Beams of electrons and ions are now fairly routinely focused to dimensions in the nanometer range. Since the beams can be used to locally alter material at the point where they are incident on a surface, they represent direct nanofabrication tools. The authors will focus here on direct fabrication rather than lithography, which is indirect in that it uses the intermediary of resist. In the case of both ions and electrons, material addition or removal can be achieved using precursor gases. In addition ions can also alter material by sputtering (milling), by damage, or by implantation. Many material removal and deposition processes employing precursor gases have been developed for numerous practical applications, such as mask repair, circuit restructuring and repair, and sample sectioning. The authors will also discuss structures that are made for research purposes or for demonstration of the processing capabilities. In many cases the minimum dimensions at which these processes can be realized are considerably larger than the beam diameters. The atomic level mechanisms responsible for the precursor gas activation have not been studied in detail in many cases. The authors will review the state of the art and level of understanding of direct ion and electron beam fabrication and point out some of the unsolved problems.

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Gas-assisted focused electron beam and ion beam processing and fabrication

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.

https://pages.jh.edu/chem/fairbr/Publications/Gas%20Phase%20EBID%20review.pdf

The role of low-energy electrons in focused electron beam induced deposition: four case studies of representative precursors.

This work introduces an additive direct-write nanofabrication technique for producing extremely conductive gold nanostructures from a commercial metalorganic precursor. Gold content of 91 atomic % (at. %) was achieved by using water as an oxidative enhancer during direct-write deposition. A model was developed based on the deposition rate and the chemical composition, and it explains the surface processes that lead to the increases in gold purity and deposition yield. Co-injection of an oxidative enhancer enabled Focused Electron Beam Induced Deposition (FEBID)-a maskless, resistless deposition method for three dimensional (3D) nanostructures-to directly yield pure gold in a single process step, without post-deposition purification. Gold nanowires displayed resistivity down to 8.8 μΩ cm. This is the highest conductivity achieved so far from FEBID and it opens the possibility of applications in nanoelectronics, such as direct-write contacts to nanomaterials. The increased gold deposition yield and the ultralow carbon level will facilitate future applications such as the fabrication of 3D nanostructures in nanoplasmonics and biomolecule immobilization.

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Highly conductive and pure gold nanostructures grown by electron beam induced deposition

While several studies have suggested that secondary electrons dominate electron beam induced deposition (EBID), we demonstrate that primary electrons (PE’s) contribute significantly to the deposition for nanoscale EBID over the electron beam energy range (500–20keV). High-aspect ratio pillar growth is a signature of EBID; W nanopillar growth on SiO2 substrate yielded a growth rate of 6nms−1 and a nanopillar aspect ratio of ∼50. A simple integration of the primary, secondary, and backscattered electron distributions versus a dissociation cross section for WF6 suggests that all three electron species should contribute to the total volume of the deposited nanopillar, contrary to reports that suggest that secondary electrons dominate the process. A three-dimensional, Monte Carlo simulation including time correlated gas dynamics and species specific deposition was developed to help elucidate which of the relevant electron species, primary (PE’s), secondary (SE’s), and/or backscattered electrons (BSE’s), induce...

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Growth and simulation of high-aspect ratio nanopillars by primary and secondary electron-induced deposition

The tip morphology in the nanoscale is discussed as a member of the one-dimensional nanostructure family. Different growth techniques used for the synthesis of the nanotips have been outlined and compared against each other including, etching, pyrolysis, physical, and chemical vapour deposition. Growth models for hollow and solid nanotips are compared and discussed in detail with a thrust on the ‘cone-helix’ model for the hollow tips and the ‘platelet stacking’ model for the solid tips. Application of the nanotips in field emission, scanning probe microscopy, sensing, anti-reflection, and nanoindentation is discussed.

Nanotips: Growth, Model, and Applications

Niobium-gated field-emitter arrays with Pt tips were fabricated using focused ion and electron beams. A promising approach, based on a two-step etch process in a Nb/SiO2/Si structure, has been implemented for the suppression of beam-induced damage and contamination in the processed area during the production of the gate openings. Only the top Nb layer was removed for gate openings by physical sputtering using the focused ion beam. The underlying SiO2 was subsequently removed by wet etching. Deposition of Pt pillars into these gate openings using electron-beam-induced chemical reaction resulted in field emission at an applied gate bias of 50 V even without any thermal annealing process.

Maskless fabrication of field-emitter array by focused ion and electron beam

To realize a high current under focusing operation we investigated the structure of double-gated field emitter arrays (FEAs) and tried to anodize the tip of the silicon emitters. It was found from these investigations that the FEAs with the thick extraction gate could maintain high emission current during focusing operation. Furthermore, the anodized Si FEAs with thick extraction gate achieved a high emission current per tip of about 300 nA under focusing operation.

High emission current double-gated field emitter arrays

Recently, various types of the field emitter arrays (FEAs) with focusing gate were investigated to realize the focusing electron beam. However, it is difficult to realize the high emission current density at the focusing operation. So we investigated to realize the high emission current density at the focusing operation.

Gate opening of metal layers in a field emitter array(FEA) was produced by physical sputtering using a focused ion beam (FIB) and subsequent wet etching of underlying silicon dioxide layer. Platinum tips were deposited into the gate opening using electron beam induced chemical reaction. Beam damage due to the FIB processing resulted in the enlargement of the gate diameter, After wet etching, the gate opening in Nb gated FEA was enlarged, while the smaller gate opening in Au gated FEA was easily fabricated because of the suppression of the enhanced etching. The gate opening could be controlled by selection of the gate metal materials. The turn-on voltage for field emission could be reduced by decreasing the diameter of the gate opening.

S. Okuda

Papers

Maskless fabrication of field-emitter array by focused ion and electron beam

High emission current double-gated field emitter arrays

High emission current double-gated field emitter arrays

Focused ion beam processing of metal gated field emitter arrays