Laser-Assisted Focused He+ Ion Beam Induced Etching with and without XeF2 Gas Assist

Condensed Matter Systems Exposed to Radiation: Multiscale Theory, Simulations, and Experiment

This roadmap reviews the new, highly interdisciplinary research field studying the behavior of condensed matter systems exposed to radiation. The Review highlights several recent advances in the field and provides a roadmap for the development of the field over the next decade. Condensed matter systems exposed to radiation can be inorganic, organic, or biological, finite or infinite, composed of different molecular species or materials, exist in different phases, and operate under different thermodynamic conditions. Many of the key phenomena related to the behavior of irradiated systems are very similar and can be understood based on the same fundamental theoretical principles and computational approaches. The multiscale nature of such phenomena requires the quantitative description of the radiation-induced effects occurring at different spatial and temporal scales, ranging from the atomic to the macroscopic, and the interlinks between such descriptions. The multiscale nature of the effects and the similarity of their manifestation in systems of different origins necessarily bring together different disciplines, such as physics, chemistry, biology, materials science, nanoscience, and biomedical research, demonstrating the numerous interlinks and commonalities between them. This research field is highly relevant to many novel and emerging technologies and medical applications.

Selected Area Manipulation of MoS2 via Focused Electron Beam-Induced Etching for Nanoscale Device Editing

We demonstrate direct-write patterning of single and multilayer MoS2 via a focused electron beam-induced etching (FEBIE) process mediated with the XeF2 precursor. MoS2 etching is performed at various currents, areal doses, on different substrates, and characterized using scanning electron and atomic force microscopies as well as Raman and photoluminescence spectroscopies. Scanning transmission electron microscopy reveals a sub-40 nm etching resolution and the progression of point defects and lateral etching of the consequent unsaturated bonds. The results confirm that the electron beam-induced etching process is minimally invasive to the underlying material in comparison to ion beam techniques, which damage the subsurface material. Single-layer MoS2 field-effect transistors are fabricated, and device characteristics are compared for channels that are edited via the selected area etching process. The source-drain current at constant gate and source-drain voltage scale linearly with the edited channel width. Moreover, the mobility of the narrowest channel width decreases, suggesting that backscattered and secondary electrons collaterally affect the periphery of the removed area. Focused electron beam doses on single-layer transistors below the etching threshold were also explored as a means to modify/thin the channel layer. The FEBIE exposures showed demonstrative effects via the transistor transfer characteristics, photoluminescence spectroscopy, and Raman spectroscopy. While strategies to minimize backscattered and secondary electron interactions outside of the scanned regions require further investigation, here, we show that FEBIE is a viable approach for selective nanoscale editing of MoS2 devices.

Selected Area Deposition of High Purity Gold for Functional 3D Architectures

Selected area deposition of high purity gold films onto nanoscale 3D architectures is highly desirable as gold is conductive, inert, plasmonically active, and can be functionalized with thiol chemistries, which are useful in many biological applications. Here, we show that high-purity gold coatings can be selectively grown with the Me₂Au (acac) precursor onto nanoscale 3D architectures via a pulsed laser pyrolytic chemical vapor deposition process. The selected area of deposition is achieved due to the high thermal resistance of the nanoscale geometries. Focused electron beam induced deposits (FEBID) and carbon nanofibers are functionalized with gold coatings, and we demonstrate the effects that laser irradiance, pulse width, and precursor pressure have on the growth rate. Furthermore, we demonstrate selected area deposition with a feature-targeting resolutions of ~100 and 5 µm, using diode lasers coupled to a multimode (915 nm) and single mode (785 nm) fiber optic, respectively. The experimental results are rationalized via finite element thermal modeling.

Emerging laser-assisted vacuum processes for ultra-precision, high-yield manufacturing

Laser technology is a cutting-edge process with a unique photothermal response, precise site selectivity, and remote controllability. Laser technology has recently emerged as a novel tool in the semiconductor, display, and thin film industries by providing additional capabilities to existing high-vacuum equipment. The in situ and in operando laser assistance enables using multiple process environments with a level of complexity unachievable with conventional vacuum equipment. This broadens the usable range of process parameters and directly improves material properties, product precision, and device performance. This review paper examines the recent research trends in laser-assisted vacuum processes (LAVPs) as a vital tool for innovation in next-generation manufacturing processing equipment and addresses the unique characteristics and mechanisms of lasers exclusively used in each study. All the findings suggest that the LAVP can lead to methodological breakthroughs in dry etching, 2D material synthesis, and chemical vapor deposition for optoelectronic devices.

Developing Anisotropy in Self-Assembled Block Copolymers: Methods, Properties, and Applications

Block copolymers (BCPs) self-assembly has continually attracted interest as a means to provide bottom-up control over nanostructures. While various methods have been demonstrated for efficiently ordering BCP nanodomains, most of them do not generically afford control of nanostructural orientation. For many applications of BCPs, such as energy storage, microelectronics, and separation membranes, alignment of nanodomains is a key requirement for enabling their practical use or enhancing materials performance. This review focuses on summarizing research progress on the development of anisotropy in BCP systems, covering a variety of topics from established aligning techniques, resultant material properties, and the associated applications. Specifically, the significance of aligning nanostructures and the anisotropic properties of BCPs is discussed and highlighted by demonstrating a few promising applications. Finally, the challenges and outlook are presented to further implement aligned BCPs into practical nanotechnological applications, where exciting opportunities exist.

A review of defect engineering, ion implantation, and nanofabrication using the helium ion microscope

The helium ion microscope has emerged as a multifaceted instrument enabling a broad range of applications beyond imaging in which the finely focused helium ion beam is used for a variety of defect engineering, ion implantation, and nanofabrication tasks. Operation of the ion source with neon has extended the reach of this technology even further. This paper reviews the materials modification research that has been enabled by the helium ion microscope since its commercialization in 2007, ranging from fundamental studies of beam-sample effects, to the prototyping of new devices with features in the sub-10 nm domain.

Recent advances in focused ion beam nanofabrication for nanostructures and devices: fundamentals and applications

The past few decades have witnessed growing research interest in developing powerful nanofabrication technologies for three-dimensional (3D) structures and devices to achieve nano-scale and nano-precision manufacturing. Among the various fabrication techniques, focused ion beam (FIB) nanofabrication has been established as a well-suited and promising technique in nearly all fields of nanotechnology for the fabrication of 3D nanostructures and devices because of increasing demands from industry and research. In this article, a series of FIB nanofabrication factors related to the fabrication of 3D nanostructures and devices, including mechanisms, instruments, processes, and typical applications of FIB nanofabrication, are systematically summarized and analyzed in detail. Additionally, current challenges and future development trends of FIB nanofabrication in this field are also given. This work intends to provide guidance for practitioners, researchers, or engineers who wish to learn more about the FIB nanofabrication technology that is driving the revolution in 3D nanostructures and devices.

Neon and helium focused ion beam etching of resist patterns

Helium ion microscopy has attracted many applications in imaging, nanofabrication and analysis. One important field of study in nanofabrication using ion beam is the milling or etching of materials using a helium or neon focused ion beam (FIB), with and without chemical gas assistance. In particular, the neon FIB has a relatively high sputtering rate with a lower probability of swelling and less re-deposition issues compared to a helium FIB. Here, both neon and helium FIB etchings are investigated for milling and repairing electron-beam lithography (EBL) defined hydrogen silsesquioxane (HSQ) and polymethyl methacrylate (PMMA) resist patterns. Different dosages of neon FIB etching result in distinct etching profiles. Using the appropriate doses, arrays of uniform gap with aspect ratio more than 20 can be achieved on HSQ nanostructures. The neon FIB etching has a resolution of 20 nm on HSQ patterns. With XeF₂ assistance, neon FIB etching can be enhanced for etching depth by a factor of ∼1.2. Whereas, helium FIB can also etch thick HSQ patterns, with much lower etch rates. But with XeF₂ assistance, helium FIB etching depth can be enhanced significantly by a factor of around 5. Furthermore, both helium and neon FIB etching methods have been employed to selectively remove residual particles in deep and narrow trenches without affecting the resist patterns. The chemical analysis of these residual particle composition and resist patterns can be also performed using helium ion microscopy coupled with secondary ion mass spectrometry (SIMS) using neon FIB. Besides, a neon FIB can also effectively etch PMMA patterns which are commonly used in nanofabrication and the unwanted connections can be etched away.

A Sub-Micron Spherical Atomic Force Microscopic Tip for Surface Measurements

We report a novel methodology for fabricating a sub-micron spherical atomic force microscope (AFM) tip controllably-a silicon sub-micron sphere atop microcantilevers, which is desired for precise nanoscale tribology measurements, biological studies, and colloid science. Silicon sub-micron spheres are fabricated through swelling of single-crystal silicon with proper high-energy helium ion dosing, a traditionally undesired phenomenon known in helium ion microscopy. Silicon sub-micron spheres with diameters from 100 nm to 1 μm are demonstrated, and the placement of silicon sub-micron spheres can be as accurate as 10 nm or even below. This AFM tip demonstrates robust measurements during friction tests on graphene/silicon oxide substrates for more than 10 000 cycles. This AFM tip overcomes a critical challenge of reducing the size of spherical AFM tips from the micrometer scale to the sub-micron scale and is promising in cross-scale mechanics studies, nanotribology, colloid science, and biology.

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.

An Experiment-Based Profile Function for the Calculation of Damage Distribution in Bulk Silicon Induced by a Helium Focused Ion Beam Process

The helium focused ion beam (He-FIB) is widely used in the field of nanostructure fabrication due to its high resolution. Complicated forms of processing damage induced by He-FIB can be observed in substrates, and these damages have a severe impact on nanostructure processing. This study experimentally investigated the influence of the beam energy and ion dose of He-FIB on processing damage. Based on the experimental results, a prediction function for the amorphous damage profile of the single-crystalline silicon substrate caused by incident He-FIB was proposed, and a method for calculating the amorphous damage profile by inputting ion dose and beam energy was established. Based on one set of the amorphous damage profiles, the function coefficients were determined using a genetic algorithm. Experiments on single-crystalline silicon scanned by He-FIB under different process parameters were carried out to validate the model. The proposed experiment-based model can accurately predict the amorphous damage profile induced by He-FIB under a wide range of different ion doses and beam energies.

High-Resolution Laser-Induced Graphene. Flexible Electronics beyond the Visible Limit

Laser-induced graphene (LIG) is a multifunctional graphene foam that is commonly direct-written with an infrared laser into a carbon-based precursor material. Here, a visible 405 nm laser is used to directly convert polyimide into LIG. This enabled the formation of LIG with a spatial resolution of ∼12 μm and a thickness of <5 μm. The spatial resolution enabled by the relatively smaller focused spot size of the 405 nm laser represents a >60% reduction in LIG feature sizes reported in prior publications. This process occurs in situ in an SEM chamber, thus allowing direct observation of LIG formation. The reduced size of the LIG features enables the direct-write formation of flexible electronics that are not visible to the unaided eye. A humidity sensor is demonstrated which could detect human breath with a response time of 250 ms. With the growing interest in LIG for flexible electronics and sensors, finer features can greatly expand its utility.

Controlled Manipulation and Multiscale Modeling of Suspended Silicon Nanostructures under Site-Specific Ion Irradiation

In this work, controlled bidirectional deformation of suspended nanostructures by site-specific ion irradiation is presented. Multiscale modeling of the bidirectional deformation of nanostructures by site-specific ion irradiation is presented, incorporating molecular dynamics (MD) simulations together with finite element analysis, to substantiate the bending mechanism. Strain engineering of the free-standing nanostructure is employed for controlled deformation through site-specific kiloelectronvolt ion irradiation experimentally using a focused ion beam. We report the detailed bending mechanism of suspended silicon (Si) nanostructures through ion-induced irradiations. MD simulations are presented to understand the ion-solid interactions, defects formation in the silicon nanowire. The atomic-scale simulations reveal that the ion irradiation-induced bidirectional bending occurs through the development of localized tensile-compressive stresses in the lattice due to defect formation associated with atomic displacements. With an increasing ion dose, the evolution of localized tensile to compressive stress is observed, developing the alternate bending directions calculated through finite element analysis. The findings of multiscale modeling are in excellent agreement with the bidirectional nature of bending observed through the experiments. The developed in situ approach for bidirectional controlled manipulation of nanostructures in this work can be used for nanofabrication of numerous novel three-dimensional configurations and can provide a route toward functional nanostructures and devices.

Pulsed Laser-Assisted Helium Ion Nanomachining of Monolayer Graphene-Direct-Write Kirigami Patterns

A helium gas field ion source has been demonstrated to be capable of realizing higher milling resolution relative to liquid gallium ion sources. One drawback, however, is that the helium ion mass is prohibitively low for reasonable sputtering rates of bulk materials, requiring a dosage that may lead to significant subsurface damage. Manipulation of suspended graphene is, therefore, a logical application for He⁺ milling. We demonstrate that competitive ion beam-induced deposition from residual carbonaceous contamination can be thermally mitigated via a pulsed laser-assisted He⁺ milling. By optimizing pulsed laser power density, frequency, and pulse width, we reduce the carbonaceous byproducts and mill graphene gaps down to sub 10 nm in highly complex kiragami patterns.

Monte Carlo simulation of nanoscale material focused ion beam gas-assisted etching: Ga+ and Ne+ etching of SiO2 in the presence of a XeF2 precursor gas

Elucidating energetic particle-precursor gas-solid interactions is critical to many atomic and nanoscale synthesis approaches. Focused ion beam sputtering and gas-assisted etching are among the more commonly used direct-write nanomachining techniques that have been developed. Here, we demonstrate a method to simulate gas-assisted focused ion beam (FIB) induced etching for editing/machining materials at the nanoscale. The method consists of an ion-solid Monte Carlo simulation, to which we have added additional routines to emulate detailed gas precursor-solid interactions, including the gas flux, adsorption, and desorption. Furthermore, for the reactive etching component, a model is presented by which energetic ions/target atoms, and secondary electrons, transfer energy to adsorbed gas molecules. The simulation is described in detail, and is validated using analytical and experimental data for surface gas adsorption, and etching yields. The method is used to study XeF₂ assisted FIB induced etching of nanoscale vias, using both a 35 keV Ga⁺, and a 10 keV Ne⁺ beam. Remarkable agreement between experimental and simulated nanoscale vias is demonstrated over a range of experimental conditions. Importantly, we demonstrate that the resolution depends strongly on the XeF₂ gas flux, with optimal resolution obtained for either pure sputtering, or saturated gas coverage; saturated gas coverage has the clear advantage of lower overall dose, and thus lower implant damage, and much faster processing.

Defect Localization and Nanofabrication for Conductive Structures with Voltage Contrast in Helium Ion Microscopy

As the dimensions of feature sizes in electronic devices decrease to nanoscale, an easy method for failure analysis and evaluation of processing steps is required. Gallium-focused ion beam (Ga-FIB) or scanning electron microscope is an efficient approach to detect voltage contrast for addressing failure analysis in semiconductor devices and processing. However, Ga-FIB may cause damage or implantation to the surface of the analyzed area, and its resolution is low. Helium ion microscopy (HIM) uses a light ion beam (helium or neon) for imaging and fabrication at nanoscale. With passive voltage contrast (PVC) in HIM images, the defect localization for failure of conductive structures can be rapidly and easily detected with a sufficient voltage contrast. Furthermore, a defect gap as narrow as sub-10 nm can be investigated with HIM imaging. PVC with HIM is an efficient method for defect localization at nanoscale with a minimal damage to the analyzed area. For circuit edit and failure analysis, it may be necessary to intentionally cut the conductive connection. In this circumstance, final results can be easily verified using PVC imaging with HIM. With XeF₂ gas assistance, both helium and neon ion beams can be used to perform nanofabrication for metal disconnection. XeF₂ gas plays an important role in preventing deposition of conductive materials on etching region and enhancing material removal rates to achieve electrically isolated structures. The etching rate with a neon ion beam is much faster than that of a helium ion beam. PVC in HIM images with controllable operation and dimensions using a helium ion beam with XeF₂ gas assistance could also be used to localize a hidden defect for a single-location-defect situation. With neon ion beam irradiation on a defective location, PVC can be used to find the defect locations in the case of a series of defects.

Simulating advanced focused ion beam nanomachining: a quantitative comparison of simulation and experimental results

A simulation study of focused ion beam (FIB) sputtering in SiO₂ is presented. The basis of this study is an enhanced version of the EnvizION Monte Carlo simulation program for FIB processing, which previously was restricted to targets composed of a single atom. A Monte Carlo method is presented for the simulation of FIB sputtering in SiO₂ in three-dimensions, with ion implantation, to elucidate the complex dynamics of nanoscale milling of compound targets. This method is applied to the simulation of sputtering experiments using both Ne⁺ and Ga⁺ ion beams. We compare simulations using experimentally derived 'measured' beam profiles for each ion species, and 'effective' beam profiles which are chosen to reproduce experimental results. Simulations using the 'measured' beam profiles produce vias which are narrower than experiments, while the 'effective' beam profiles for both Ne⁺ and Ga⁺ are significantly wider than the 'measured' profiles. The difference between the 'measured' and 'effective' beam profiles is attributed to widening of the milling effects of the beam beyond its static dimensions, due to platform level artifacts such as vibrations and, possibly, charging. Simulations using the 'effective' beam profiles are found to accurately reproduce the depths and overall shape of experimental FIB sputtered vias in test cases, which vary in ion species, beam energy, total dose, and raster parameters. This comparison is the most extensive validation of the EnvizION simulation against experiments to date. However, the location of implanted ions in simulations is shallower than experiments, which is attributed to the fact that implanted species are required to find nearest neighbor vacancies and not allowed to occupy interstitial positions.

Exploring Photothermal Pathways via in Situ Laser Heating in the Transmission Electron Microscope: Recrystallization, Grain Growth, Phase Separation, and Dewetting in Ag0.5Ni0.5 Thin Films

A new optical delivery system has been developed for the (scanning) transmission electron microscope. Here we describe the in situ and "rapid ex situ" photothermal heating modality of the system, which delivers &gt;200 mW of optical power from a fiber-coupled laser diode to a 3.7 μm radius spot on the sample. Selected thermal pathways can be accessed via judicious choices of the laser power, pulse width, number of pulses, and radial position. The long optical working distance mitigates any charging artifacts and tremendous thermal stability is observed in both pulsed and continuous wave conditions, notably, no drift correction is applied in any experiment. To demonstrate the optical delivery system's capability, we explore the recrystallization, grain growth, phase separation, and solid state dewetting of a Ag0.5Ni0.5 film. Finally, we demonstrate that the structural and chemical aspects of the resulting dewetted films was assessed.

Superplastic nanoscale pore shaping by ion irradiation

Exposed to ionizing radiation, nanomaterials often undergo unusual transformations compared to their bulk form. However, atomic-level mechanisms of such transformations are largely unknown. This work visualizes and quantifies nanopore shrinkage in nanoporous alumina subjected to low-energy ion beams in a helium ion microscope. Mass transport in porous alumina is thus simultaneously induced and imaged with nanoscale precision, thereby relating nanoscale interactions to mesoscopic deformations. The interplay between chemical bonds, disorders, and ionization-induced transformations is analyzed. It is found that irradiation-induced diffusion is responsible for mass transport and that the ionization affects mobility of diffusive entities. The extraordinary room temperature superplasticity of the normally brittle alumina is discovered. These findings enable the effective manipulation of chemical bonds and structural order by nanoscale ion-matter interactions to produce mesoscopic structures with nanometer precision, such as ultra-high density arrays of sub-10-nm pores with or without the accompanying controlled plastic deformations.

When nanomaterials are exposed to ionizing radiation, they often sustain mesoscopic changes not seen in their bulk form. Here, the authors use a helium ion microscope to induce and examine transformations in nanoporous alumina, drawing connections between atomic structure and nano- and microscale behavior in materials under irradiation.

Switchable reflection/transmission utilizing polarization on a plasmonic structure consisting of self-assembly polystyrene spheres with silver patches

We report a plasmonic structure for switchable reflection and transmission by polarization. The structure is composed of a hexagonal-packed polystyrene sphere array with silver patches on them. Simulations and experiments demonstrated that the conversions between reflected beams and transmitted ones can be performed when the polarization directions of incident beams vary from 0° to 90°. A switchable reflection and transmission at a given wavelength can be obtained, as long as sizes of PS spheres and azimuthal angles are properly chosen. Such a patchy plasmonic structure serving as a switch between reflection and transmission have potential applications in photoelectric control devices.