Helium Ion Microscopy

The key technologies that comprise the helium ion microscope are described in detail. Specific attention is given the cryogenic cooling system, the vacuum system, the gas delivery system, the ion-optical column, the detector and imaging system, and vibrational considerations.

This chapter discusses fabrication and experimental evaluation of W(111) single atom tips (SATs) for gas field ion source applications. Firstly, a brief history of field ion microscopy (FIM) will be given since it will be heavily relied on throughout the text. We will discuss ion current generation in FIM and carry that knowledge over to fabricated SATs. Secondly, gas assisted etching and evaporation process will be discussed in detail. It will be shown that nanotip shape, and therefore SAT characteristics, can be controlled and modified to achieve desirable ion beam properties. Lastly, we will evaluate ion beam width as a function of tip voltage and temperature as examples of experimental efforts to better understand gas field ion source performance.

The family of two–dimensional (2D) materials is an attractive subject for modern microscopy techniques such as helium and neon ion microscopy. In this chapter, we provide a theoretical treatment on the effects of light ion irradiation on the structure of 2D materials, foremost graphene, using methods from the binary collision model to molecular dynamics. While reviewing the current literature on the topic, we point out that helium and neon irradiation can be used to create specifically small point defects (single and double vacancies) or to drill features into graphene. We also point out the current lack of studies involving non-graphene 2D materials.

Focused ion beam technologies have revolutionized the modern material research, development and production. It has offered new possibilities for materials modification and fabrication with a higher spatial resolution by using helium and neon ions. In recent years, various experimental and numerical simulation approaches have been developed and implemented to broaden the applications of focused ion beam technology. The Monte Carlo (MC) simulation approach is one of the useful techniques to study the ion-solid interactions which provides crucial quantitative information which cannot be achieved, in some cases, from the experiments. The MC approaches have a number of advantages over analytical calculations. It allows a more rigorous treatment of scattering events, energy distribution of incident ions, recoil target atoms or molecules and secondary electrons as well as their angular distributions. This chapter presents a brief introduction of a Monte Carlo simulator called EnvizION and some simulation results related to focused ion beam induced physical sputtering, Extreme Ultraviolet (EUV) mask repairs, and sputtering-limiting as well as resolution-limiting effects.

The theories, modeling and experiments of the processes of secondary electron (SE) generation and SE usage in helium ion microscopy (HIM) are reviewed and discussed. Conventional and recently introduced SE imaging modes in HIM utilizing SE energy filtering and ion-to-SE conversion, such as scanning transmission ion microscopy and reflection ion microscopy, are described.

The helium ion microscope (HIM), as the name implies, is primarily an imaging tool. This chapter serves as an introduction to imaging with the HIM and explores the various ways this is implemented by first describing the numerous imaging signals and contrast mechanisms available and then giving an overview of some practical HIM imaging techniques. Several examples from the literature are used to illustrate the important imaging modes including high resolution secondary electron imaging, backscattered ion imaging, ionoluminescence imaging and imaging with transmitted or reflected ions. Key concepts such as ion channeling, charge neutralization and utilizing the large depth of field are introduced, setting the scene for the subsequent chapters in this section that focus on particular aspects of HIM imaging.

Due to its charge compensation capabilities the imaging of insulating sample is a natural application of Helium Ion Microscopy. The imaging of biological samples often requires complicated sample preparation methods who’s influence on the sample structure is not always fully understood. In this chapter we will present a number of recent studies from the aforementioned field that make use and demonstrate the benefits of Helium Ion Microscopy for these research topics. These examples also demonstrate the large depth of focus that distinguishes the method.

In this chapter, selected applications of helium ion microscopy are presented which demonstrate the strength of this method in characterizing systems relevant for the field of combustion science: nascent soot and transition metal oxide catalysts. Soot consists of carbon-based nanoparticles which were generated in flames. The knowledge about soot growth at very early stages is of high interest in combustion science, where HIM yields a new lower size limit for imaging such particles with high contrast. In fact, the contrast on samples composed of light elements, in combination with the high lateral resolution, are key advantages of HIM which are relevant for studying soot. HIM allowed time efficient probing of a high number of particles with sizes and shapes down to 2 nm. In the first part of this chapter, this is demonstrated for soot from different flames. Their sizes and shapes were simultaneously analyzed for typically 1000 particles per sample. Due to the high resolution and high depth of focus, HIM is also well suited to investigate the surface morphology of highly corrugated catalysts, which is shown here for transition metal oxide films grown by pulse spray evaporation CVD. In the second part of this chapter, examples of such catalyst surfaces are presented. In particular, the morphologie of oxides of iron, iron-cobalt and iron-copper films are characterized by HIM with high resolution.

While the default imaging mode in HIM uses secondary electrons, backscattered helium or neon contains valuable information about the sample composition and structure. In this chapter, we will discuss how backscattered helium can be used to obtain information about buried structures and provide qualitative elemental contrast. The discussion is extended to the use of channeling to increase image quality and obtain crystallographic information. As an example, we demonstrate that the period of a dislocation network in a film only two monolayers thick can be obtained with atomic precision.

Carbon Nanomembranes (CNMs) are extremely thin (0.5–3.0 nm), synthetic two-dimensional (2D) layers or sheets with tailored physical, chemical or biological function. With two opposing surfaces they interface and link different environments by their distinct physical and chemical properties, which depend on their thickness, molecular composition, structure and the environment on either side. Due to their nanometer thickness and 2D architecture, they can be regarded as “surfaces without bulk” separating regions with different gaseous, liquid or solid components and controlling any materials exchange between them. Helium Ion Microscopy is very well suited to investigate CNMs. Its main advantage is its high surface sensitivity that generates high contrast images, even in samples where there is very little material available. It is shown that HIM imaging is an effective tool for the characterization of “free-standing” as well as “supported” CNMs. Effects of sample charging, the imaging of multilayer-CNMs and the identification of image artefacts are discussed. It will be shown that at even low magnification, single sheets of CNM can be clearly detected and HIM images show a lot of detail. Folds, wrinkles and pores in the membrane are clearly seen and can be used to characterize the quality of CNMs. In addition, the high depth of focus eases the HIM operation. CNMs can also be milled with the HIM, and nanopores of very small diameter (down to 1.3 nm) have been fabricated.

The ability to modify materials at the atomic scale is crucial for the fabrication of functional nanoscale building blocks for novel nanodevices. Consistency and reproducibility of modifications represent the greatest technical challenges, which demand finer structuring capability than the currently available methods. In this chapter, we will present our recent results of sub-nanometer characterization and modification of two-dimensional (2D) materials enabled by an ultrafine helium ion beam. Characterization will be presented not only in terms of lateral dimension measurement, but also with quantitative thickness and work function extraction. This information is obtained from secondary electron imaging. The effect of contamination and charging on our ability to extract such quantitative information will also be assessed. We will demonstrate that structural defects and stoichiometry modification can be controllably introduced in a few-layer molybdenum disulfide (MoS₂) sample at a few-nanometer scale. Consequently, localized tuning of the physical properties of MoS₂ can be realized. Fabrication of MoS₂ nanostructures with 7-nm dimensions and pristine crystal structure has also been achieved, and the effects of beam dose and profile on the modification will be clarified. This nanoscale modification technique is a generalized approach which can be applied to various 2D materials to produce a new range of 2D metamaterials.

The idea of using backscattered helium particles to access chemical information on the surface in a helium ion microscope came up right from the early days of this relatively young imaging technique. From the basic principles of backscattering spectrometry, ion solid interaction and particle detection it became clear rapidly that this attempt will suffer many difficulties in terms of technical realization and physical limitations. This chapter is about describing those difficulties and working out different scenarios of how to apply backscattering spectrometry to the HIM anyways. It will be shown that an actual technical realization exist enabling laterally resolved chemical analysis in a HIM with a resolution down to \(55\,\)nm.

Secondary Ion Mass Spectrometry (SIMS) is an extremely powerful technique for analysing surfaces, owing in particular to its excellent sensitivity, high dynamic range, very high mass resolution, and ability to differentiate between isotopes. The combination of He/Ne microscopy and SIMS makes it possible not only to obtain SIMS information limited only by the size of the probe–sample interaction (~10 nm), but also to directly correlate such SIMS images with high-resolution (0.5 nm) secondary electron images of the same zone taken at the same time. This chapter will discuss the feasibility of combining SIMS with Helium Ion Microscopy from a fundamental and instrumental point of view.

After a sample has been excited by ion irradiation it has several ways to release the excess energy. One of them is photon emission. This process is called ionoluminescence (IL). The IL signal contains information on the electronic structure of the sample. Furthermore, it can help to reveal the processes occurring in the sample under the influence of an ion beam. Analysis of IL is significantly complicated by the fact that ion beam not only induces light emission, but also modifies the luminescence properties of the material. Several types of materials were investigated in HIM in terms of ionoluminescence: semiconductors, minerals, organic compounds. Analysis of the IL signal and its behavior allowed not only to identify the origin of the signal, but also to study the formation of ion-induced defects, their migration and interaction with each other. The effect of crystal coloration in case of the alkali halides led to a possibility for the creation of nano–scaled luminescent patterns using the He\(^+\) ion beam. Such ionoluminescent patterns allowed the visualization and direct experimental measurements of the ion beam interaction volume.

The performance of He-FIB milling and direct-write He-FIB induced deposition is compared to the performance achieved by other noble gas ions as well as the more conventionally used beams of electrons and Ga-ions. Experimental results, simulations, and in-depth discussions of mechanisms highlight the peculiarities of each ion species with respect to nanostructuring issues like lateral sputter resolution, sputter rate, damage, amorphization, implantation, high-aspect ratio nanostructuring, deposition rate, chemical composition of deposits, and nanostructure shape fidelity. He-FIB is peculiar with respect to its small primary interaction volume and high secondary electron yield leading to excellent small milling and deposition features. The other noble gas ions perform better than He with respect to unwanted ion implantation (leading to swelling), higher sputter yields, or deposition of purer material from organometallic precursors.

Helium ion beam lithography (HIL) has been demonstrated as a promising alternative to electron beam lithography (EBL) for R&D purposes, offering high-resolution lithography at high pattern densities. This chapter reviews focused He ion beam lithography, providing a detailed discussion on the ion beam-resist interaction mechanisms and latest experimental results in this field. In addition, impact of ion shot noise is examined, a comparison to He-ion beam milling is made, and future directions are mentioned.

The ability of gas field ion sources (GFIS) to produce controllable inert gas ion beams with atomic level precision opens up new applications in nanoscale direct-write material modification. Two areas where this has recently been demonstrated is focused helium ion beam production of high-transition temperature (high-T _C) superconductor electronics and magnetic spin transport devices. The enabling advance in the case of superconducting electronics is the ability to use the GFIS to make features on the small length-scale of quantum mechanical tunnel barriers. Because the tunneling probability depends exponentially on distance, tunnel barriers must be less than a few nanometers wide, which is beyond the limits of other nanofabrication techniques such as electron beam lithography. In magnetism, the GFIS has recently been used to generate chemical disordering and modify magnetic properties at the nanoscale. The strongest effect is observed in materials where ion-induced chemical disordering leads to increased saturation magnetization, enabling positive magnetic patterning. In this chapter, we review the latest results and progress in GFIS ion beam modification of (high-T _C) superconductors and magnetic materials.

Solid-state nanopores are an emerging technology for the detection and analysis of biomolecules at the single-molecule level. Consisting of one or more nanometer-scale apertures in a thin, solid-state membrane, a number of methods have been utilized to make these devices. However, conventional approaches are either non-trivial to scale up or lack sufficient precision for many applications. In this chapter, we describe the use of the helium ion microscope to produce nanopores. We demonstrate control over diverse aspects of the device and discuss a range of applications that have been enabled by their implementation.

Electron beam and ion beam based techniques such as SEM, TEM, and FIB are used extensively by the semiconductor industry to provide analytical, metrology, and debug capabilities for process development, manufacturing yield monitoring, and new product ramp. The unique imaging and nanomachining attributes of the helium and neon Gas Field Ionization Source (GFIS) technology may extend beyond what electron and gallium beams can achieve alone. In this chapter, emerging semiconductor applications for helium and neon GFIS are reviewed and the key imaging and nanomachining limitations and attributes of both are discussed.