Electron scattering in Pt(PF3)4: Elastic scattering, vibrational, and electronic excitation

Dissociative ionization and electron beam induced deposition of tetrakis(dimethylamino)silane, a precursor for silicon nitride deposition

Motivated by the use of tetrakis(dimethylamino)silane (TKDMAS) to produce silicon nitride-based deposits and its potential as a precursor for Focused Electron Beam Induced Deposition (FEBID), we have studied its reactivity towards low energy electrons in the gas phase and the composition of its deposits created by FEBID. While no negative ion formation was observed through dissociative electron attachment (DEA), significant fragmentation was observed in dissociative ionization (DI). Appearance energies (AEs) of fragments formed in DI were measured and are compared to the respective threshold energies calculated at the DFT and coupled cluster (CC) levels of theory. The average carbon and nitrogen loss per DI incident is calculated and compared to its deposit composition in FEBID. We find that hydrogen transfer reactions and new bond formations play a significant role in the DI of TKDMAS. Surprisingly, a significantly lower nitrogen content is observed in the deposits than is to be expected from the DI experiments. Furthermore, a post treatment protocol using water vapour during electron exposure was developed to remove the unwanted carbon content of FEBIDs created from TKDMAS. For comparison, these were also applied to FEBID deposits formed with tetraethyl orthosilicate (TEOS). In contrast, effective carbon removal was achieved in post treatment of TKDMAS, while his approach only marginally affected the composition of deposits made with TEOS.

Dissociation of the FEBID precursor cis-Pt(CO)2Cl2 driven by low-energy electrons

In this study, we present experimental and theoretical results on dissociative electron attachment and dissociative ionisation for the potential FEBID precursor cis-Pt(CO)₂Cl₂. UHV surface studies have shown that high purity platinum deposits can be obtained from cis-Pt(CO)₂Cl₂. The efficiency and energetics of ligand removal through these processes are discussed and experimental appearance energies are compared to calculated thermochemical thresholds. The present results demonstrate the potential effectiveness of electron-induced reactions in the deposition of this FEBID precursor, and these are discussed in conjunction with surface science studies on this precursor and the design of new FEBID precursors.

Electron interactions with the heteronuclear carbonyl precursor H2FeRu3(CO)13 and comparison with HFeCo3(CO)12: from fundamental gas phase and surface science studies to focused electron beam induced deposition

In the current contribution we present a comprehensive study on the heteronuclear carbonyl complex H2FeRu3(CO)13 covering its low energy electron induced fragmentation in the gas phase through dissociative electron attachment (DEA) and dissociative ionization (DI), its decomposition when adsorbed on a surface under controlled ultrahigh vacuum (UHV) conditions and exposed to irradiation with 500 eV electrons, and its performance in focused electron beam induced deposition (FEBID) at room temperature under HV conditions. The performance of this precursor in FEBID is poor, resulting in maximum metal content of 26 atom % under optimized conditions. Furthermore, the Ru/Fe ratio in the FEBID deposit (≈3.5) is higher than the 3:1 ratio predicted. This is somewhat surprising as in recent FEBID studies on a structurally similar bimetallic precursor, HFeCo3(CO)12, metal contents of about 80 atom % is achievable on a routine basis and the deposits are found to maintain the initial Co/Fe ratio. Low temperature (≈213 K) surface science studies on thin films of H2FeRu3(CO)13 demonstrate that electron stimulated decomposition leads to significant CO desorption (average of 8-9 CO groups per molecule) to form partially decarbonylated intermediates. However, once formed these intermediates are largely unaffected by either further electron irradiation or annealing to room temperature, with a predicted metal content similar to what is observed in FEBID. Furthermore, gas phase experiments indicate formation of Fe(CO)4 from H2FeRu3(CO)13 upon low energy electron interaction. This fragment could desorb at room temperature under high vacuum conditions, which may explain the slight increase in the Ru/Fe ratio of deposits in FEBID. With the combination of gas phase experiments, surface science studies and actual FEBID experiments, we can offer new insights into the low energy electron induced decomposition of this precursor and how this is reflected in the relatively poor performance of H2FeRu3(CO)13 as compared to the structurally similar HFeCo3(CO)12.

Electron interaction with copper(II) carboxylate compounds

In the present study we have performed electron collision experiments with copper carboxylate complexes: [Cu2(t-BuNH2)2(µ-O2CC2F5)4], [Cu2(s-BuNH2)2(µ-O2CC2F5)4], [Cu2(EtNH2)2(µ-O2CC2F5)4], and [Cu2(µ-O2CC2F5)4]. Mass spectrometry was used to identify the fragmentation pattern of the coordination compounds produced in crossed electron - molecular beam experiments and to measure the dependence of ion yields of positive and negative ions on the electron energy. The dissociation pattern of positive ions contains a sequential loss of both the carboxylate ligands and/or the amine ligands from the complexes. Moreover, the fragmentation of the ligands themselves is visible in the mass spectrum below m/z 140. For the studied complexes the metallated ions containing both ligands, e.g., Cu2(O2CC2F5)(RNH2)+, Cu2(O2CC2F5)3(RNH2)2+ confirm the evaporation of whole complex molecules. A significant production of Cu+ ion was observed only for [Cu2(µ-O2CC2F5)4], a weak yield was detected for [Cu2(EtNH2)2(µ-O2CC2F5)4] as well. The dissociative electron attachment processes leading to formation of negative ions are similar for all investigated molecules as the highest unoccupied molecular orbital of the studied complexes has Cu-N and Cu-O antibonding character. For all complexes, formation of the Cu2(O2CC2F5)4-• anion is observed together with mononuclear DEA fragments Cu(O2CC2F5)3-, Cu(O2CC2F5)2- and Cu(O2CC2F5)-•. All dominant DEA fragments of these complexes are formed through single particle resonant processes close to 0 eV.

Suppression of low-energy dissociative electron attachment in Fe(CO)5 upon clustering

In this work, we probe anion production upon electron interaction with Fe(CO)5 clusters using two complementary cluster-beam setups. We have identified two mechanisms that lead to synthesis of complex anions with mixed Fe/CO composition. These two mechanisms are operative in distinct electron energy ranges. It is shown that the elementary decomposition mechanism that has received perhaps the most attention in recent years (i.e., dissociative electron attachment at energies close to 0 eV) becomes suppressed upon increasing aggregation of iron pentacarbonyl. We attribute this suppression to the electrostatic shielding of a long-range interaction that strongly enhances the dissociative electron attachment in isolated Fe(CO)5.

Interactions of low-energy electrons with the FEBID precursor chromium hexacarbonyl (Cr(CO)6)

Interactions of low-energy electrons with the FEBID precursor Cr(CO)₆ have been investigated in a crossed electron-molecular beam setup coupled with a double focusing mass spectrometer with reverse geometry. Dissociative electron attachment leads to the formation of a series of anions by the loss of CO ligand units. The bare chromium anion is formed by electron capture at an electron energy of about 9 eV. Metastable decays of Cr(CO)₅^- into Cr(CO)₄^-, Cr(CO)₄^- into Cr(CO)₃^- and Cr(CO)₃^- into Cr(CO)₂^- are discussed. Electron-induced dissociation at 70 eV impact energy was found to be in agreement with previous studies. A series of Cr(CO) _n C⁺ (0 ≤ n ≤ 3) cations formed by C-O cleavage is described for the first time. The metastable decay of Cr(CO)₆⁺ into Cr(CO)₅⁺ and collision-induced dissociation leading to bare Cr⁺, are discussed. In addition, doubly charged cations were identified and the ration between doubly and singly charged fragments was determined and compared with previous studies, showing considerable differences.

Dissociative electron attachment to coordination complexes of chromium: chromium(0) hexacarbonyl and benzene-chromium(0) tricarbonyl

Here we report the results of dissociative electron attachment (DEA) to gas-phase chromium(0) hexacarbonyl (Cr(CO)₆) and benzene-chromium(0) tricarbonyl ((η⁶-C₆H₆)Cr(CO)₃) in the energy range of 0-12 eV. Measurements have been performed utilizing an electron-molecular crossed beam setup. It was found that DEA to Cr(CO)₆ results (under the given experimental conditions) in the formation of three fragment anions, namely [Cr(CO)₅]^-, [Cr(CO)₄]^-, and [Cr(CO)₃]^-. The predominant reaction channel is the formation of [Cr(CO)₅]^- due to the loss of one CO ligand from the transient negative ion. The [Cr(CO)₅]^- channel is visible via two overlapping resonant structures appearing in the energy range below 1.5 eV with a dominant structure peaking at around 0 eV. The peak maxima of the fragments generated by the loss of two or three CO ligands are blue-shifted and the most intense peaks within the ion yield curves appear at 1.4 eV and 4.7 eV, respectively. (η⁶-C₆H₆)Cr(CO)₃ shows a very rich fragmentation pattern with decomposition leading to the formation of seven fragment anions. Three of them are generated from the cleavage of one, two or three CO ligand(s). The energy of the peak maxima of the [(C₆H₆)Cr(CO)₂]^-, [(C₆H₆)Cr(CO)]^-, and [(C₆H₆)Cr]^- fragments is shifted towards higher energy with respect to the position of the respective fragments generated from Cr(CO)₆. This phenomenon is most likely caused by the fact that chromium-carbonyl bonds are stronger in the heteroleptic complex (η⁶-C₆H₆)Cr(CO)₃ than in homoleptic Cr(CO)₆. Besides, we have observed the formation of anions due to the loss of C₆H₆ and one or more CO units. Finally, we found that Cr^-, when stripped of all ligands, is generated through a high-energy resonance, peaking at 8 eV.

Electron interactions with the focused electron beam induced processing (FEBID) precursor tungsten hexachloride

RATIONALE

Secondary electrons with an energy distribution below 100 eV are formed when high-energy particles interact with matter. In the focused electron beam induced deposition, high-energy beams are used to decompose organometallic compounds on surfaces. We investigated the electron ionisation of WCl6 and dissociative electron attachment to WCl6 in the gas phase in order to better understand the decomposition mechanism driven by secondary electrons.

METHODS

A double-focusing mass spectrometer coupled with a Nier-type ion source was used to perform the present studies. The electron ionisation studies were performed with an electron energy of 70 eV and dissociative electron attachment studies in the energy range of ~0-14 eV.

RESULTS

Tungsten hexachloride rapidly oxidises, leading to the formation of a mixture of pure WCl6 and WCl4 O together with WCl2 O2 species. The fragmentation of the three chlorinated compounds is effective, although electron ionisation to WCl6 leads to W(+) in contrast with WCl2 O2 and WCl4 O leading to WO2 (+) and WO(+) , respectively, as lighter fragments. With regard to electron attachment, decomposition of the precursor molecules is observed; however, W(-) was not detected within the detection limit of the instrument.

CONCLUSIONS

Electron ionisation and dissociative electron attachment (DEA) to WCl6 , WCl4 O and WCl2 O2 lead to strong fragmentation. In electron ionisation, the fragmentation by loss of chlorine atoms was observed for both WCl6 and the oxidised species. Additionally, the loss of all chlorine ligands is observable for WCl6 as well as the oxidised species. The DEA results have shown dissociation by the scission of chlorine atoms as well as by the scission of an oxygen atom. The formation of chlorine and oxygen anions was observed, indicating the formation of a neutral counterpart containing the metal atom, free to be attacked by the next electron.

P. R, Fairbrother DH, Ingólfsson O. The role of low-energy electrons in focused electron beam induced deposition: four case studies of representative precursors

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.

Low-energy electron interactions with tungsten hexacarbonyl--W(CO)6

RATIONALE

Low-energy secondary electrons are formed when energetic particles interact with matter. High-energy electrons or ions are used to form metallic structures from adsorbed organometallic molecules like W(CO)(6) on surfaces. We investigated low-energy electron attachment to W(CO)(6) in the gas phase to elucidate possible reactions during surface modification.

METHODS

Two crossed electron/molecular beam setups were utilised: (i) a high-resolution electron monochromator combined with a quadrupole mass spectrometer which was used for the measurement of relative cross sections as a function of the electron energy, and (ii) a double focusing mass spectrometer used for measurements of metastable decays of anions.

RESULTS

The study was performed in the electron energy range between ~0 and 14 eV. W(CO)(6) efficiently decomposed upon attachment of a low-energy electron and no stable W(CO)(6)(-) anion was observed on mass spectrometric time scales. The transient negative ion formed lost instead sequentially CO ligands. The fragment anions W(CO)(5)(-), W(CO)(4)(-), W(CO)(3)(-), and W(CO)(2)(-) were observed. However, no W(-) was detectable.

CONCLUSIONS

Dissociative electron attachment (DEA) to W(CO)(6) led to strong dissociation but a complete loss of all CO ligands was not observed in DEA. Deposit contaminations might be a direct result of DEA reactions close to the irradiation spot in beam deposition techniques.