Ion Milling Research Articles

Highly spin-polarized field emission from ⟨100⟩-oriented Co2MnGa tips

A highly spin-polarized electron beam is extracted via field emission from a ⟨100⟩-oriented Heusler alloy Co2MnGa emitter, which is fabricated through ion milling of a crystallized thin film and well characterized through H2-promoted field evaporation and field ion microscopy (FIM). Crystalline facets of low-index planes such as the (100) and {011} planes can be observed as ring structures in the H2-Ne FIM images, despite the ternary nature of the alloy. The well-defined (100) surface at the emitter apex is the emission site, having higher emission current than other locations. The maximum spin polarization of the field-emitted electrons from the Co2MnGa(100) surface is 61%–76% and can be observed under a 1-nA probe current at 44 K. Although decreased spin polarization is observed with elapsed time, cleaning performed by the field evaporation process restores the spin polarization to the initial value. The spin polarization of more than 60% obtained even at room temperature surpasses those previously reported for transition metals such as Fe, Co, and Cr on W tips.

Unravelling atomically resolved structure of a high-k dielectric oxide-semiconductor interface: Exit wave reconstruction and ab-initio calculation insights

Heterostructure interfaces play a major role in defining the performance of thin-film devices. High-k dielectric oxide-semiconductor heterostructures are being extensively investigated as promising candidates for future integrated circuits, thus it becomes important to precisely probe the interfaces at the atomic scale for technological advancements. In this work, a high-k dielectric oxide (Gd2O3)-semiconductor (Ge) interface was characterized at the atomic scale using complex-valued exit wave reconstructed from a set of focal series high-resolution transmission electron microscopy (HRTEM) images acquired without objective lens spherical aberration correction. The complexity of this characterization lies in removing image artefacts produced by amorphous layer deposited on the imaged region during ion milling which was successfully solved using an algorithm to remove amorphous background developed recently. The final result reveals that the interface of the present study is atomically sharp and flat. The thickness of the imaged region along viewing direction was estimated from channelling map. Comparing reconstructed amplitude of experimental data with that of simulated one generated using Density Functional Theory (DFT) optimized interface structure, it was found that the Gd2O3 layers were terminated at the Gd atoms in the interface.

Shape-imposed anisotropy in antiferromagnetic complex oxide nanostructures

In this study, we report on a shape-imposed magnetic anisotropy in micro- and nanostructures defined in antiferromagnetic (AF) LaFeO3 (LFO) thin films. Two distinct types of structures are investigated: embedded magnets created via ion implantation and free-standing magnets created via ion milling. Using a combination of x-ray photoemission electron microscopy and x-ray absorption spectroscopy, we examine the impact of the structure type, AF layer thickness, and crystal geometry on the Néel vector orientation in these structures. We demonstrate a distinct shape-imposed anisotropy in embedded and free-standing structures alike and show that both parallel and perpendicular alignments of the AF spin axis with respect to structure edges can be achieved by variation of the AF layer thickness and the orientation of the structure edges with respect to the LFO crystalline axes. This work demonstrates how the fabrication procedure affects the magnetic order in thin film AF nanostructures and shows how nanoscale patterning can be used to control the orientation of the Néel vector in epitaxial oxide thin films.

X-Ray Photoelectron Study of the Binder/Electrode Interface on Silicon Anodes for Lithium Ion Batteries

Polymeric binders are a critical component of all composite electrodes used in today's lithium ion batteries (LIB). Binders keep active material and conductive additive mixture together, while also adhering this mixture to the current collector. At the particle level the binder forms bridges between particles of active material. Failure of these bridges contributes to electrical isolation of active material and thus poor cyclic life. the mechanical strength of these binder bridges is of key interest to understand the failure mechanisms of LIBs. Failure of binder bridges is highlighted in LIBs that incorporate Li-alloy forming anode materials such as Si which are known to degrade due to expansion and contraction during cycling. Meanwhile, the formation of solid electrolyte interphase (SEI) can also exacerbate capacity loss in novel anode materials, however the contribution of the binder to SEI formation and stability of alloying anodes has not been emphasized. Favorable interface properties between binder and active material resulting in good adhesion is desirable for anodes which encounter larger stresses during cycling. The objective of this study is to understand the effect different polymer binders have on the nature, formation, and location of the SEI layer of silicon based anodes. In this study crystalline Si wafers coated with thin films of CMC and PVDF binders were utilized. No conductive additives were used. Half cells containing either bare crystal Si or a wafer with a thin film of CMC or PVDF were cycled in a standard lithium-ion electrolyte (1 molar LiPF6 in 1:1:1 vol. ratio of EC:DC:DMC) to form a stable SEI. After cycling, the cells were opened in an inert atmosphere and x-ray photoelectron spectroscopy (XPS) was carried out to analyze and compare the surface chemistries. Depth profiling was then conducted utilizing C-60 fullerene ions opposed to the conventional Ar ion milling probe due to the known issue of chemical degradation caused by the Ar ion milling. In addition, the morphology of the substrates and binder films were characterized by SEM. Preliminary results indicate that binder layers on the order of hundreds of nm thick do not impede the lithiation and delithiation of Si (Figure 1A) but do play a role in the SEI composition (Figure B-D). These results can help inform the optimization of current and advanced composite electrodes for commercial LIB. Figure 1

Sample preparation by focused ion beam without argon ion milling for quantitative electron holography of p-n junctions

A method is suggested to prepare lamella for quantitative electron holography by focused ion beam without the need for postprocessing. It relies on thinning of a lamella from the back side of the Si substrate and removal of the protective layers with Ga ions with progressively lower energies down to 2 kV. It is shown that variations of the dopant potential across a one-dimensional p+/n junction in Si, which are derived by electron holography and from the results of secondary ion mass spectrometry, agree to much better values than 50 mV. The effect of the protective layer deposited during TEM sample preparation on the results of electron holography was evaluated. The Si oxide protective layer, which was deposited onto the Si surface, showed a limited charging effect while oxidizing the top of the Si surface. The carbon protective layer showed no surface erosion, yet it has revealed strong charging effects. The deposition process of the protective layer needs to be optimized for a given application in future work.

Identification of collagen fibrils in cross sections of bone by electron energy loss spectroscopy (EELS)

Transmission electron microscopic (TEM) images of ion-milled bovid cortical bone cut approximately normal to the axes of fibrils show that mineral occurs in the form of plates surrounding and laying between circular or elliptical features about 50 nm in diameter. The classification of these features as either pores or collagen fibrils is highly debated. Electron energy loss spectroscopy (EELS) mapping of these features in ion milled sections shows that they are lacking significant amounts of mineral or collagen, although their appearance suggests that they are cross sections of collagen fibrils. However, analogous sections prepared using an ultramicrotome show that, while these circular features show reduced concentrations of calcium and phosphorus, some of them contain quantities of carbon and nitrogen in bonding states comparable to the composition of collagen. This work demonstrates that the observed circular features are sections of collagen fibrils, but that bombardment by argon ions during broad beam ion milling destroys the collagen and associated gap-zone mineral.

Transmission Electron Microscopy Sample Preparation of INCONEL 738 Nickel-Base Superalloy

Size, shape, volume fraction and distribution of embedded g/ phase in g phase has direct effect on strength of INCONEL alloy. Microstructure parameters of INCONEL phases are quantified from microstructure images using transmission electron microscopy (TEM). Different TEM sample preparation techniques were used to study INCONEL 738 alloy microstructure for transmission electron microscopy (TEM). The INCONEL 738 was first cut into a 1 × 1 cm slice with 600 µm thickness with diamond wire cut. INCONEL sample was mounted by wax (M135), after initial grinding and polishing. The molded INCONEL sample was further polished by different grit size SiC paper to reduce the thickness below 80 micron. At this stage, 3 mm discs were cut from the thin slice of INCONEL alloy by mechanical punch machine. The 3 mm discs of INCONEL alloy were used for TEM sample preparation. Three methods of electro-jet polishing, ion milling and micro control dimpling were employed to prepare transparent TEM sample to observe the surface microstructure details of INCONEL 738 alloy. Electro-jet polishing TEM sample preparation technique could reveal microstructure details of INCONEL alloy g and g/ phases using 42% H3PO4, 34% H2SO4, 24% H2O electrolyte at about – 400C bath temperature and applied voltage of 30 V.

Spin-dependent scattering induced negative magnetoresistance in topological insulator Bi2Te3 nanowires

Studies of negative magnetoresistance in novel materials have recently been in the forefront of spintronic research. Here, we report an experimental observation of the temperature dependent negative magnetoresistance in Bi2Te3 topological insulator (TI) nanowires at ultralow temperatures (20 mK). We find a crossover from negative to positive magnetoresistance while increasing temperature under longitudinal magnetic field. We observe a large negative magnetoresistance which reaches −22% at 8 T. The interplay between negative and positive magnetoresistance can be understood in terms of the competition between dephasing and spin-orbit scattering time scales. Based on the first-principles calculations within a density functional theory framework, we demonstrate that disorder (substitutional) by Ga+ ion milling process, which is used to fabricate nanowires, induces local magnetic moments in Bi2Te3 crystal that can lead to spin-dependent scattering of surface and bulk electrons. These experimental findings show a significant advance in the nanoscale spintronics applications based on longitudinal magnetoresistance in TIs. Our experimental results of large negative longitudinal magnetoresistance in 3D TIs further indicate that axial anomaly is a universal phenomenon in generic 3D metals.

Direct-write crosslinking in vacuum-deposited small-molecule films using focussed ion and electron beams

Recent advances in helium ion microscopy (HIM) have enabled the use of fine-focused He+ beams to image and shape materials at the nanoscale. In addition to traditional ion milling, the beam can also be used to induce reactions, such as cross-linking, in films of organic molecules. Here, we compare the use of focused ion and electron beams to fabricate spatially-defined cross-linked features in nanometre-thick films of tetracene. Ion and electron beam treatments were performed using the focussed energetic beams in a HIM and a scanning electron microscope, respectively. The patterned samples were analysed by optical microscopy, HIM, atomic force microscopy and nanoindentation. For samples fabricated using both energetic beams, the total deposited particle dose could be used to modify the optical properties, thickness and hardness of the dosed regions. X-ray photoelectron spectroscopy revealed that the dosed regions exhibited a higher sp3 content, consistent with crosslinking; rinsing in solvent showed that the patterned regions were insoluble and could be isolated by removing the unmodified film through dissolution. These molecular nanopatterns demonstrate the promise for ultrahigh resolution chemical lithography, and for fabrication of nanocomponents with tailored physical properties.

Ion Beam Induced Artifacts in Lead-Based Chalcogenides.

Metal chalcogenides have attracted great attention because of their broad applications. It has been well acknowledged that microstructure can alter the intrinsic properties and performance of metal chalcogenides. The structure-property-performance relationships can be investigated at atomic scale with scanning transmission and transmission electron microscopy (STEM and TEM). Nevertheless, careful specimen preparation is paramount for accurate analyses and interpretations. In this work, we compare the effects of a variety of well-established TEM specimen preparation methods on the observed microstructure of an ingot stoichiometric lead telluride (PbTe). Most importantly, from aberration corrected STEM and first principles calculations, we discovered that argon (Ar) ion milling can lead to surface irradiation damage in the form of Pb vacancy clusters and self-interstitial atom (SIA) clusters. The SIA clusters appear as orthogonal nanoscale features when characterized along the crystal orientation of the rock salt structured PbTe. This obfuscates the interpretation of the intrinsic microstructure of metal chalcogenides, especially lead chalcogenides. We demonstrate that with sufficiently low energy (300 eV) Ar ion cleaning or appropriate high-temperature annealing, the surface damage layer can be properly cleaned and the orthogonal nanoscale features are significantly reduced. This reveals the materials' intrinsic structure and can be used as the standard protocol for future TEM specimen preparation of lead-based chalcogenide materials.

High field-emission current density from β-Ga2O3 nanopillars

Field emission from gallium oxide (β-Ga2O3) nanopillars, etched by Ne+ ion milling on β-polymorph (100) single crystals, is reported. A stable field emission current, with a record density over 100 A/cm2 and a turn on field of ∼ 30 V/μm, is achieved. We expect that the high field enhancement factor of about 200 at a cathode-anode distance of 1 μm can be further increased by optimizing the shape of the nanopillar apex. This work demonstrates that the material properties combined with an appropriate nano-patterning can make β-Ga2O3 competitive or better than other well-established field emitters.

Removing Stripes, Scratches, and Curtaining with Nonrecoverable Compressed Sensing.

Highly-directional image artifacts such as ion mill curtaining, mechanical scratches, or image striping from beam instability degrade the interpretability of micrographs. These unwanted, aperiodic features extend the image along a primary direction and occupy a small wedge of information in Fourier space. Deleting this wedge of data replaces stripes, scratches, or curtaining, with more complex streaking and blurring artifacts-known within the tomography community as "missing wedge" artifacts. Here, we overcome this problem by recovering the missing region using total variation minimization, which leverages image sparsity-based reconstruction techniques-colloquially referred to as compressed sensing (CS)-to reliably restore images corrupted by stripe-like features. Our approach removes beam instability, ion mill curtaining, mechanical scratches, or any stripe features and remains robust at low signal-to-noise. The success of this approach is achieved by exploiting CS's inability to recover directional structures that are highly localized and missing in Fourier Space.

Terahertz time domain spectroscopy of GdBCO superconducting thin films

We present terahertz optical properties of GdBa2Cu3O7-x (GdBCO) superconducting thin films. GdBCO films with a thickness of about 105 nm were grown on a LaAlO3 (LAO) single crystal substrate using a conventional pulsed laser deposition (PLD) technique. Using an Ar ion milling system, the thickness of the GdBCO film was reduced to 58 nm, and its surface was also smoothened. Terahertz (THz) transmission spectra through two different GdBCO films are measured over the range between 0.2 and 1.5 THz using THz time domain spectroscopy. Interestingly, the THz transmission of the thinner GdBCO film has been increased to six times larger than that of the thicker one, while the thinner film is still maintaining its superconducting property at below 90 K.

Micro-machining of PMN-PT Crystals with Ultrashort Laser Pulses

Lead–magnesium niobate lead titanate (PMN-PT) has been proven as an excellent material for sensing and actuating applications. The fabrication of advanced ultra-small PMN-PT-based devices relies on the availability of sophisticated procedures for the micro-machining of PMN-PT thin films or bulk substrates. Approaches reported up to date include chemical etching, excimer laser ablation, and ion milling. To ensure an excellent device performance, a key mandatory feature for a micro-machining process is to preserve as far as possible the crystalline quality of the substrates; in other words, the fabrication method must induce a low density of cracks and other kind of defects. In this work, we demonstrate a relatively fast procedure for the fabrication of high-quality PMN-PT micro-machined actuators employing green femtosecond laser pulses. The fabricated devices feature the absence of extended cracks and well-defined edges with relatively low roughness, which is advantageous for the further integration of nanomaterials onto the piezoelectric actuators.

Characterization of a high-brightness, laser-cooled Li+ ion source.

Ion sources based on laser cooling have recently provided new pathways to high-resolution microscopy, ion milling, and ion implantation. Here, we present the design and detailed characterization of a 7Li magneto-optical trap ion source (MOTIS) with a peak brightness of (1.2 ± 0.2) × 105 A m−2 sr−1 eV−1 and a maximum continuous current over 1 nA. These values significantly surpass previous Li MOTIS performance benchmarks. Using simple models, we discuss how the performance of this system relates to fundamental operating limits. This source will support a range of projects using lithium ion beams for surface microscopy and nanostructure characterization, including Li+ implantation for studies of ionic transport in energy storage materials.

In situ micropillar compression of Al/SiC nanolaminates using laboratory-based nanoscale X-ray microscopy: Effect of nanopores on mechanical behavior

In situ studies using X-ray tomography have gained immense popularity in the recent past owing to their non-destructive nature and ability to capture the microstructure of a wide range of materials in 3D. In this work, we have conducted in situ micropillar compression and studied damage evolution in Al/SiC nanolaminates using a laboratory-based nanoscale X-ray microscope. Nanoscale defects present in the microstructure were characterized in 3D. The effect of these nanopores on damage initiation was quantified and is discussed. Additionally, the effect of Ga+ ion milling on micropillar fabrication was characterized by performing a comparative experiment on pillars fabricated using Ga+ and Ne+ ion source based focused ion beams.

Electron beam damage of epoxy resin films studied by scanning transmission X-ray spectromicroscopy

Focused ion beam coupled with scanning electron microscopy (FIB-SEM) is a popular technique for advanced electron microscopy with applications such as, high-precision site-specific lamella sample preparation for transmission electron microscopy (TEM) and slice-and-view FIB 3-dimensional tomography. Damage caused by the electron imaging component of FIB-SEM may be compounded with damage from the ions during the ion milling process. There are known strategies for mitigating damage from ions and electrons (cryo-SEM, dose-control, voltage control), but the electron damage on common embedding resins for EM has not been explored in detail beyond their resistance to shape-change. The relationship between beam parameters and damage mechanisms remains unclear. Since we are relying on the physical, chemical and thermal stability of embedded samples during ion-beam milling, it is important to distinguish electron beam damage from ion beam damage. Scanning transmission X-ray microscopy (STXM) has been used for analyzing the electron beam radiation damage on polymer films by characterizing the chemical bonding changes. In this paper, we focus on the effect of beam voltage and electron dose on electron beam damage to epoxy resin thin films. Irradiated areas on polymer thin films were characterized by near edge X-ray absorption fine structure (NEXAFS) in STXM. We found that, even when using low current and voltage, the electron beam can still cause noticeable chemical changes within the polymer film. The degree of electron beam damage depends not only on the beam energy, but also on the amount of inelastic scattering occurring within the material, as determined by the sample thickness.

Correlative Microscopy-Based Approach for Analyzing Microscopic Impurities in Carbon Black for Lithium-Ion Battery Applications

A novel method based on correlative microscopy for analyzing metal-rich particulate impurities randomly distributed in large volumes of carbon black powder is described here. Approximately 5 g of carbon black of tap density of 0.075 g cm−3 was used. The powder was encapsulated in epoxy and a combination of 3D X-ray computed tomography (X-CT), laser/ion beam milling and energy dispersive X-ray spectroscopy (EDS) was used to locate and analyze particles of interest. The locations of impurities were identified using X-CT and a focused ion beam (FIB) system with ion and laser beam milling capabilities was then used to analyze regions of interest in the encapsulated sample. The use of a nanosecond pulsed laser for milling allowed for enhanced material removal rates of up to 3 × 105 μm3 s−1. Finally, scanning electron microscopy (SEM) and EDS were used to image and analyze the composition of the exposed particles. This correlative workflow is shown to be capable of providing detailed spatial, morphological and compositional information about large metal-rich impurities in a carbon black product or a device with carbon black as additives, which may be used for identifying the source of impurities or predicting failure modes in the device.

Co/Pd-Based synthetic antiferromagnetic thin films on Au/resist underlayers: towards biomedical applications.

Thin film stacks consisting of multiple repeats M of synthetic antiferromagnetic (SAF) [Co/Pd]N/Ru/[Co/Pd]N units with perpendicular magnetic anisotropy were explored as potential starting materials to fabricate free-standing micro/nanodisks, which represent a promising candidate system for theranostic applications. The films were directly grown on a sacrificial resist layer spin-coated on SiOx/Si(100) substrates, required for the preparation of free-standing disks after its dissolution. Furthermore, the film stack was sandwiched between two Au layers to allow further bio-functionalization. For M ≤ 5, the samples fulfill all the key criteria mandatory for biomedical applications, i.e., zero remanence, zero field susceptibility at small fields and sharp switching to saturation, together with the ability to vary the total magnetic moment at saturation by changing the number of repetitions of the multi-stack. Moreover, the samples show strong perpendicular magnetic anisotropy, which is required for applications relying on the transduction of a mechanical force through the micro/nano-disks under a magnetic field, such as the mechanical cell disruption, which is nowadays considered a promising alternative to the more investigated magnetic hyperthermia approach for cancer treatment. In a further step, SAF microdisks were prepared from the continuous multi-stacks by combining electron beam lithography and Ar ion milling, revealing similar magnetic properties as compared to the continuous films.

Evidence for multiple mechanisms underlying surface electric-field noise in ion traps

Electric-field noise from ion-trap electrode surfaces can limit the fidelity of multiqubit entangling operations in trapped-ion quantum information processors and can give rise to systematic errors in trapped-ion optical clocks. The underlying mechanism for this noise is unknown, but it has been shown that the noise amplitude can be reduced by energetic ion bombardment, or "ion milling," of the trap electrode surfaces. Using a single trapped $^{88}$Sr$^{+}$ ion as a sensor, we investigate the temperature dependence of this noise both before and after ex situ ion milling of the trap electrodes. Making measurements over a trap electrode temperature range of 4 K to 295 K in both sputtered niobium and electroplated gold traps, we see a marked change in the temperature scaling of the electric-field noise after ion milling: power-law behavior in untreated surfaces is transformed to Arrhenius behavior after treatment. The temperature scaling becomes material-dependent after treatment as well, strongly suggesting that different noise mechanisms are at work before and after ion milling. To constrain potential noise mechanisms, we measure the frequency dependence of the electric-field noise, as well as its dependence on ion-electrode distance, for niobium traps at room temperature both before and after ion milling. These scalings are unchanged by ion milling.