Gas-assisted Focused Ion Beam Research Articles

Review of Recent Advances in Gas-Assisted Focused Ion Beam Time-of-Flight Secondary Ion Mass Spectrometry (FIB-TOF-SIMS).

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is a powerful chemical characterization technique allowing for the distribution of all material components (including light and heavy elements and molecules) to be analyzed in 3D with nanoscale resolution. Furthermore, the sample's surface can be probed over a wide analytical area range (usually between 1 µm2 and 104 µm2) providing insights into local variations in sample composition, as well as giving a general overview of the sample's structure. Finally, as long as the sample's surface is flat and conductive, no additional sample preparation is needed prior to TOF-SIMS measurements. Despite many advantages, TOF-SIMS analysis can be challenging, especially in the case of weakly ionizing elements. Furthermore, mass interference, different component polarity of complex samples, and matrix effect are the main drawbacks of this technique. This implies a strong need for developing new methods, which could help improve TOF-SIMS signal quality and facilitate data interpretation. In this review, we primarily focus on gas-assisted TOF-SIMS, which has proven to have potential for overcoming most of the aforementioned difficulties. In particular, the recently proposed use of XeF2 during sample bombardment with a Ga+ primary ion beam exhibits outstanding properties, which can lead to significant positive secondary ion yield enhancement, separation of mass interference, and inversion of secondary ion charge polarity from negative to positive. The implementation of the presented experimental protocols can be easily achieved by upgrading commonly used focused ion beam/scanning electron microscopes (FIB/SEM) with a high vacuum (HV)-compatible TOF-SIMS detector and a commercial gas injection system (GIS), making it an attractive solution for both academic centers and the industrial sectors.

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.

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

Focused helium ion (He+) milling has been demonstrated as a high-resolution nanopatterning technique; however, it can be limited by its low sputter yield as well as the introduction of undesired subsurface damage. Here, we introduce pulsed laser- and gas-assisted processes to enhance the material removal rate and patterning fidelity. A pulsed laser-assisted He+ milling process is shown to enable high-resolution milling of titanium while reducing subsurface damage in situ. Gas-assisted focused ion beam induced etching (FIBIE) of Ti is also demonstrated in which the XeF2 precursor provides a chemical assist for enhanced material removal rate. Finally, a pulsed laser-assisted and gas-assisted FIBIE process is shown to increase the etch yield by ∼9× relative to the pure He+ sputtering process. These He+ induced nanopatterning techniques improve material removal rate, in comparison to standard He+ sputtering, while simultaneously decreasing subsurface damage, thus extending the applicability of the He+ probe as a nanopattering tool.

Enhanced Etching, Surface Damage Recovery, and Submicron Patterning of Hybrid Perovskites using a Chemically Gas-Assisted Focused-Ion Beam for Subwavelength Grating Photonic Applications.

The high optical gain and absorption of organic-inorganic hybrid perovskites have attracted attention for photonic device applications. However, owing to the sensitivity of organic moieties to solvents and temperature, device processing is challenging, particularly for patterning. Here, we report the direct patterning of perovskites using chemically gas-assisted focused-ion beam (GAFIB) etching with XeF2 and I2 precursors. We demonstrate etching enhancement in addition to controllability and marginal surface damage compared to focused-ion beam (FIB) etching without precursors. Utilizing the GAFIB etching, we fabricated a uniform and periodic submicron perovskite subwavelength grating (SWG) absorber with broadband absorption and nanoscale precision. Our results demonstrate the use of FIB as a submicron patterning tool and a means of providing surface treatment (after FIB patterning to minimize optical loss) for perovskite photonic nanostructures. The SWG absorber can be patterned on perovskite solar cells to enhance the device efficiency through increasing light trapping and absorption.

Structural and composition investigations at delayered locations of low k integrated circuit device by gas-assisted focused ion beam

The authors report a new delayering technique – gas-assisted focused ion beam (FIB) method and its effects on the top layer materials of integrated circuit (IC) device. It demonstrates a highly efficient failure analysis with investigations on the precise location. After removing the dielectric layers under the bombardment of an ion beam, the chemical composition of the top layer was altered with the reduced oxygen content. Further energy-dispersive x-ray spectroscopy and Fourier transform infrared analysis revealed that the oxygen reduction lead to appreciable silicon suboxide formation. Our findings with structural and composition alteration of dielectric layer after FIB delayering open up a new insight avenue for the failure analysis in IC devices.

Gas-assisted focused ion beam fabrication of gold nanoelectrode arrays in electron-beam evaporated alumina films for microfluidic electrochemical sensors

This work reports the fabrication details used to prepare gold nanoelectrode arrays as working electrodes in a microfluidic electrochemical sensor system. We emphasize on the design criteria and the methods used to construct the nanoelectrode arrays for the purpose of enhancing mass transport of the redox species to the electrodes. The fabrication techniques include electron-beam evaporation of Ti and Au to create the electrode platform on a glass substrate, followed by electron-beam evaporation of thick alumina films as a passivation layer. I2-assisted focused ion beam (FIB) milling is employed to produce high-aspect-ratio pores in the alumina films. The final nanopores with diameters around 60–120nm are achieved by ion beam sculpting after the initial milling process. The nanopores are filled with Au via electrodeposition to obtain a nanoelectrode structure. The current responses of Ru(NH3)63+/Ru(NH3)62+ redox species measured at three-electrode cells in polydimethylsiloxane (PDMS) microfluidic chips are investigated by cyclic voltammetry (CV). The current density obtained at the nanoelectrode arrays is increased by two orders of magnitude compared to that obtained from the microelectrode system. In addition, due to the effect of enhanced mass transport, the voltammetric response curve acquired from the nanoelectrode array exhibits large deviations from the classical diffusion-limited current transport theory.

In-situ monitoring of gas-assisted focused ion beam and focused electron beam induced processing

The state of the art of several in-situ monitoring techniques for gas assisted focused ion beam and focused electron beam induced processing (FIBIP and FEBIP) is presented. The monitoring techniques discussed comprise (a) the stage current and secondary electron signal for topographic information, (b) mass sensors to determine process yields, deposit density, molecule coverage and adsorption, (c) electrical resistance measurements, (d) reflectometry, (e) annular dark field sensing, and (f) SEM integrated mechanical measurements.

Optical Output Power Enhancement of Gallium Nitride Light-Emitting Diodes with Microlens Array on p-GaN Layer and Wave-Patterned Sidewalls by Gas-Assisted Focused Ion Beam Etching

By using iodine gas-assisted focused ion beam etching, a gallium nitride light-emitting diode with wave-patterned sidewalls and a microlens array on a p-GaN layer was successfully fabricated without an additional lithography process. The gallium nitride light-emitting diode after gas-assisted focused ion beam etching on the sidewalls and p-GaN layer shows 24% enhancement from the sidewalls and 53% from the top surface in optical output power with an operating voltage increase of 0.06 V at 20 mA.

Gas-Assisted Focused Ion Beam Etching of Indium–Tin Oxide Film

Gas-assisted focused ion beam etching is a very useful extension of the focused ion beam technique. In this study, gas-assisted focused ion beam etching is performed on indium–tin oxide (ITO) films using the halide gases xenon difluoride and iodine. The results from this study show that higher etching rates of ITO films may be obtained using xenon difluoride gas at a short beam dwelling time and a negative beam overlap percentage. They also show that gas-assisted focused ion beam etching has several advantages over physical sputtering in focused ion beam milling: increase in etching rate, minimization of redeposition on sidewalls, and minimization of ion implantation in samples.

Gas-assisted focused ion beam etching characteristics of niobium

The gas-assisted focused ion beam etching characteristics of niobium have been investigated systematically in I2 and XeF2 gas atmosphere, respectively. The effects of the etching parameters, such as: ion beam current, dwell time, overlap percentage and gas species, on the niobium etching rate and on the gas-assisted etching enhancement have been studied. It was found that the overlap percentage had little effect on the etching characteristics of niobium. It also showed that the I2 radicals are more reactant with niobium than XeF2 radicals, and to obtain maximum improvement from the gas-assisted etching process one needs to select a smaller ion beam current with a shorter beam dwell time.

Fast prototyping of high-aspect ratio, high-resolution X-ray masks by gas-assisted focused ion beam

The capacity of chemically-assisted focused ion beam (FIB) etching systems to undertake direct and highly anisotropic erosion of thin and thick gold (or other high atomic number {Z}) coatings on X-ray mask membranes/substrates provides new levels of precision, flexibility, simplification and rapidity in the manufacture of mask absorber patterns, allowing for fast prototyping of high-aspect ratio, high-resolution masks for deep X-ray lithography for the LIGA process. In preliminary demonstrations, an automated FIB system operating at 30 keV with a gallium liquid metal source and an iodine gas injection system was used for direct milling into a few micrometer thick gold of microstructures into the sub-hundred nanometer regime. Three-dimensional micromachining in bulk diamond is also reported to illustrate the capability of the technique.

Low-damage specimen preparation technique for transmission electron microscopy using iodine gas-assisted focused ion beam milling

A novel method is presented for the preparation of high-resolution electron microscopy specimens with gas-assisted focused ion beam (FIB) milling. An etching rate of InP is enhanced 11–13 times with iodine gas-assisted FIB milling, compared to conventional FIB milling. Specimens with lower radiation damage can be prepared due to lower ion dose during FIB milling, and show no anomalous contrast which comes from the artifacts during FIB milling. Damaged layer introduced during FIB milling was also investigated.

Characteristics of gas-assisted focused ion beam etching

Gas-assisted etching with a finely focused ion beam has been studied. The presence of a reactive gas, in this case Cl2, results in an enhanced etch rate compared to the rate for sputtering for many materials, including Si, Al, and GaAs. Other advantages over sputtering are the absence of redeposited material and the high etch selectivity possible with some material combinations, which has been exploited in the etching of microstructures. In some applications of this technique, a protective layer of low etch rate material is used over the substrate to improve the quality of the etched structure. The characteristics of the etching process have been studied with variation in the scan speed, gas flux, and current density into the scanned area. In general, a high scan speed, high gas flux, and low current density were found to give the maximum enhancement in the etch rate over sputtering. The application of these results to etching over a wide range of experimental conditions is discussed, to give a basis for estimating the etching results that would occur under other conditions.

Gas-assisted focused ion beam etching for microfabrication and inspection

A focused ion beam system has been applied to the etching of semiconductor materials. The etching can be carried out by sputtering with the ion beam alone or by a combination of the ion beam and a reactive gas, in this case chlorine. The flexibility of the system allows beam currents from 5.10 -7 to 5.10 -12 A to be used, with a typical beam current density of 1–2 A.cm -2 . A number of diverse applications that make use of the large current range available have been identified and characterised in the system. These range from large-area layer removal and TEM specimen preparation with high beam currents to the formation of fine, smooth features using small-diameter, low-current ion probes.