CO) 2 − formation.…”

“…7,20 At those energies, dissociative electron attachment (DEA) is the only efficient fragmentation mechanism and, in that context, its potential role in FEBID has been discussed in some detail. [21][22][23][24][25] Although DEA is the only effective electron induced dissociation mechanism below the ionization limit of the respective molecules, SEs and BSEs, with incident energies close to and above the ionization limit of the respective molecules, can also cause fragmentation through dissociative ionization (DI). This is a non-resonant process, but can nonetheless be very efficient at fairly low incident energies (typically, in the range 10-50 eV).…”

Electron driven processes in sulphur containing compounds CH3SCH3 and CH3SSCH3

“…Such processes can be both bond and site selective [2]. This selectivity is of high importance, e.g., in chemical lithography [3], nanofabrication [4,5] and for modification and functionalization of the surfaces [6,7]. Besides technological applications this specificity is relevant for radiation chemistry [8].…”