Focused Helium Ion Beam Research Articles

Damage profile evolution model based on the Boltzmann transport equation for silicon micromachining with the focused helium ion beam

In this work, a damage profile evolution model based on the Boltzmann transport equation was developed for silicon micromachining using the focused helium ion beam. The energy grouping method, the spherical harmonic function expansion method, and the discrete ordinate method were used to solve the model numerically to obtain the implanted ion distribution and the amorphous damage profile. The influence of the ion beam energy and the ion dose on the evolution of the amorphous damage profile and the helium ion distribution in the silicon substrate were investigated. The implantation depth of the ions was positively correlated with the beam energy, and the influence of the ion dose on the implantation depth of the ions was very slight. The amorphous damage profiles predicted by the model were in good agreement with the experimental transmission electron microscope images. The results showed that the amorphous damage profile evolved into an approximately cylindrical shape for low beam energy (<10 keV), and an approximate “vase” shape for high beam energy (>10 keV). For the specified beam energy, the maximum amorphous width increased linearly and the amorphous depth first increased rapidly and then slowly increased with the increasing ion dose, and there was a limit on the amorphous depth. The computational efficiency of the model was not limited by the number of particles, which made up for a common shortcoming of the Monte Carlo and molecular dynamics methods.

YBa$_{2}$Cu$_{3}$O$_{7-\delta }$Single Flux Quantum Flip Flop Directly Written With a Focused Helium Ion Beam

Superconducting logic circuits based on low transition temperature (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) are attractive due to their intrinsic low power consumption and high operating frequency. They have a wide variety of applications (particularly in future computing), but are difficult to use because they require cooling with liquid helium to operate. The current high T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> logic circuit based on YBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-δ</sub> (YBCO) already has the ability to operate at a temperature above 4 K. However, this material is difficult to integrate into large-scale circuits by the traditional YBCO junction fabrication process. The limits of fabrication methods are the bottleneck stifling this material's potential for integration into logic circuits. As such, in this paper we aim to advance the state of the art in junction fabrication technology. Here we report the demonstration of a high T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> superconducting single flux quantum (SFQ) circuit made by a focused helium ion beam that is easily fabricated and can readily be incorporated into a large-scale integration with nano-scale junctions for future applications. An inductively coupled readout superconducting quantum interference device (SQUID) is employed to reduce the circuit complexity. The circuit parameters and performance are discussed.

Branched High Aspect Ratio Nanostructures Fabricated by Focused Helium Ion Beam Induced Deposition of an Insulator.

Helium ion beam induced deposition using the gaseous precursor pentamethylcyclopentasiloxane is employed to fabricate high aspect ratio insulator nanostructures (nanopillars and nanocylinders) that exhibit charge induced branching. The branched nanostructures are analyzed by transmission electron microscopy. It is found that the side branches form above a certain threshold height and that by increasing the flow rate of the precursor, the vertical growth rate and branching phenomenon can be significantly enhanced, with fractalesque branching patterns observed. The direct-write ion beam nanofabrication technique described herein offers a fast single-step method for the growth of high aspect ratio branched nanostructures with site-selective placement on the nanometer scale.

Large-Scale Focused Helium Ion Beam Lithography

Focused ion beam sources have recently been commercialized and are increasingly utilized for applications in nanotechnology. There is a desire to pattern large designs, millimeter in scale, while still achieving sub-nanometer resolution in the smallest features. However, focused helium ion beams are restricted to fields of view of roughly 100 micrometers. Recently, we have developed an automated process for layout designs with feature sizes from sub-nanometer to millimeter scales, using Raith lithography software in a Zeiss Orion Plus helium ion microscope. The technique relies on stitching writefields together and incorporating automated focusing algorithms in the Raith software. This procedure was demonstrated by scaling and automating the process of producing ion-damaged Josephson junctions in arrays in the high-temperature superconducting material YBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-δ</sub> . We demonstrate that the automated process reduces operator input while maintaining consistency at the smallest resolutions.

Fabrication of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ ab-Plane Josephson Junctions by a Focused Helium Ion Beam

We report the fabrication and measurement of an ab-plane Josephson junction created in an 80-nm thick thin-film-like exfoliated Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> CaCu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8+x</sub> $ single crystal flake. The junctions are formed using ion irradiation from a focused helium ion beam which locally converts the material from a superconductor to an insulator. Current-voltage characteristics show the device to be a weak link with I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</sub> product of 40 μA. The critical current exhibits a well-behaved Fraunhofer pattern with magnetic field applied perpendicular to the film confirming the DC Josephson effect. Simulations reveal a large amount of ion straggle deep into the film which is likely influencing the electrical properties. Combination of these devices with the intrinsic junctions could provide a method for multi-dimensional Josephson devices.

Selective patterning of out-of-plane piezoelectricity in MoTe2 via focused ion beam

Two-dimensional transition-metal dichalcogenides (TMDs) have a strain-sensitive nature and can only exhibit in-plane piezoelectricity, owing to their in-plane inversion symmetry breaking, which limits their practical applications for vertical stimulations. In this study, we demonstrated the capability of focused ion beams to create out-of-plane piezoelectricity on multi-layered MoTe 2 . We utilized a focused helium ion beam to selectively pattern the out-of-plane piezoelectricity via defect engineering in a layered MoTe 2 flake. The generated out-of-plane piezoelectricity in the desired area was quantitatively examined using atomic force microscopy, and ion beam irradiation-induced defect formation that gave rise to inversion symmetry breaking was confirmed. These results indicated that the out-of-plane piezoelectricity can be selectively patterned through a focused helium ion beam, and it is expected that this approach can also be applied to other classes of TMDs and can expand the application fields of TMD-based devices. • Selective patterning of out-of-plane piezoelectricity in 2D TMDs. • Generation/improvement of piezoelectricity using ion beam unlike usual reports on the ion beam irradiation. • Atomic-scale inversion symmetry breaking via local defect formation.

Helium Ion Microscopy for Reduced Spin Orbit Torque Switching Currents.

Spin orbit torque driven switching is a favorable way to manipulate nanoscale magnetic objects for both memory and wireless communication devices. The critical current required to switch from one magnetic state to another depends on the geometry and the intrinsic properties of the materials used, which are difficult to control locally. Here, we demonstrate how focused helium ion beam irradiation can modulate the local magnetic anisotropy of a Co thin film at the microscopic scale. Real-time in situ characterization using the anomalous Hall effect showed up to an order of magnitude reduction of the magnetic anisotropy under irradiation, with multilevel switching demonstrated. The result is that spin-switching current densities, down to 800 kA cm-2, can be achieved on predetermined areas of the film, without the need for lithography. The ability to vary critical currents spatially has implications not only for storage elements but also neuromorphic and probabilistic computing.

Effect of localized helium ion irradiation on the performance of synthetic monolayer MoS2 field-effect transistors.

Helium ion irradiation is a known method of tuning the electrical conductivity and charge carrier mobility of novel two-dimensional semiconductors. Here, we report a systematic study of the electrical performance of chemically synthesized monolayer molybdenum disulfide (MoS2) field-effect transistors irradiated with a focused helium ion beam as a function of increasing areal irradiation coverage. We determine an optimal coverage range of approx. 10%, which allows for the improvement of both the carrier mobility in the transistor channel and the electrical conductance of the MoS2, due to doping with ion beam-created sulfur vacancies. Larger areal irradiations introduce a higher concentration of scattering centers, hampering the electrical performance of the device. In addition, we find that irradiating the electrode–channel interface has a deleterious impact on charge transport when contrasted with irradiations confined only to the transistor channel.

Dynamic properties of high-Tc superconducting nano-junctions made with a focused helium ion beam

The Josephson junction (JJ) is the corner stone of superconducting electronics and quantum information processing. While the technology for fabricating low Tc JJ is mature and delivers quantum circuits able to reach the “quantum supremacy”, the fabrication of reproducible and low-noise high-Tc JJ is still a challenge to be taken up. Here we report on noise properties at RF frequencies of recently introduced high-Tc Josephson nano-junctions fabricated by mean of a Helium ion beam focused at sub-nanometer scale on a YBa2Cu3O7 thin film. We show that their current-voltage characteristics follow the standard Resistively-Shunted-Junction (RSJ) circuit model, and that their characteristic frequency fc = (2e/h)IcRn reaches ~300 GHz at low temperature. Using the “detector response” method, we evidence that the Josephson oscillation linewidth is only limited by the thermal noise in the RSJ model for temperature ranging from T ~ 20 K to 75 K. At lower temperature and for the highest He irradiation dose, the shot noise contribution must also be taken into account when approaching the tunneling regime. We conclude that these Josephson nano-junctions present the lowest noise level possible, which makes them very promising for future applications in the microwave and terahertz regimes.

Micrometer Scale Y–Ba–Cu–O SQUID Arrays Fabricated With a Focused Helium Ion Beam

We present the fabrication and testing of microarrays of superconducting quantum interference devices (SQUIDs) fabricated from the high-transition temperature superconductor YBa $_2$ Cu $_3$ O $_{7-\delta }$ (YBCO). The arrays consisted of 84 SQUIDs, each with loop area 25 $\mu \text{m}^{2}$ , connected electrically in series with a meander line structure. The entire array was contained in an area of just $\text{100}\,\mu \text{m}\times \text{100}\,\mu \text{m}$ . The Josephson junctions in the arrays were directly written into YBCO thin films with ion irradiation from a focused helium ion beam. We found that the electrical properties can be controlled by changing the diameter of the ion beam through selection of either a 20 or 5 $\mu \text{m}$ beam aperture. In particular, larger aperture or beam size results in a weaker link junction with higher operating temperature and smaller critical voltage. The voltages across the dc-biased arrays were measured as a function of magnetic field. The data exhibited maximum voltage modulations of 300 $\mu \text{V}$ at 65 K and 1 mV at 52 K for the 20 and 5 $\mu \text{m}$ beam aperture, respectively. The magnetic field periodicity for both arrays was approximately 30 $\mu$ T per flux quantum.

Inductance of YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ Thin-Films With and Without Superconducting Ground Planes

We experimentally measured and compared the sheet inductance of YBa $_{2}$ Cu $_{3}$ O $_{7-\delta }$ (YBCO) films with and without superconducting ground planes. Specifically, we fabricated several superconducting quantum interference devices from single and multilayer films with different geometries using a focused helium ion beam. Measurements of the device electrical transport properties were analyzed to determine the sheet inductance. Additionally, temperature dependent measurements of the inductance were used in order to separate the contributions from geometric and kinetic inductance. We found that the presence of a ground plane in our multilayer structure significantly reduces the contribution of the geometric inductance with no detectable change in the kinetic inductance.

Direct Write of 3D Nanoscale Mesh Objects with Platinum Precursor via Focused Helium Ion Beam Induced Deposition.

The next generation optical, electronic, biological, and sensing devices as well as platforms will inevitably extend their architecture into the 3rd dimension to enhance functionality. In focused ion beam induced deposition (FIBID), a helium gas field ion source can be used with an organometallic precursor gas to fabricate nanoscale structures in 3D with high-precision and smaller critical dimensions than focused electron beam induced deposition (FEBID), traditional liquid metal source FIBID, or other additive manufacturing technology. In this work, we report the effect of beam current, dwell time, and pixel pitch on the resultant segment and angle growth for nanoscale 3D mesh objects. We note subtle beam heating effects, which impact the segment angle and the feature size. Additionally, we investigate the competition of material deposition and sputtering during the 3D FIBID process, with helium ion microscopy experiments and Monte Carlo simulations. Our results show complex 3D mesh structures measuring ~300 nm in the largest dimension, with individual features as small as 16 nm at full width half maximum (FWHM). These assemblies can be completed in minutes, with the underlying fabrication technology compatible with existing lithographic techniques, suggesting a higher-throughput pathway to integrating FIBID with established nanofabrication techniques.

Topology Optimization‐Based Inverse Design of Plasmonic Nanodimer with Maximum Near‐Field Enhancement

AbstractThe near‐field enhancement factor is one of the most significant parameters to evaluate the performance of plasmonic nanostructures. Numerous efforts have been made to maximize the enhancement factor through optimizing the size, shape, and spatial arrangement of metallic nanostructures with simple geometries, such as disk, triangle, and rod. This work implements topology optimization to inversely design a metallic nanoparticle dimer with the goal of optimizing the near‐field enhancement factor in its sub‐10 nm gap. By optimizing the material layout within a given design space, the topology optimization algorithm results in a plasmonic nanodimer of two heart‐shaped particles having both convex and concave features. Full‐wave electromagnetic analysis reveals that the largest near‐field enhancement in the heart‐shaped nanoparticle dimer is originated from the greatest concentration of surface charges at the nano‐heart apex. Inversely designed heart‐, bowtie‐, and disk‐shaped nanodimers are fabricated by using focused helium ion beam milling with a “sketch and peel” strategy, and their near‐field enhancement performances are characterized with nonlinear optical spectroscopies at the single‐particle level. Indeed, the heart‐shaped nanodimer exhibits much stronger signal intensities than the other two structures. The present work corroborates the validity and effectiveness of topology optimization‐based inverse design in achieving desired plasmonic functionalities.

Present and future of high-temperature superconductor quantum-based voltage standards

This paper presents a brief overview of the current state-of-the-art of Josephson junctions for Quantum-based Voltage Standards fabricated with High-Temperature Superconductors (HTS). A short introduction on the history and technical evolution of Low Temperature Superconductors (LTS) technology is provided for non-specialists. Then HTS technology is summarized and discussed in the context of quantum voltage standard applications. Finally, the two most promising technologies: bicrystal and Focused Helium-Ion Beam junctions are discussed with more detail, analyzing strength, limitations and perspectives in both cases.

Helium focused ion beam irradiation with subsequent chemical etching for the fabrication of nanostructures

In this paper we demonstrate a nanofabrication technique based on local ion irradiation of silicon dioxide with a focused helium ion beam. The wet etching of silicon dioxide irradiated with a focused helium ion beam is described in a two-dimensional case both numerically and experimentally. We suggest a model for the etching process based on the distribution of ion induced defects in the irradiated material. The profile of the surface of the etched silicon dioxide is simulated and compared with the results from scanning electron microscopy. Fabrication of a suspended nanostring with a diameter of less than 20 nm by means of etching ion-irradiated material is demonstrated.

High-transition-temperature nanoscale superconducting quantum interference devices directly written with a focused helium ion beam

In this work, we present nanoscale superconducting quantum interference devices (SQUIDs) with dimensions as small as 10 nm from the high-transition-temperature superconductor YBa2Cu3O7−δ (YBCO). The SQUID features and Josephson junctions are directly written into a 35-nm thick YBCO film with a focused helium ion beam. We integrate these nano-SQUIDs with directly written nano-isolated inductively coupled control lines to demonstrate a low power superconducting output driver capable of transimpedance conversion over a very wide temperature range of 4–50 K.

On-site tuning of the carrier lifetime in silicon for on-chip THz circuits using a focused beam of helium ions

In this study, we demonstrate that a focused helium ion beam allows the local adjustment and optimization of the carrier lifetime in silicon-based photoswitches integrated in ultrafast on-chip terahertz-circuits. Starting with a carrier lifetime of 5.3 ps for as-grown silicon on sapphire, we monotonously reduce the carrier lifetime in integrated switches to a minimum of ∼0.55 ps for a helium ion fluence of 20 × 1015 ions/cm2. By introducing an analytical model for the carrier lifetimes in the photoswitches, we particularly demonstrate that the carrier lifetime can be adjusted locally even within single photoswitches. In turn, the demonstrated on-site tuning allows optimizing ultrafast high-frequency circuits, into which radiation-sensitive nanoscale materials, such as two-dimensional materials, are embedded.

Height patterning of nanostructured surfaces with a focused helium ion beam: a precise and gentle non-sputtering method

This work presents a new technique for surface patterning with focused ion beams. The technique is based on chemical decomposition in the bulk of a polymer substrate with negligible surface sputtering effects. By using a focused helium ion beam, generated in a helium ion microscope, we show that the surface height of polymethyl methacrylate substrates can be patterned with nanometer depth precision, while preserving the essential features of the nanostructures prefabricated on this surface. The key factors that control this patterning process are discussed.

MoS2 Memtransistors Fabricated by Localized Helium Ion Beam Irradiation.

Two-dimensional (2D) layered semiconductors have recently emerged as attractive building blocks for next-generation low-power nonvolatile memories. However, challenges remain in the controllable fabrication of bipolar resistive switching circuit components from these materials. Here, the experimental realization of lateral memtransistors from monolayer single-crystal molybdenum disulfide (MoS2) utilizing a focused helium ion beam is reported. Site-specific irradiation with the focused probe of a helium ion microscope creates a nanometer-scale defect-rich region, bisecting the MoS2 lattice. The reversible drift of these defects in the applied electric field modulates the resistance of the channel, enabling versatile memristive functionality. The device can reliably retain its resistance ratios and set/reset biases for 1180 switching cycles. Long-term potentiation and depression with sharp habituation are demonstrated. This work establishes the feasibility of ion irradiation for controllable fabrication of 2D memristive devices with promising key performance parameters, such as low power consumption. The applicability of these devices for synaptic emulation may address the demands of future neuromorphic architectures.

Miniaturized fractal optical nanoantennas defined by focused helium ion beam milling

It has been shown in the past that fractal geometries are beneficial for radio and communication antenna designs in terms of bandwidth and gain. Recently, this concept was extended to plasmonic nanoantennas. Here, we present a fabrication method based on electron beam lithography and focused helium ion beam milling to further miniaturize dimer nanoantennas of 0th, 1st and 2nd order Sierpiński fractals. With this state-of-the-art approach, it becomes feasible to experimentally move their resonance conditions into the sub-micron wavelength regime, while maintaining excellent pattern definition and achieving sub-10 nm gap sizes for high near-field enhancement. These highly sophisticated nanostructures are numerically simulated and analyzed by dark-field scattering spectroscopy to monitor the effects of the fractal structuring on the scattering spectra and near-field enhancement.