Helium-ion-beam nanofabrication: extreme processes and applications

Helium ion beam (HIB) technology plays an important role in the extreme fields of nanofabrication. This paper reviews the latest developments in HIB technology as well as its extreme processing capabilities and widespread applications in nanofabrication. HIB-based nanofabrication includes direct-write milling, ion beam- induced deposition, and direct-write lithography without resist assistance. HIB nanoscale applications have also been evaluated in the areas of integrated circuits, materials sciences, nano-optics, and biological sciences. This review covers four thematic applications of HIB: (1) helium ion microscopy imaging for biological samples and semiconductors; (2) HIB milling and swelling for 2D/3D nanopore fabrication; (3) HIB-induced deposition for nanopillars, nanowires, and 3D nanostructures; (4) additional HIB direct writing for resist, graphene, and plasmonic nanostructures. This paper concludes with a summary of potential future applications and areas of improvement for HIB extreme nanofabrication technology.


Introduction
Due to high resolution and sensitivity, nanofabrication technology is widely used to pattern nanostructures into components, devices, or systems for integrated circuits, materials sciences, nano-optics, and bio-sciences applications. The focused positive ion beam with a sub-nanometer spot-size is developed by using the gas field ion source (GFIS), which equips a three-sided pyramidal tip consisting of only three atoms at the apex (called a trimer) [1]. With its advantage of a subnanometer spot-size, helium ion microscopy (HIM) is a promising method for high-resolution imaging with secondary electron (SE) emission by generating a focused helium ion beam (HIB) from a GFIS. Helium gas molecules are field ionized on a cryogenically cooled tungsten (W) tip with a trimer. One of the HIBs emitted from three atoms of the trimer is chosen for the high-resolution HIM imaging. Changing the gas pressure allows for controlled operation of the HIB current between fA and pA levels to meet the high-resolution requirements of various applications [2,3]. HIM has significant advantages over a focused ion beam (FIB) with gallium source and scanning electron microscope (SEM), including highresolution, high-sensitivity, high-SE yield, and long-depth of focus with several novel contrast mechanisms for imaging applications [4]. For example, HIM is used for failure analysis of semiconductor devices by using voltage contrast theory [5]. Since scanning HIB slows SE charge accumulation, HIM can successfully image poorly conducting samples, such as uncoated biological samples [6,7]. Therefore, HIM can be used for high-sensitivity and high-contrast imaging in the fields of semiconductors, materials sciences, and biological sciences fields with less damage to target samples. Except for high-resolution imaging, HIM can also perform extremely complex direct-write nanofabrication [8]. While equipped with a gas injection system (GIS), HIM can pattern special structures through HIB-assisted milling and HIBinduced deposition processes [9]. The small focal spot and sharp beam source with the development of GFIS, and the unique interaction of the high-energy helium ions with the target at and just below the surface, enables HIM to perform ultrahigh-resolution nanopore fabrication on thin films or bulk materials to meet the requirements of single molecular detection in bioscience [10][11][12][13]. HIB bombardment can easily produce sub-10 nm diameter nanopores on freestanding material film with high reproducibility [14]. Sub-5 nm diameter nanopores can be obtained using optimized HIB milling parameters on suspended monolayer graphene, which is valuable for single-base biomolecule analysis to achieve DNA/RNA sequencing functions [15]. Using HIB technology, amorphization is one of the most important parts of the bulk material milling process. Amorphization during the HIB milling process and materials swelling pave the way towards the fabrication of nano-volcano structures for potential nanooptical and biological science applications [16].
The energy and momentum of HIB promote the chemical reactions of precursor gas molecules on the surface. Scanning the HIB during continuous decomposition precursor gas molecules can result in direct deposition of materials with programmed three-dimensional (3D) structures. The HIB induces material deposition to create wires for future circuit editing and 3D structures for nano-electronic device applications [17]. The HIB-assisted milling process also increases the material removal rate because the chemical reaction requires a smaller dose of ions between the precursor gas molecules and the target-specific materials [18]. The maximum 41% platinum (Pt) content nanopillar and 10 nm resolution multiple cobalt (Co) lines are deposited by HIB-induced deposition at a rate of 0.6 µm 3 /nC with precursor gases [19]. Another major HIB nanofabrication mode is the direct writing pattern which works with or without resist assistance. Compared with FIB direct writing technique using heavy gallium ions, HIB direct writing is used in high pattern-density nanolithography with highresolution and sensitivity due to its sub-nanometer spot size. The proximity effect has a less significant impact on patterning because the forward scattering is weaker and the yielded SEs diffuse small laterally [20]. Sub-10 nm scale nanochannels and nanoribbons can be directly patterned on a graphene membrane using HIB direct writing techniques for functional nanoscale graphene devices. Due to its high-resolution, HIM can pattern 1.5 nm structures by correcting the proximity effect [21,22]. Plasmonic nanostructures fabricated with HIB direct writing techniques are used for molecular spectroscopy, nearfield or nanoscale imaging, quantum mechanical phenomena applications [23,24].
This review examines the extreme processes and applications of HIB nanofabrication. We discuss the advantages of HIM, including its high-resolution and high-sensitivity for extreme nanostructures fabrications. We follow with a review of studies using HIM for biological samples and semiconductors and an introduction of HIM system with GFIS. Then, HIB-related nanofabrication technologies are presented, which includes milling and swelling, ion beam induced deposition, and additional HIB nanofabrication, such as direct writing techniques. The nanostructures fabricated by the above technologies and their properties will be used to verify the feasibility and resolution of HIB nanofabrication. Finally, this review concludes with a summary of the latest progress in HIB nanofabrication and potential future developments in extreme processes and applications.

HIM system with GFIS
The basic components of a focus HIM system consist of an ion source, a column, a sample chamber with a substrate stage, and a vacuum chamber with auxiliary equipment (figure 1). The extremely bright GFIS in the HIB system was developed to achieve a smaller, sub-nanometer probe size based on a needle-shaped W electrode called an 'emitter'. Under a very larger electric field, high voltage is applied between the emitter and an adjacent grounded electrode. The emitter, with a sharp tip, prompts electrons from the helium atoms to escape because of the quantum-mechanical tunneling effect. The source produces positive ions by field ionization. These positive ions pass through the extractor plate. A continuous beam of helium ions is successfully obtained by accelerating these ions through the GFIS column. In the GFIS column, three atoms at the apex of the emitter, called a trimer, are used to ionize the helium gas, which forms the ion beam. A single beam is aligned to the optics column by carefully adjusting the lens 1, quadrupoles, octopoles, and lens 2. With the ultrahigh brightness GFIS, the extremely narrow beam due to the atomic scale source, the use of helium ions, and the low energy spread makes HIB ideal as a high-resolution probe for imaging and nanofabrication [20].

HIM imaging
SEM is one of the basic techniques for imaging the exterior shape and structure of a sample [25]. Numerous advances in SEM imaging have been developed by controlling charge and maintaining the biological surface structure to increase topographical contrast and surface sensitivity [26]. However, SEM is limited in terms of resolution and contrast in the nanoscale applications. Recently, focused HIM has become an important microscopy technology for ultrahigh-resolution and high-contrast imaging [27,28]. Since one atom at the apex is selected as the ion source for imaging, a very small He + ion beam spot size defines the aperture. Therefore, the HIM exhibits a higher resolution with a larger depth of field, neglecting the spherical and chromatic aberrations of the ion optical column [29,30]. Since its development by Ward B W et al [31] in 2006, HIM has been applied in high-contrast imaging of conducting, semiconducting, and insulating materials, as well as biological samples [32].

HIM imaging of biological samples
Bazou D et al demonstrated the possibility of HIM imaging for uncoated human colon cancer cells and its advantages in biological samples. Detailed morphological information, such as pores on the cell surface and overall cell surface roughness of uncoated human colon cancer cells, verifies the effectiveness of HIM imaging in biological samples, as shown in figure 2(a) [33]. As shown in figure 2(b), HIM can also acquire high-resolution images with 3D-like nanoscale fibril morphology details and fibril connections in samples without conductive coatings [34]. Based on its advantages of higher resolution and contrast, the HIM technique visualizes the surface ultrastructure of challenging biological samples without requiring metal coatings. Therefore, HIM preserves the subtle surface features that are typically obscured by metal coatings in SEM imaging. Figures 2(c) and (d) show that the low sputtering rate of HIB produces no discernable beam damage on the small and delicate surface features of biological samples [35][36][37]. HIM was used to imaging bacterial colonies and bacteriaphage interactions on a natural agar growth substrate at subnanometer resolution. For the first time, images depicting the (c) HIM image of drosophila melanogaster tarsus with the empodium (big arrowhead) that is flanked dorso-laterally by two tarsal claws (thick arrows) and ventrally by two sets of pulivilli (one pulivillus, group of thin arrows). Reprinted from [36] with permission from Elsevier. (d) HIM image of pristionchus pacificus, the delicate nature of the neon milling was demonstrated by the patterning of a small horizontal line using a dose of 0.3 nC mm −3 . Despite being strong enough to penetrate the outer cuticle of the nematode, minimal ablation and thermal damage are visible on the surrounding area [35]. different stages of a viral infection within host cells were obtained with HIM imaging technology [38]. HIM technology has also been used to show the detailed characteristics of conducting nanostructures on insulating cells. The above results highlight the potential of HIM technology to further understand the organic-inorganic interfaces of nanomaterials used in the biological field [39].

HIM imaging of semiconductors
Due to the high sensitivity of contrast attributes, HIM is widely used in semiconductors alongside voltage contrast imaging and doped contrast imaging [5]. The failure analysis of dynamic random access memory was developed with HIM by using the voltage contrast in semiconductor devices. The contrast between the silicon bulk and the oxide liners in the HIM image was more pronounced than in the SEM images, which was difficult to distinguish the continuous liner [29]. Furthermore, the voltage contrast in the HIM images of graphene nanoconductors fabricated by HIB maskless lithography was also used to test the electrical conductivity characteristics of nanoelectronic devices in situ [40]. SE images of cross-sections of multilayer ceramic capacitors demonstrate that active voltage contrast can be used to investigate electrical potential differences between a ground electrode and a positively biased internal electrode [41]. HIM imaging technology can also map the nanometer-scale electrical potential distribution of Li-ion rechargeable batteries [42]. The defect localization for failure analysis of conductive structures, especially electronic devices, can be detected with passive voltage contrast in HIM images at the nanoscale, as shown in figures 3(a) and (b) [43]. It can detect defect gaps smaller than 10 nm. HIM imaging is an effective method for conducting structural failure analysis due to its higher resolution and ability to analyze a surface with minimal damage or implantation [44].
In the case of doped contrast imaging, Jepson M A E et al showed that HIM imaging can improve the lateral resolution in quantitative dopant mapping. For high doping levels, the contrast in HIM images is highly sensitive to changes in dopant concentration relative to SEM [46,47]. With surface sensitivity and high lateral resolution, HIM can obtain information about the lattice structure of surface-confined alloys (figures 3(c) and (d)), including the single atomic layer steps between terraces and the periodicities of the hcp/fcc pattern formed in a 2-3 layer thick alloy film [45]. Except for the acquisition of crystallographic information, HIM imaging can also be used to obtain information about ultrathin organic and inorganic layers, such as crystal defects and membrane thickness. Therefore, a new contrast mechanism for HIM imaging has been developed to suppress absorbed thin films from channeling into crystalline matter. It can visualize ultrathin layers of light elements on top of heavier substrates [48]. Using a custom scanning transmission ion microscopy stage, Hall A R [49]. measured the local thickness of a freely suspended solidstate membrane thinned by a focused HIB. The relative brightness of the transmission image collected by the SE detector from the ions transmitted through the suspended membrane can be used to estimate the membrane thickness. To solve the challenge of measuring critical dimensions of complicated nanostructures, such as an atomic force microscope (AFM) tip characterizer, the scanning HIM can be used to characterize focal depth with SE images [50].

HIB milling and swelling
In top-down micro-and nano-fabrications, the ion beam milling process involves bombarding the substrate surface with a beam of accelerated ions, which transfer their momentum to the target atoms. Then, the target atoms sputter out of the substrate in a controlled manner. The processing technology includes physical sputtering, material redeposition, and amorphization (e.g. swelling) processes. The development of GFIS technology can produce high brightness HIB. When HIB dwells on a target sample, the helium ions lose energy as they interact with the target material and sputter atoms during the milling process. Compared to gallium FIB milling, He + ions have relatively low mass, which results in less damage to the sample. Therefore, the HIB milling process generates a lower milling yield but a more controlled fabrication process with a higher aspect ratio and higher accuracy. The focused HIB technology has distinct advantages in nanofabrication, including milling processes for local thickness control and nanostructure fabrication in free-standing membranes and bulk materials [51]. However, the amorphization and helium implantation during HIB milling may damage bulk substrates. Therefore, the optimization of ion dose, beam energy, and HIB dose rate is critical for local thickness manipulation and topography accuracy control in nanostructures fabrication [52].

Milling for nanopore fabrications
In recent years, solid-state nanopore technology has become an important tool for detecting single biomolecules [10], especially in DNA/RNA sequencing applications [53]. The highresolution requirements of solid-state nanopore technology prompted efforts to achieve precise control over nanopore pore size and membrane thickness. The characteristics of HIB milling technology, such as its small focal spot, sharp beam source, and the relatively low mass of He + , make it is useful for manipulating the local thickness of membranes and fabricating nanoscale pores.
Marshall M M et al [54] demonstrated that membrane thickness significantly effects the expansion rate of pores, which follows a reproducible trend in nanopore fabrication. The silicon nitride membranes were locally thinned to the appropriate thickness with HIM, and the nanopores were precisely fabricated in a fixed position by the controlled breakdown technique for single-molecule sensing applications [55,56]. Marshall M M et al [54] also investigated aspects of direct and transmission HIB milling on suspended silicon nitride membranes for accurate thickness control. In addition to local thickness manipulation, topography accuracy control of nanostructures is another advantage of the HIB milling process. Xia D et al [14] used HIB milling technology to fabricate excellent, reproducible solid-state nanopores on the silicon nitride membranes over a large area (figure 4)(A). However, the bubble phenomenon often occurs during the HIB milling process of bulk materials. A volcanolike nanostructure, surrounded the milled region on the silicon nitride membranes and gradually decreased during nanopore fabrication [54].
Due to its highly focused GFIS and small beam-sample interaction volume, nanopore size is easily controlled when using a focused HIB with a diameter in the range of 10 nm or less, and as low as 1.3 nm in carbon nanomembranes (CNM), as shown in figure 4(B) [12]. Additionally, HIB milling technology can be used to rapidly fabricate nanopores on the two-dimensional (2D) membrane materials, such as molybdenum disulfide (MoS 2 ) and hexagonal boron nitride (h-BN) for highly sensitive biomolecule analyses [58,59]. The sub-10 nm nanopores in the h-BN lattice membrane passivated the dangling bonds of graphene quantum dots (GQDs). This made the GQDs highly stable while maintaining their intrinsic quantum properties [57] (figure 4)(C).
The thinness of 2D materials, such as graphene and MoS 2 , promotes the use of HIM-based rapid fabrication processes in nanopore technology applications. The impact of helium ion bombardment on freestanding thin membranes is essential for nanoscale accuracy control and biomolecule detection applications. Therefore, Raman spectroscopy was used to analyze the amorphization and defects on HIB-damaged free-standing graphene and supported graphene [60,61]. A large fraction of helium ions passed through the free-standing graphene membrane, causing minimal damage to its lattice state. However, helium ion collisions can destroy the lattice, forming vacancy defects in the freestanding 2D materials membranes. Therefore, optimizing HIB parameters is crucial for fabricating high-quality graphene nanopores, while minimizing the effect of amorphization on nanopore fabrication during HIB milling. HIB technology can produce high-quality, sub-5 nm graphene nanopores by precisely controlling the milling parameters (figure 5). The diameters of graphene nanopores expand at a faster rate during short periods of HIB bombardment whereas longer exposure times decrease the expansion rate [15]. Theoretical and experimental investigations confirm that HIB milling technology can effciently generate reproducible nano-   scale nanopore patterns on freestanding 2D graphene [62]. Moreover, the graphene nanomesh formed by patterning nanopore arrays directly through HIB milling in the suspended monolayer graphene has potential to control phonons, water filtration, and semiconductor applications [63].

Milling for 3D nanostructures
In addition to nanopore fabrication, HIB milling can also be used to direct pattern other nanostructures, such as special 3D nanostructures and plasmon nanostructures. First, a set of parallel lines, 3D nanopyramid, and nanocone structures are patterned by HIB milling on a Si substrate, as shown in figure 6 [64]. The small focal spot size helps to confine the interaction between high-energy helium ions with Si. 3.5 nm half-pitch parallel lines can be achieved by direct writing nanofabrication [65]. Unlike FIB milling based on gallium ions, when HIB bombardment is focused on a Si substrate, the implanted helium ions tumefaction leads the surface to swell. At the same time, Zhang L et al demonstrated that these nanostructures are stable at room temperature, which were generated through diffusion, coalescence, and nanobubble formation [64]. Furthermore, direct-write HIB milling is used to fabricate metrology test structures with programmed imperfections. Due to the high resolution of HIM, features around 5 nm are resolvable. The direct-write HIB milling process can then pattern 1.5 nm structures by correcting the proximity effect. It can offers a promising alternative method for fabricating programmed defects and test structures for sub-7 nm advanced metrology solutions [66,67].

Swelling for specific nanopore fabrications
Helium ion bombardment in the HIB milling process can damage bulk materials, resulting in defects, such as bubbles, that have a significant impact effect on special applications. On the other hand, we can take advantage of the bubble phenomenon to form special nanostructures. Due to its high resolution, great depth of field, and minimal damage on the substrate, HIB technology was used to create ultra-high aspect ratio vias and 3D nanovolcanic nanopores on gold film [68]. HIM imaging can characterize the feature size, lateral milling resolution, and sidewall angle for specific topography. The results show that in addition to physical sputtering, amorphization (i.e. swelling) is another key method of preparing nanostructures on crystalline substrates by the HIB milling process. Generally, amorphization occurs in the bombarded area when the HIB dose is not sufficient enough to induce sputtering. During the amorphization process, the incident helium ions displace the target atoms from their lattice sites. The movement of displaced atoms relocating to the nearby areas expands the substrate.
Giri P K et al demonstrated the swelling mechanism of silicon irradiated by low energy ions through AFM and TEM experiments, as shown in figure 7 [69,70]. For SiC at room temperature and low flux, the implanted helium ions relaxed the local strain, thereby promoting implantation. Leclerc S et al reported that the amorphization phenomenon contributes 15% of the thickness of the amorphous layer in the swelling process [71]. Tseng, A. A. also demonstrated that the maximum swelling range due to amorphization can reach tens of nanometers [72]. Therefore, we should consider amorphization in nanofabrication to control the dimensional accuracy of nanostructures. Swelling caused by amorphization has been used to form special nanostructures in HIB-milled gold films, such as nanovolcanic nanopores (figure 8), which have the potential to enhance fluorescence and scattered light in chemical sensing and biophysical applications [16,68,73]. Because of its light mass, helium ions propagated into the Au layer over a long distance, resulting in significant ion implantation, as shown in figure 8(g). The implantation of helium ions induced further swelling around the nanopore and formed 3D nanovolcanic nanopores. When the dose of HIB is 10 nC µm −2 , the height of fabricated 3D structures can reach about 50 nm (figure 8(f)), which is higher than the depth of the HIB milling process [16]. However, Marshall M M et al [54] suggests that the volcanic structures are the result of charge-induced fluidization of the material and ionic pressure.

HIB-induced deposition
Ion-beam-induced deposition is an important nanofabrication technology that can modify the properties of materials according to the interactions between the ion beam and the materials. The advantages of direct writing and growth make the deposition process useful for specific nanofabrication applications. Tseng A A recently reported that by adding GIS in the chamber, gallium FIB induced deposition has been widely used in circuit editing, device modification, and nanoelectronic device fabrication [72]. Following the scanning ion beam, the deposition process occurs in the interaction between the volatile compounds (e.g. organometallic, halides, and halogenides), the gaseous precursor, and the target material in the vacuum chamber. The gaseous precursors are physically adsorbed on the target substrate and then locally dissociated by the high-energy scanning gallium ion beam where the ion beam strikes the substrate. Following Moore's Law, the scale of integrated circuits have progressively increased with the development of semiconductor technology. However, gallium FIB-induced deposition technology cannot meet the nanofabrication requirements of future circuit editing and nanoelectronic devices fabrications due to its problems, because the resolution of the gallium ion beam is low and the substrate could be damaged by the exposure of gallium ion beam. The development of HIBinduced deposition is a reasonable, appropriate technique for these specific nanofabrication applications because of the light mass of helium ions and the different electrical properties between inert helium and electroactive gallium [8, 9, 17, and 74]. This chapter outlines the latest work on HIB-induced deposition.

Deposition parameters optimization and model simulation
When HIB scans gaseous precursors at the substrate surface, the competition between physical and chemical kinetics induces metal deposition on the substrate. The chemical reaction rate of the gaseous precursor under HIB exposure is greater than the removal rate of parasitic material from the substrate by HIB bombardment. The deposition rate strongly depends on the precursor gas coverage of the substrate surface in HIB-induced deposition. The shape and volume of the deposit on the substrate varies with the competition between deposition and sputtering processes, the thermal and beam-enhanced diffusion, and the implantation of primary ions. Shadowing-which results from partial exposure to the precursor flux-has a specific effect on the shape and volume of the deposit. Therefore, it is necessary to optimize the deposition parameters of a specific nanoprocess for HIB-induced-deposition technology. Initially, Alkemade P F group achieved HIB-induced Pt deposition using a Carl Zeiss ORION™ HIM equipment with OmniGIS [75,76]. For Pt deposition with (Methylcyclopentadientyl) trimethyl platinum (CH 3 ) 3 Pt(C P CH 3 ) as the gaseous precursor, various controllable parameters such as beam current, beam overlap, and deposition size were investigated. Statistical experiments show that during HIB-induced Pt deposition, HIB current, scanning pixel pitch, and total deposit size have a significant effect on the metal deposition rate and composition (figure 9(a)) [77].
Alkemade P F et al also demonstrated that the growth of nanopillars via HIB-induced deposition strongly depends on the consumption of gaseous precursor molecules [78]. Compared to continuous HIB exposure, the pulsed HIB-induced deposition was developed as a new nanofabrication method. Pulsed HIB can reduce depletion, thereby increasing the growth rates of PtC nanopillars. The high growth rate and rapid vertical growth produced nanopillars with high aspect ratios. The deposition efficiency of short dwell times is 20 times higher than that of continuous HIB exposure conditions. The non-damaging dose optimizes the parameters of the HIB-induced deposition process to reduce the damage caused by high-energy helium ion bombardment [79]. The proximity effect-which is caused by charging or discharging neighboring structures-causes the deposited pillars to growth both vertically and laterally as the pitch decreases [8]. Ultimately, the optimization of HIB-induced deposition process parameters, such as beam current, dwell time, beam focus, and refresh time of gaseous precursor, is necessary to efficiently and precisely fabricate extended, complex nanostructures.
The simulation of HIB-induced deposition qualitatively reflects the relevant processes that guide extreme nanofabrication. In this article, both Monte Carlo simulation and analysis models are used to quantify and understand the complex processes involved in HIB-induced deposition [80]. Taking the fabrication of nanopillars deposited by HIB-induced deposition as an example, the competition between helium ions, SE, and dispersed ions in the vertical and horizontal column, induced nanopillar growth nanopillars, as shown in figures 9(b) and (c) [76]. When (CH 3 ) 3 Pt(C P CH 3 ) is used as the gaseous precursor, the growth rate of PtC pillars is related to the HIM beam current. The Monte Carlo simulation of HIB-induced deposition also shows that the reaction rate can affect the vertical growth and the lateral broadening with incident primary helium ions [81]. However, due to the scattered ions and SEs, the proximity effect limits mass transport, which decreases the vertical growth velocity but broadens the deposition structures. The Monte Carlo simulations were carried out with actual values obtained from HIB-induced Pt deposition to predict the growth characteristics of PtC nanopillars [82].

Nanopillars deposition with different gaseous precursors
Ion beam-induced deposition of different nanostructures depends on the nature of the injected gaseous precursors and noble gas ion beams. Various gas compounds have been reported for focused gallium ion beam [73] and electron beam [9] induced deposition processes. Due to the different electrical properties of the inert helium and electroactive gallium, only a small amount of volatile compounds like organometallics were selected as gaseous precursors for HIB-induced deposition. Among them, (CH 3 ) 3 Pt(C P CH 3 ) was first used as a gaseous precursor to fabricate nanopillars with the HIBinduced deposition process [75]. The parameter optimization and theoretical simulation of HIB-induced Pt deposition make it possible for nanofabrication of future circuit editing and nanoelectronic devices.
Other gaseous precursors aside from (CH 3 ) 3 Pt(C P CH 3 ), such as W(CO) 6 and dicobalt octacarbonyl (Co 2 (CO) 8 ), are used in the HIB-induced deposition process. Kohama K et al successfully deposited 40 nm wide and 2 µm tall W-based pillars on carbon and silicon substrates using W(CO) 6 as the gaseous precursor for the HIB-induced deposition process [83]. Their experiments suggest that at least three phenomena occur during the formation of W-based pillar under HIB exposure: pillar deposition, sputter-etching, and Si blistering [83]. Wu H et al selected Co 2 (CO) 8 as the gas precursor for HIB-induced Co nanowire deposition induced [84]. The resistivity and contact resistance of the deposited nanowire between the metal nanowire and the electrical substrate verify the feasibility of depositing high-quality Co metal lines through the gaseous precursor Co 2 (CO) 8 using HIB-induced deposition.

Nanowire deposition for circuit editing application
Circuit editing is a powerful technique, widely used in the field of integrated circuits, to rapidly debug and modify nanoelectronic devices. In conjunction with GIS, HIM is used to deposit complex patterns of sub-10 nm metals to fill and wire connections on the surfaces of nanoelectronic devices, due to its ultimately smaller probe size and beam profile with much less beam tail contribution. The purity of the deposited metal is high enough to achieve the low resistivity required in circuit editing applications. Drezner Y et al characterized the pillar morphology, structure, and composition of HIB-induced deposited metal by using TEM and energy dispersive spectroscopy (EDS) [19]. The maximum Pt content for HIB-induced deposition on the Si substrate was 41% ( figure 10(A)). A lower HIB current is suitable for metal deposition because it involves less subsurface damage to the Si substrate in future circuit editing applications. The Co nanowires are deposited on the electrical test structure using the HIB-induced deposition process with the gaseous precursor Co 2 (CO) 8 . The extremely small spot size of HIB allowed for the effective patterning of single and multiple Co lines with 10 nm resolution by a scanning HIM equipment with Omniprobe GIS, as shown in figure 10(B). Good repeatability was obtained in the deposition of Co metal wires, and their purity was extremely high. HIM imaging, high-resolution TEM imaging, and electron energy-loss spectroscopy also show that there was almost no overspray around the deposition area [84]. The investigation of deposit composition and the subsurface damage with HIB-induced deposition in sub 10 nm patterns fabrication at high densities provides a new solution for future circuit editing application.
Owing to its excellent electrical properties, graphene has been chosen as a new material to fabricate next generation nanoscale electronic devices. In the case of HIB-induced deposition, W and Pt wires are deposited on graphene nanodevices assisted with gaseous precursors, forming an electrical contact in quantum cellular automaton devices [85]. Since the deposition rate is a function of the HIB current, the growth of deposition wires is related to the dose of helium ions. HIB direct writing can also be used to directly pattern graphene without significantly damaging the substrate. Therefore, the combination of HIB direct writing with HIB-induced deposition can fabricate subnanometer-scale graphene nanoelectronic devices with optimized process parameters. Superconducting W nanowires grown by HIB-induced deposition technology are used in hybrid coplanar waveguide microwave resonators, which are combined with sputtered niobium thin films in high-frequency superconducting circuits [86]. SEM images of (a) 5 nm, (b) 4 nm, and (c) 3.5 nm half-pitch nested L's formed by helium ion beam lithography in an HSQ layer that was subsequently developed to remove the unexposed resist. Five nanometers and 4 nm half-pitch patterns were resolved. Although the 3.5 nm half-pitch patterns were not completely resolved, there were regions in which individual lines are distinct. Reprinted with permission from [92]. Copyright 2012 American Vacuum Society.

3D nanostructures deposition for nanoelectronic devices fabrications
Córdoba R et al first fabricated 3D superconducting W carbide (WC) hollow nanowire with a diameter of 32 nm and a maximum aspect ratio (length/diameter) of 200 by using the HIB-induced deposition technique with the gaseous precursor W(CO) 6 , as shown in figures 11(a)-(c) [65]. Because of their quasi-one-dimensional superconductivity, the fabricated nanowires demonstrated a large critical magnetic field and current density when they were superconducting at 6.4 K. As shown in figures 11(d)-(e), through the direct writing method, Córdoba's team also fabricated novel 3D WC nanohelices by controlling key deposition parameters for superconducting applications. Córdoba R et al experimentally identified the characteristics of the vortex and phase-slip patterns based on its helical 3D geometry, which induces specific superconducting properties [87]. The advantages of using HIB-induced complex 3D nanofabrication methods provide more opportunities for nanoelectronic devices based on other sophisticated 3D nano-superconductors.

Additional HIB nanofabrication
Electron beam lithography (EBL) is a supplement to photolilography and one of the most well-established technologies for fabricating prototype nanoelectronic devices through top-down processes. Focused electron beams can modify the chemical properties of thin resist materials coated on substrates into which the arbitrary pattern nanoscale structures are created upon exposure [88]. Due to its sub-nanometer spot-size, the focused HIB is a new, high-resolution direct writing exposure beam for nanofabrication. Considering its high resolution, high SE yield, and low proximity effect, HIB direct writing is equal to or better than EBL for nanoelectronic devices fabrication. Moreover, helium ions have relatively low mass and are less damaging to target substrates compared to other particles, such as electrons and gallium ions [89,90]. Therefore, the HIB direct writing technique can be considered as a new alternative 'bottom-up' nanofabrication method.

Resist assisted nanostructures
Resist assisted HIB direct writing uses a focused HIB to modify chemical properties of resist materials and to alter the solubility of the exposed resist. The subsequent development step with a chemical solvent is carried out and a topographical pattern can be formed in the resist. The minimum feature size achievable with resist assisted HIB direct writing depends on the interaction of HIB with the resist and its properties. The focused HIB with a sub-nanometer spot-size and GFIS produces high-resolution direct writing comparable to or superior than that achieved with electron beams. Many resist materials demonstrate their potential for use in HIB direct writing owing to their ability to be patterned down to ∼10 nm or less. Shi X and Kalhor N et al discussed and reviewed the HIBresist interaction mechanisms and the latest experimental results using HIB direct writing with resist assistance [20,91]. By combining high-resolution HIB patterning and nanoimprint lithography Li W D et al [92] fabricated a series of sub-10 nm line patterns with 4 nm half-pitches by exposing a layer of hydrogen silsesquioxane (HSQ) resist with a scanning focused HIB, as shown in figure 12. A novel, negative tone fullerene-derivative molecular resist was investigated with HIB direct writing to fabricate sparse line features with 7.3 nm line widths using standard processing conditions with minimal proximity effect [93]. The isolated features as small as 5 nm were patterned using focused HIB direct writing with a recently developed alumina-based negative-tone resist. The alumina-based resist was synthesized using a sol-gel method that nearly turned into a completely inorganic alumina system when exposed to the ion beam [94]. In field-emission device fabrication, a negative tone metal-organic (MOC) resist was presented which can be patterned to produce sub-10 nm features on a sub-20 nm pitch in Si and W when exposed to HIB direct writing at line doses on the order of tens of pico coulombs per centimeter [95]. A negative tone resist incorporating nickel-based MOC clusters (Ni-MOC) was investigated under HIB direct writing for the first time. Highresolution ∼9 nm line patterns were well developed at a sensitivity of 22 µC cm −2 and significantly lower line-edge/width roughness of 1.81 ± 0.06 nm and 2.90 ± 0.06 nm. These findings elucidate the potential for sub-10 nm technology nodes, under standard processing conditions [96].
The ultimate resolution of lithographic features depends on both the point-spread function and resist contrast. The proximity effect is reduced to a single-digit nanometer resolution for single-pixel exposures over small areas. Flatabo R et al showed that HIB direct writing can fabricate precision highdensity gratings on 100 × 100 µm planar surfaces with a pitch as small as 35 nm using an area dose for exposure without any proximity effect corrections while maintaining a large focus tolerance [97]. The proximity effect due to the interaction between the high-energy HIB and the resist, and the lithographic effect at sites nearby the original beam incidence site can be exploited for nanofabrication.
As shown in figure 13, Cai J et al experimentally investigated the 3D interaction volume between incident helium ions with HSQ [98]. They developed a new, flexible 3D nanofabrication technique by using a novel throughmembrane exposure method of HIB direct writing. The 3D volumetric energy deposition of incident helium ions and it is local crosslinking with HSQ under focused HIB exposure enable the fabrication of complex crosslinked HSQ nanostructures, such as embedded nanochannels and suspended grids. The proposed crosslinked HSQ method expands normal HIB into 3D lithography for nanofabrication. HIB direct writing can also be used to pattern other nanostructures without the aid of resists, such as special graphene devices and plasmon nanostructures. Thereafter, we discuss the nonresist-assisted HIB direct writing technology and its nanofabrication applications, which offer rapid device prototyping without the use of photo-masks, resist, or other expensive equipment.

Graphene devices
Due to its stability, high strength, resistance to defects, and unique electronic energy band structure, graphene material is widely used in nanoscale electronic, optoelectronic, and mechanical applications. HIB was used to controllably modify the electrical properties of graphene-based electronic devices by Marcus C M group with the direct writing technique [13]. As shown in figure 14(A), a channel about 10 nm wide was etched in a suspended graphene device to isolate different parts of the graphene field effect transistor. The electron transport performance of graphene nanoribbons was measured on an encapsulated graphene device with a 10 nm wide insulation barrier, which was etched by a 30 keV HIB [21]. As shown in figure 14(B), the HIB direct writing process was used to pattern graphene nanoribbon arrays as low as 5 nm wide for the first time. HIB direct writing technique was also used to create band gaps in graphene field effect transistors for electronic sensing applications [22]. Naitou Y et al also patterned sub-10 nm wide nanoribbons on suspended monolayer graphene by HIB direct writing [99]. Scanning capacitance microscopy measurement results show that the spatial resolution of HIB pattern has a non-monotonic relationship with its dose. As shown in figure 14(C), the superstructures graphene nanoribbons with a pitch of 20 nm are directly written with a singlestep pattern of defect lines with a width of 5 nm by HIB direct writing [100]. A set of graphene triangles were patterned using HIB direct writing technology for plasmon nanoresonator applications. The dose of helium ions greatly effects the plasmon response of graphene structures [101]. Although a lower dose of helium ions was bombarded on the graphene device, the residual conductivity attributed to hydrocarbon contamination should still be considered. Therefore, by using an improved HIM equipped with a commercial beam pattern package, Bell D C et al precisely cut and nanoscale patterned graphene by controlling the alignment, patterning, and exposure processes with a computer [102]. HIB allows for dosage control, pattern configurability, and precise alignment of existing features, all of which make HIB direct writing as fast

Plasmonic nanostructures
Plasmonic nanostructures like metallic nano-antennas are widely used functional elements, such as molecular spectroscopy, near-field nanoscale imaging, and quantum mechanical phenomena. Coaxial optical antennae structures with a critical dimension less than 10 nm were directly fabricated using HIB direct writing technology for nano-optical applications. Due to the high-resolution fabrication capabilities of HIB, for the same design geometry, the quality factor of the fabricated coaxial antennae is higher than that of gallium FIB direct written resonators, as shown in figure 15(A) [104]. Figure 15(B) shows a 6 nm gap on the gold bowtie antennas, which were milled by HIB direct writing to investigate the quantum optical properties in a near-field plasmon nanoantenna with the third harmonic spectroscopy [23,105]. Combined with electron lithography, HIB direct writing can fabricate the smallest size parts at low milling rates. Then, plasmonic nanoantennas could be patterned with the maximum precision in a reasonable time on the polycrystalline gold film or single-crystalline gold flakes [106]. As shown in figure 15(C), these strategies are used to further miniaturize zero-, first-, and second-order Sierpiński fractal dimer nanoantennas and to investigate the scattering spectrum and high near-field enhancement [107]. Plasmonic dipole nanoantennas with 5 nm-wide gaps were also fabricated by focused HIB direct writing. These antennas could have a wavelength shift of about 250 nm per refractive index unit [24]. Moreover, a high-quality metal plasma heptamer nanohole array with an aspect ratio of 4:1 was fabricated by focused HIB direct writing [108]. For the plasmonic nanoantenna fabrication [109], HIB is astate-of-the-art nanofabrication technology, but its subtractive patterning strategy in nature and extremely low patterning efficiency make it unsuitable for plasmonic applications based on particle and assemblies. Therefore, Chen Y et al developed a sketch and peel strategy [110] enabling HIB to be utilized in the fabrication of plasmonic nanodimer with sub-10 nm nanogaps, as shown in figure 16. The two heart-shaped plasmonic nanodimers had both convex and concave features [111], which were obtained by topology optimization-based inverse designing. The heart-shaped nanodimers exhibited significantly stronger near-field enhancement performance than the bowtie-, and disk-shaped nanodimers structures in its sub-10 nm gap.

Outlook
The latest developments in HIB technology have been summarized, as shown in table 1. Generally, due to its high sensitivity and high resolution, HIB technology has been rapidly developed to fabricate more complicated nanostructures that can be used in a variety of applications. HIB technology is used for high-contrast, high-resolution imaging of conducting, semiconducting, and insulating materials, and biological samples. Although the ions collide with the target sample, HIB is superior to conventional SEM imaging. For extreme nanofabrication, nanometer-scale nanopores that are beneficial for single base recognition of DNA/RNA sequences can be fabricated by HIB milling on thinned silicon nitride membranes or suspended graphene. Amorphization during the milling process promotes the formation of specific 3D nanopores, which can be used for potential nano-optics and bioscience applications. The chemical reaction of the precursor gas molecules adsorbed on the surface induced by HIB results in the direct deposition of programmed 3D structures at the nanoscale. HIB direct writing without resist assistance is used to pattern sub-10 nm nanochannels, nanoribbons, and nanostructures for nanoscale functional devices. Both HIM imaging and HIB nanofabrication must take into account the inevitable damage caused by helium ions colliding with the probe substrate. We can conclude that HIB technology is an attractive method for extreme nanofabrication because of its comprehensive advantages in sensitivity, resolution, and precision. For example, this technology takes advantage of the unique physical properties of graphene to facilitate graphene-based nanoelectronic devices used in a variety of applications, such as plasmon devices, one-step fabrication of solid-state nanopores in ultrathin membranes, etc. HIB technology has a lower sputtering yield but can produce larger damage on the substrate in nanofabrication processing, such as bubbles, implantation, and amorphization. More in-depth theoretical research on the interaction mechanism between helium ions and materials has promoted the improvement of the processing capability of extreme nanofabrication with HIB technology. The stability and repeatability of the HIB milling process will be enhanced to meet the requirements of sub-nanometer resolution and highthroughput fabrication in special applications. Besides, when optimizing the nanofabrication process, the positive or negative impact of helium ions bombardment on the material properties should be considered, so that HIB technology can be used to directly fabricate nanostructures with fewer defects and excellent performance. For direct-write HIB technique and HIB-induced deposition processes, the common challenge is to increase the complexity of nanostructures while maintaining the nanoscale feature size for special applications. To increase the complexity of nanostructures and their applications in production, HIB direct writing must be improved through careful optimization of parameters. Futhermore, the proximity effect should also be taken into consideration in the HIB direct writing and HIB-induced deposition processes. In the end, HIB technology is expected to play an integral role in extreme nanofabrication because it has the advantages of high sensitivity, resolution, and precision for direct writing milling, patterning, assisted-milling and deposition processes with fewer damages to the samples.