Formation of pure Cu nanocrystals upon post-growth annealing of Cu-C material obtained from focused electron beam induced deposition: comparison of different methods

Ligand Size and Carbon-Chain Length Study of Silver Carboxylates in Focused Electron-Beam-Induced Deposition

Gas-assisted focused electron-beam-induced deposition is a versatile tool for the direct writing of complex-shaped nanostructures with unprecedented shape fidelity and resolution. While the technique is well-established for various materials, the direct electron beam writing of silver is still in its infancy. Here, we examine and compare five different silver carboxylates, three perfluorinated: [Ag₂(µ-O₂CCF₃)₂], [Ag₂(µ-O₂CC₂F₅)₂], and [Ag₂(µ-O₂CC₃F₇)₂], and two containing branched substituents: [Ag₂(µ-O₂CCMe₂Et)₂] and [Ag₂(µ-O₂C^tBu)₂], as potential precursors for focused electron-beam-induced deposition. All of the compounds show high sensitivity to electron dissociation and efficient dissociation of Ag-O bonds. The as-deposited materials have silver contents from 42 at.% to above 70 at.% and are composed of silver nano-crystals with impurities of carbon and fluorine between them. Precursors with the shortest carbon-fluorine chain ligands yield the highest silver contents. In addition, the deposited silver content depends on the balance of electron-induced ligand co-deposition and ligand desorption. For all of the tested compounds, low electron flux was related to high silver content. Our findings demonstrate that silver carboxylates constitute a promising group of precursors for gas-assisted focused electron beam writing of high silver content materials.

Mechanical Properties of 3D Nanostructures Obtained by Focused Electron/Ion Beam-Induced Deposition: A Review

This article reviews the state-of-the -art of mechanical material properties and measurement methods of nanostructures obtained by two nanoscale additive manufacturing methods: gas-assisted focused electron and focused ion beam-induced deposition using volatile organic and organometallic precursors. Gas-assisted focused electron and ion beam-induced deposition-based additive manufacturing technologies enable the direct-write fabrication of complex 3D nanostructures with feature dimensions below 50 nm, pore-free and nanometer-smooth high-fidelity surfaces, and an increasing flexibility in choice of materials via novel precursors. We discuss the principles, possibilities, and literature proven examples related to the mechanical properties of such 3D nanoobjects. Most materials fabricated via these approaches reveal a metal matrix composition with metallic nanograins embedded in a carbonaceous matrix. By that, specific material functionalities, such as magnetic, electrical, or optical can be largely independently tuned with respect to mechanical properties governed mostly by the matrix. The carbonaceous matrix can be precisely tuned via electron and/or ion beam irradiation with respect to the carbon network, carbon hybridization, and volatile element content and thus take mechanical properties ranging from polymeric-like over amorphous-like toward diamond-like behavior. Such metal matrix nanostructures open up entirely new applications, which exploit their full potential in combination with the unique 3D additive manufacturing capabilities at the nanoscale.

The radiation chemistry of focused electron-beam induced etching of copper in liquids

Well-controlled, focused electron-beam induced etching of copper thin films has been successfully conducted on bulk substrates in an environmental scanning electron microscope by controlling liquid-film thickness with an in situ correlative interferometry system. Knowledge of the liquid-film thickness enables a hybrid Monte Carlo/continuum model of the radiation chemistry to accurately predict the copper etch rate using only electron scattering cross-sections, radical yields, and reaction rates from previous studies. Etch rates depended strongly on the thickness of the liquid film and simulations confirmed that this was a result of increased oxidizing radical generation. Etch rates also depended strongly, but non-linearly, on electron beam current, and simulations showed that this effect arises through the dose-rate dependence of reactions of radical species.

Impact of Electron-Beam Heating during 3D Nanoprinting

An artifact limiting the reproduction of three-dimensional (3D) designs using nanoprinting has been quantified. Beam-induced heating was determined through complementary experiments, models, and simulations to affect the deposition rate during the 3D nanoprinting of mesh objects using focused electron beam induced deposition (FEBID). The mesh objects are constructed using interconnected nanowires. During nanowire growth, the beam interaction driving deposition also causes local heating. The temperature at the beam impact region progressively rises as thermal resistance increases with nanowire growth. Heat dissipation resembles the classical mode of heat transfer from extended surfaces; heat must flow through the mesh object to reach the substrate sink. Simulations reveal that beam heating causes an increase in the rate of precursor desorption at the BIR, causing a concomitant decrease in the deposition rate, overwhelming an increase in the deposition rate driven by thermally enhanced precursor surface diffusion. Temperature changes as small as 10 K produce noticeable changes in deposit geometry; nanowires appear to deflect and curve toward the substrate because the vertical growth rate decreases. The 3D FEBID naturally ensues from the substrate surface upward, inducing a vertical temperature gradient along the deposit. Simulations, experiments, temperature-controlled studies, and process current monitoring all confirm the cause of nanowire distortion as beam-induced heating while also revealing the rate-determining physics governing the final deposit shape.

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.

Transmission XMCD-PEEM imaging of an engineered vertical FEBID cobalt nanowire with a domain wall

Using focused electron-beam-induced deposition, we fabricate a vertical, platinum-coated cobalt nanowire with a controlled three-dimensional structure. The latter is engineered to feature bends along the height: these are used as pinning sites for domain walls, which are obtained at remanence after saturation of the nanostructure in a horizontally applied magnetic field. The presence of domain walls is investigated using x-ray magnetic circular dichroism (XMCD) coupled to photoemission electron microscopy (PEEM). The vertical geometry of our sample combined with the low incidence of the x-ray beam produce an extended wire shadow which we use to recover the wire's magnetic configuration. In this transmission configuration, the whole sample volume is probed, thus circumventing the limitation of PEEM to surfaces. This article reports on the first study of magnetic nanostructures standing perpendicular to the substrate with XMCD-PEEM. The use of this technique in shadow mode enabled us to confirm the presence of a domain wall without direct imaging of the nanowire.

A novel copper precursor for electron beam induced deposition

A fluorine free copper precursor, Cu(tbaoac)₂ with the chemical sum formula CuC₁₆O₆H₂₆ is introduced for focused electron beam induced deposition (FEBID). FEBID with 15 keV and 7 nA results in deposits with an atomic composition of Cu:O:C of approximately 1:1:2. Transmission electron microscopy proved that pure copper nanocrystals with sizes of up to around 15 nm were dispersed inside the carbonaceous matrix. Raman investigations revealed a high degree of amorphization of the carbonaceous matrix and showed hints for partial copper oxidation taking place selectively on the surfaces of the deposits. Optical transmission/reflection measurements of deposited pads showed a dielectric behavior of the material in the optical spectral range. The general behavior of the permittivity could be described by applying the Maxwell-Garnett mixing model to amorphous carbon and copper. The dielectric function measured from deposited pads was used to simulate the optical response of tip arrays fabricated out of the same precursor and showed good agreement with measurements. This paves the way for future plasmonic applications with copper-FEBID.

Comparative study of post-growth annealing of Cu(hfac)2, Co2(CO)8 and Me2Au(acac) metal precursors deposited by FEBID

Non-noble metals, such as Cu and Co, as well as noble metals, such as Au, can be used in a number modern technological applications, which include advanced scanning-probe systems, magnetic memory and storage, ferroelectric tunnel junction memristors, metal interconnects for high performance integrated circuits in microelectronics and nano-optics applications, especially in the areas of plasmonics and metamaterials. Focused-electron-beam-induced deposition (FEBID) is a maskless direct-write tool capable of defining 3-dimensional metal deposits at nanometre scale for above applications. However, codeposition of organic ligands when using organometallic precursors is a typical problem that limits FEBID of pure metal nanostructures. In this work, we present a comparative study using a post-growth annealing protocol at 100, 200, and 300 °C under high vacuum on deposits obtained from Co₂(CO)₈, Cu(II)(hfac)₂, and Me₂Au(acac) to study improvements on composition and electrical conductivity. Although the as-deposited material was similar for all precursors, metal grains embedded in a carbonaceous matrix, the post-growth annealing results differed. Cu-containing deposits showed the formation of pure Cu nanocrystals at the outer surface of the initial deposit for temperatures above 100 °C, due to the migration of Cu atoms from the carbonaceous matrix containing carbon, oxygen, and fluorine atoms. The average size of the Cu crystals doubles between 100 and 300 °C of annealing temperature, while the composition remains constant. In contrast, for Co-containing deposits oxygen release was observed upon annealing, while the carbon content remained approximately constant; the cobalt atoms coalesced to form a metallic film. The as-deposited Au-containing material shows subnanometric grains that coalesce at 100 °C, maintaining the same average size at annealing temperatures up to 300 °C. Raman analysis suggests that the amorphous carbonaceous matrix of the as-written Co, Cu and Au deposits turned into nanocrystalline graphite with comparable crystal sizes of 12-14 nm at 300 °C annealing temperature. However, we observed a more effective formation of graphite clusters in Co- than in Cu- and Au-containing deposits. The graphitisation has a minor influence on the electrical conductivity improvements of Co-C deposits, which is attributed to the high as-deposited Co content and the related metal grain percolation. On the contrary, electrical conductivity improvements by factors of 30 and 12 for, respectively, Cu-C and Au-C deposits with low metal content are mainly attributed to the graphitisation. This relatively simple vacuum-based post-growth annealing protocol may be useful for other precursors as it proved to be efficient in reliably tuning the electrical properties of as-deposited FEBID materials. Finally, a H₂-assisted gold purification protocol is demonstrated at temperatures around 300 °C by fully removing the carbon matrix and drastically reducing the electrical resistance of the deposit.

Focused electron beam induced deposition of copper with high resolution and purity from aqueous solutions

Electron-beam induced deposition of high-purity copper nanostructures is desirable for nanoscale rapid prototyping, interconnection of chemically synthesized structures, and integrated circuit editing. However, metalorganic, gas-phase precursors for copper introduce high levels of carbon contamination. Here we demonstrate electron beam induced deposition of high-purity copper nanostructures from aqueous solutions of copper sulfate. The addition of sulfuric acid eliminates oxygen contamination from the deposit and produces a deposit with ∼95 at% copper. The addition of sodium dodecyl sulfate (SDS), Triton X-100, or polyethylene glycole (PEG) improves pattern resolution and controls deposit morphology but leads to slightly reduced purity. High resolution nested lines with a 100 nm pitch are obtained from CuSO₄-H₂SO₄-SDS-H₂O. Higher aspect ratios (∼1:1) with reduced line edge roughness and unintended deposition are obtained from CuSO₄-H₂SO₄-PEG-H₂O. Evidence for radiation-chemical deposition mechanisms was observed, including deposition efficiency as high as 1.4 primary electrons/Cu atom.

3D Nanoprinting via laser-assisted electron beam induced deposition: growth kinetics, enhanced purity, and electrical resistivity

We investigate the growth, purity, grain structure/morphology, and electrical resistivity of 3D platinum nanowires synthesized via electron beam induced deposition with and without an in situ pulsed laser assist process which photothermally couples to the growing Pt-C deposits. Notably, we demonstrate: 1) higher platinum concentration and a coalescence of the otherwise Pt-C nanogranular material, 2) a slight enhancement in the deposit resolution and 3) a 100-fold improvement in the conductivity of suspended nanowires grown with the in situ photothermal assist process, while retaining a high degree of shape fidelity.