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.

Monte Carlo simulations of nanoscale Ne+ ion beam sputtering: investigating the influence of surface effects, interstitial formation, and the nanostructural evolution.

We present an updated version of our Monte-Carlo based code for the simulation of ion beam sputtering. This code simulates the interaction of energetic ions with a target, and tracks the cumulative damage, enabling it to simulate the dynamic evolution of nanostructures as material is removed. The updated code described in this paper is significantly faster, permitting the inclusion of new features, namely routines to handle interstitial atoms, and to reduce the surface energy as the structure would otherwise develop energetically unfavorable surface porosity. We validate our code against the popular Monte-Carlo code SRIM-TRIM, and study the development of nanostructures from Ne+ ion beam milling in a copper target.

In Situ Mitigation of Subsurface and Peripheral Focused Ion Beam Damage via Simultaneous Pulsed Laser Heating.

Focused helium and neon ion (He(+)/Ne(+)) beam processing has recently been used to push resolution limits of direct-write nanoscale synthesis. The ubiquitous insertion of focused He(+)/Ne(+) beams as the next-generation nanofabrication tool-of-choice is currently limited by deleterious subsurface and peripheral damage induced by the energetic ions in the underlying substrate. The in situ mitigation of subsurface damage induced by He(+)/Ne(+) ion exposures in silicon via a synchronized infrared pulsed laser-assisted process is demonstrated. The pulsed laser assist provides highly localized in situ photothermal energy which reduces the implantation and defect concentration by greater than 90%. The laser-assisted exposure process is also shown to reduce peripheral defects in He(+) patterned graphene, which makes this process an attractive candidate for direct-write patterning of 2D materials. These results offer a necessary solution for the applicability of high-resolution direct-write nanoscale material processing via focused ion beams.

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.

Observation of synchronized atomic motions in the field ion microscope.

For over half a century, the field ion microscope (FIM) has been used to visualize atomic structures at the apex of a sharpened needle by way of the ion beams which are created at the most protruding atoms. In this paper we used a conventional FIM to study the emission characteristics of the neon ion beams produced within the FIM. The neon emission pattern is observed to be relatively short lived and subject to temporal and angular fluctuations. The nature of these fluctuations is complex, often with different parts of the emission pattern changing in a synchronized fashion over timescales spanning from milliseconds to a few tens of seconds. In this paper, we characterize the observed instability of the neon emission. We also offer a simple model of adsorbed atom mobility that explains much of these observations. And finally, we present a method by which the stability can be greatly improved so that the produced neon beam can be used effectively for nanomachining applications.

The prospects of a subnanometer focused neon ion beam.

The success of the helium ion microscope has encouraged extensions of this technology to produce beams of other ion species. A review of the various candidate ion beams and their technical prospects suggest that a neon beam might be the most readily achieved. Such a neon beam would provide a sputtering yield that exceeds helium by an order of magnitude while still offering a theoretical probe size less than 1-nm. This article outlines the motivation for a neon gas field ion source, the expected performance through simulations, and provides an update of our experimental progress.