Focused Helium Ion Beam Research Articles

Selective area growth of InAs nanowires from SiO2/Si(1 1 1) templates direct-written by focused helium ion beam technology

We report on the selective area growth of InAs nanowires on patterned SiO2/Si (1 1 1) nano-holes, prepared by focused helium ion beam technology. We used a single spot mode, in which the focused helium ion beam was fixed on a single point with a He+-ion dosage, ranging from 1.5 pC to 8 pC, to drill the nano-holes. The smallest hole diameter achieved is ∼8 nm. We found that low He+-ion dosage is able to facilitate the nucleation of (1 1 1)B InAs on the highly mismatched Si, leading to the vertical growth of InAs nanowires (NWs). High He-ion dosage, on the contrary, severely damaged Si surface, resulting in tilted and stripe-like NWs. In addition to titled NW grown from (1 1 1)A InAs domain, a new titled growth direction due to defect induced twinning was observed. Cross-sectional TEM images of vertical NWs show mixed wurtizite (WZ) and zincblende (ZB) phases, while WZ phase dominants. The stacking faults resulting from the phase change is proportional to NW diameter, suggesting that the critical diameter of phase turning is larger than 110 nm, the maximum diameter of our NWs. Period of misfit dislocation at the InAs/Si interface of vertical NW is also found larger than the theoretical value when the diameter of heterointerface is smaller than 50 nm, indicating that the small contact area is able to accommodate the large lattice and thermal mismatch between InAs and Si.

Precise milling of nano-gap chains in graphene with a focused helium ion beam

A focused helium ion beam was used to introduce nano-sized gap chains in graphene. The effect of beam scanning strategies in the fabrication of the nano-gap chains was investigated. The tuning of graphene conductivity has been achieved by modulating the magnitude and uniformity of the ion dose and hence the morphology of the nano-gap chains. A model based on the site-specific and dose-dependent conductivity was built to understand the tuning of the conductivity, taking into account the nanoscale non-uniformity of irradiation.

Focused helium-ion beam irradiation effects on electrical transport properties of few-layer WSe2: enabling nanoscale direct write homo-junctions.

Atomically thin transition metal dichalcogenides (TMDs) are currently receiving significant attention due to their promising opto-electronic properties. Tuning optical and electrical properties of mono and few-layer TMDs, such as tungsten diselenide (WSe2), by controlling the defects, is an intriguing opportunity to synthesize next generation two dimensional material opto-electronic devices. Here, we report the effects of focused helium ion beam irradiation on the structural, optical and electrical properties of few-layer WSe2, via high resolution scanning transmission electron microscopy, Raman spectroscopy, and electrical transport measurements. By controlling the ion irradiation dose, we selectively introduce precise defects in few-layer WSe2 thereby locally tuning the resistivity and transport properties of the material. Hole transport in the few layer WSe2 is degraded more severely relative to electron transport after helium ion irradiation. Furthermore, by selectively exposing material with the ion beam, we demonstrate a simple yet highly tunable method to create lateral homo-junctions in few layer WSe2 flakes, which constitutes an important advance towards two dimensional opto-electronic devices.

Nanopore fabrication and characterization by helium ion microscopy

The Helium Ion Microscope (HIM) has the capability to image small features with a resolution down to 0.35 nm due to its highly focused gas field ionization source and its small beam-sample interaction volume. In this work, the focused helium ion beam of a HIM is utilized to create nanopores with diameters down to 1.3 nm. It will be demonstrated that nanopores can be milled into silicon nitride, carbon nanomembranes, and graphene with well-defined aspect ratio. To image and characterize the produced nanopores, helium ion microscopy and high resolution scanning transmission electron microscopy were used. The analysis of the nanopores' growth behavior allows inferring on the profile of the helium ion beam.

A circuit model for defective bilayer graphene transistors

This paper investigates the behaviour of a defective single-gate bilayer graphene transistor. Point defects were introduced into pristine graphene crystal structure using a tightly focused helium ion beam. The transfer characteristics of the exposed transistors were measured ex-situ for different defect concentrations. The channel peak resistance increased with increasing defect concentration whilst the on–off ratio showed a decreasing trend for both electrons and holes. To understand the electrical behaviour of the transistors, a circuit model for bilayer graphene is developed which shows a very good agreement when validated against experimental data. The model allowed parameter extraction of bilayer transistor and can be implemented in circuit level simulators.

Focused particle beam-induced processing.

Focused particle beam-induced processing.

Progression of Focused Helium Ion Beam Milling in Gold Substrates

The focused helium ion beam-solid state material interaction volumes differ in thin membranes and in bulk substrates [1, 2]. While nanometer-scale structures with feature sizes of 5 nm or smaller have been machined in thin membranes [3-5], inconsistent results have been observed in bulk substrates where the irradiated region often swells rather than mills [6]. The depth and breadth of the interaction volume scales with beam energy, and the sputter yield also varies significantly. In this study, we seek to explore and understand these differences by observing the progression of focused helium ion beam milling. For this we used transmission electron microscope images of the beam-irradiated cross sections of 100-nm-thick gold membrane and 400-nm-thick (essentially bulk) gold substrates. We varied the ion beam landing energies and the dose as well.

YBa2Cu3O7−δ superconducting quantum interference devices with metallic to insulating barriers written with a focused helium ion beam

In this work, we demonstrate the ability to fabricate superconducting quantum interference devices (SQUIDs) by directly writing Josephson junctions into the plane of YBa2Cu3O7−δ thin films with a focused helium ion beam. This technique allows for the control of the Josephson barrier transport properties through the single parameter, ion dose. SQUIDs written with a dose of 4 × 1016 ions/cm2 had metallic barrier junctions that exhibited nearly ideal electrical transport characteristics at 50 K and a flux noise of 20 μΦ0/Hz at 10 Hz. At higher irradiation doses, the SQUIDs had insulating barrier Josephson junctions with a quasi particle energy gap edge at 20 meV.

New views of the Toxoplasma gondii parasitophorous vacuole as revealed by Helium Ion Microscopy (HIM).

The Helium Ion Microscope (HIM) is a new technology that uses a highly focused helium ion beam to scan and interact with the sample, which is not coated. The images have resolution and depth of field superior to field emission scanning electron microscopes. In this paper, we used HIM to study LLC-MK2 cells infected with Toxoplasma gondii. These samples were chemically fixed and, after critical point drying, were scraped with adhesive tape to expose the inner structure of the cell and parasitophorous vacuoles. We confirmed some of the previous findings made by field emission-scanning electron microscopy and showed that the surface of the parasite is rich in structures suggestive of secretion, that the nanotubules of the intravacuolar network (IVN) are not always straight, and that bifurcations are less frequent than previously thought. Fusion of the tubules with the parasite membrane or the parasitophorous vacuole membrane (PVM) was also infrequent. Tiny adhesive links were observed for the first time connecting the IVN tubules. The PVM showed openings of various sizes that even allowed the observation of endoplasmic reticulum membranes in the cytoplasm of the host cell. These findings are discussed in relation to current knowledge on the cell biology of T. gondii.

Nano Josephson superconducting tunnel junctions in YBa2Cu3O(7-δ) directly patterned with a focused helium ion beam.

Since the discovery of the high-transition-temperature superconductors (HTSs), researchers have explored many methods to fabricate superconducting tunnel junctions from these materials for basic science purposes and applications. HTS circuits operating at liquid-nitrogen temperatures (∼77 K) would significantly reduce power requirements in comparison with those fabricated from conventional superconductors. The difficulty is that the superconducting coherence length is very short and anisotropic in these materials, typically ∼2 nm in the a-b plane and ∼0.2 nm along the c axis. The electrical properties of Josephson junctions are therefore sensitive to chemical variations and structural defects on atomic length scales. To make multiple uniform HTS junctions, control at the atomic level is required. In this Letter we demonstrate all-HTS Josephson superconducting tunnel junctions created by using a 500-pm-diameter focused beam of helium ions to directly write tunnel barriers into YBa2Cu3O(7-δ) (YBCO) thin films. We demonstrate the ability to control the barrier properties continuously from conducting to insulating by varying the irradiation dose. This technique could provide a reliable and reproducible pathway for scaling up quantum-mechanical circuits operating at liquid-nitrogen temperatures, as well as an avenue to conduct novel planar superconducting tunnelling studies for basic science.

Backscattered helium spectroscopy in the helium ion microscope: Principles, resolution and applications

We demonstrate the possibilities and limitations for microstructure characterization using backscattered particles from a sharply focused helium ion beam. The interaction of helium ions with matter enables the imaging, spectroscopic characterization, as well as the nanometer scale modification of samples. The contrast that is seen in helium ion microscopy (HIM) images differs from that in scanning electron microscopy (SEM) and is generally a result of the higher surface sensitivity of the method. It allows, for instance, a much better visualization of low-Z materials as a result of the small secondary electron escape depth. However, the same differences in beam interaction that give HIM an edge over other imaging techniques, also impose limitations for spectroscopic applications using backscattered particles. Here we quantify those limitations and discuss opportunities to further improve the technique.

Membrane thickness dependence of nanopore formation with a focused helium ion beam.

Solid-state nanopores are emerging as a valuable tool for the detection and characterization of individual biomolecules. Central to their success is the realization of fabrication strategies that are both rapid and flexible in their ability to achieve diverse device dimensions. In this paper, we demonstrate the membrane thickness dependence of solid-state nanopore formation with a focused helium ion beam. We vary membrane thickness in situ and show that the rate of pore expansion follows a reproducible trend under all investigated membrane conditions. We show that this trend shifts to lower ion dose for thin membranes in a manner that can be described quantitatively, allowing devices of arbitrary dimension to be realized. Finally, we demonstrate that thin, small-diameter nanopores formed with our approach can be utilized for high signal-to-noise ratio resistive pulse sensing of DNA.

Focused helium ion beam deposited low resistivity cobalt metal lines with 10 nm resolution: implications for advanced circuit editing

Helium ion beam induced cobalt nanowire deposition using dicobalt octacarbonyl (Co2(CO)8) as a precursor is described. 10 nm wide metal lines were fabricated with good repeatability and extremely high purity. The metal lines were deposited on electrical test structures to determine the nanowire resistivity and contact resistance. Measurements reveal that these metal lines have 50–100 μΩ-cm resistivity. The contact resistance of a Co line on a gold pad is 15 Ω with a 0.14 × 1.4 μm contact area. The resulting metal deposit size was determined by helium ion microscopy (HIM) because of its high imaging resolution, and further characterized by high resolution transmission electron microscopy (HR-TEM) and electron energy-loss spectroscopy (EELS). HR-TEM images reveal that these metal lines are composed of 6 ± 2 nm cobalt crystallite grains; and EELS analysis shows that no measurable carbon signal was observed. Single-line and multiple-line patterns were prepared to examine proximity effects around the metal line depositions. Both HIM imaging and electrical measurements of the patterns verify that the collateral proximity deposition between individual lines can be minimized. Finally, factors determining the ultimate line width achievable with the He ion beam are discussed.

Sub-10 nm patterning by focused He-ion beam milling for fabrication of downscaled graphene nano devices

In this work, a novel hybrid fabrication method for graphene quantum dot devices with minimum feature sizes of ∼3nm and high yield is described. It is a combination of e-beam lithography and direct milling with the sub-nm focused helium ion beam generated by a helium ion microscope. The method is used to fabricate graphene quantum dot devices contacted with metal to allow electrical characterization. An annealing step is described that reduces hydrocarbon contamination on the sample surface and allows complete removal of graphene by the helium ion beam and therefore successful isolation of side gates. The electrical characterization of the final device demonstrates the successful fabrication of the first electrically characterized He-ion beam patterned graphene device. The highly controllable, fine scale fabrication capabilities offered by this approach could lead to a more detailed understanding of the electrical characteristics of graphene quantum devices and pave the way towards room-temperature operable graphene quantum dot devices.

Diamond nitrogen-vacancy centers created by scanning focused helium ion beam and annealing

We demonstrate a method to create nitrogen-vacancy (NV) centers in diamond using focused helium ion microscopy. Near-surface NV centers can be created with spatial resolution below 0.6 μm. We studied the density, creation efficiency, and spectral linewidths at optical and microwave frequencies for NV centers produced using various helium ion implantation doses. The optical linewidths are narrower than those of similar nitrogen-vacancy centers produced using nitrogen ion implantation.

In Situ Thickness Assessment During Ion Milling of a Free-Standing Membrane Using Transmission Helium Ion Microscopy

We describe a novel method for in situ measurement of the local thickness of a freely suspended solid-state membrane after thinning with a focused helium ion beam. The technique utilizes a custom stage for the helium ion microscope that allows the secondary electron detector used for normal imaging to collect information from ions transmitted through the sample. We find that relative brightness in the transmission image scales directly with the membrane thickness as determined by atomic force microscopy measurements.

Local modification of magnetic anisotropy and ion milling of Co/Pt multilayers using a He+ ion beam microscope

We use a focused helium ion beam microscope to controllably remove material from Pt-capped Co/Pt thin film multilayers with perpendicular magnetic anisotropy. The dose required to remove several nm of the cap layer is similar to that required to induce a spin reorientation transition. Milling depth for the Pt overlayer is found to scale with the logarithm of ion dose, with very high removal yields of hundreds of atoms per He+ ion at low doses. While the deposited energy is of order of previous broad beam helium irradiations, the current density impacting on the surface of the sample is several tens of A cm−2, orders of magnitude higher. Patterning of the platinum capping layer with 20 nm resolution is demonstrated.

Combined helium ion beam and nanoimprint lithography attains 4 nm half-pitch dense patterns

The authors demonstrated a promising technique that yielded single-digit nanometer features for nanotechnology research and possible future electronic circuit fabrication by combining high resolution helium ion beam patterning and nanoimprint lithography. They fabricated a series of line patterns with single-digit nanometer half-pitches by exposing a layer of hydrogen silsesquioxane (HSQ) resist with a scanning focused helium ion beam. The smallest half-pitch of clearly resolved line patterns was 4 nm. Using the HSQ patterns as a nanoimprint template, nanoscale patterns down to 4 nm half-pitch were transferred into nanoimprint resist through a UV-curable nanoimprint process.

Nano-structuring, surface and bulk modification with a focused helium ion beam

We investigate the ability of a focused helium ion beam to selectively modify and mill materials. The sub nanometer probe size of the helium ion microscope used provides lateral control not previously available for helium ion irradiation experiments. At high incidence angles the helium ions were found to remove surface material from a silicon lamella leaving the subsurface structure intact for further analysis. Surface roughness and contaminants were both reduced by the irradiation process. Fabrication is also realized with a high level of patterning acuity. Implantation of helium beneath the surface of the sample is visualized in cross section allowing direct observation of the extended effects of high dose irradiation. The effect of the irradiation on the crystal structure of the material is presented. Applications of the sample modification process are presented and further prospects discussed.

Nano-structuring and modification with a focused helium ion beam

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.