Helium Ion Microcopy (HIM) based on Gas Field Ion Sources (GFIS) represents a new ultra high resolution microscopy and nano-fabrication technique. It is an enabling technology that not only provides imagery of conducting as well as uncoated insulating nano-structures but also allows to create these features. The latter can be achieved using resists or material removal due to sputtering. The close to free-form sculpting of structures over several length scales has been made possible by the extension of the method to other gases such as Neon. A brief introduction of the underlying physics as well as a broad review of the applicability of the method is presented in this review.

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Helium ion microscopy

Additive manufacturing of three-dimensional objects on the nanoscale is a very relevant topic but still a highly challenging task. Among the pool of nanofabrication techniques, focused electron beam induced deposition (FEBID) has recently developed from a trial-and-error laboratory method to a predictable 3D nanoprinting technology with unique advantages. This perspective article first introduces the basic principles of 3D-FEBID, followed by an overview of historical developments with a particular emphasis on the last three years. Here, we examine different aspects of 3D nanoprinting such as the instrumental setup, fundamental growth mechanisms, simulations, computer aided design software solutions, material properties, and application studies. For each aspect, the individual challenges and limitations are discussed. In addition, we share our outlook about possible solutions and studies currently under investigation. As a perspective, we also address the most urgent milestones of the future and speculate on applications ranging from optics to mechanics, magnetics, and electronics, all of them benefiting from the recently improved 3D FEBID synthesis technique.

3D nanoprinting via focused electron beams

Secondary ion mass spectrometry (SIMS) constitutes an extremely sensitive technique for imaging surfaces in 2D and 3D. Apart from its excellent sensitivity and high lateral resolution (50 nm on state-of-the-art SIMS instruments), advantages of SIMS include high dynamic range and the ability to differentiate between isotopes. This paper first reviews the underlying principles of SIMS as well as the performance and applications of 2D and 3D SIMS elemental imaging. The prospects for further improving the capabilities of SIMS imaging are discussed. The lateral resolution in SIMS imaging when using the microprobe mode is limited by (i) the ion probe size, which is dependent on the brightness of the primary ion source, the quality of the optics of the primary ion column and the electric fields in the near sample region used to extract secondary ions; (ii) the sensitivity of the analysis as a reasonable secondary ion signal, which must be detected from very tiny voxel sizes and thus from a very limited number of sputtered atoms; and (iii) the physical dimensions of the collision cascade determining the origin of the sputtered ions with respect to the impact site of the incident primary ion probe. One interesting prospect is the use of SIMS-based correlative microscopy. In this approach SIMS is combined with various high-resolution microscopy techniques, so that elemental/chemical information at the highest sensitivity can be obtained with SIMS, while excellent spatial resolution is provided by overlaying the SIMS images with high-resolution images obtained by these microscopy techniques. Examples of this approach are given by presenting in situ combinations of SIMS with transmission electron microscopy (TEM), helium ion microscopy (HIM) and scanning probe microscopy (SPM).

http://iopscience.iop.org/article/10.1088/0957-4484/26/43/434001/pdf

High-resolution high-sensitivity elemental imaging by secondary ion mass spectrometry: from traditional 2D and 3D imaging to correlative microscopy.

Ion beam nanoscale fabrication and lithography—A review

Summary

The helium ion microscope has recently emerged as a commercially available instrument. However, its roots go back more than 60 years to the development of the field ion microscope in Berlin, first reported in 1951. Over the intervening years, numerous researchers have pursued the development of a gas field ionization source with the goal of producing a suitable source for an ion microscope. This proved to be an elusive goal until early in this century when a number of discoveries led to a successful source, and shortly thereafter, an instrument fully able to exploit its advantages. Many individuals and many technical advances have come together to make this new class of microscope. The long history of this quest is reviewed along with the recent advances that led to the achievement of this milestone. A brief summary of the current status of the technology and its applications are given. SCANNING 33: 83–89, 2012. © 2011 Wiley Periodicals, Inc.

The history and development of the helium ion microscope.

An analytical model for the growth of nanopillars by helium ion-beam-induced deposition is presented and compared to experimental data. This model describes the competition between pillar growth in vertical and lateral directions. It assumes that vertical growth is induced by incident primary ions and type-1 secondary electrons, whereas lateral growth is induced by scattered ions and type-2 secondary ions. An essential element of the model is the notion that depletion of adsorbed precursor molecules occurs only at the pillars’ apex. Depletion impedes vertical growth at the apex, allowing more time for lateral outgrowth of the pillar’s sidewalls. The model describes qualitatively the trends in measured vertical, lateral, and volumetric growth rates of PtC pillars as functions of the ion-beam current. It can be used to design growth experiments and Monte Carlo simulations.

https://repository.tudelft.nl/islandora/object/uuid%3Af81b4384-c151-4299-a0f0-9dc1f48e863d/datastream/OBJ/download

Model for nanopillar growth by focused helium ion-beam-induced deposition

The determination of the quality of an imaging system is not an easy task for, in general, at least three parameters, strictly interdependent, concur in defining it: resolution, contrast, and signal-to-noise ratio. The definition of resolution itself in scanning microscopy is elusive and the case of scanning ion microscopy is complicated by the damage of the sample under the ion beam, which, especially for small features, can be the limiting factor. This is indeed the case for most focused ion beam systems, which exploit beams of Ga +. The only way to overcome this limit is to exploit sources of low mass ions, such as H+ and He+. In this article the authors analyze the way the sputtering may affect the resolution, defined as smallest detectable feature in an image, of a scanning ion microscope, for heavy and light ions, in the case of spherical features. It appears that the fundamental limit to the resolution in scanning microscopy is not given by the spot size, but by the dynamics of the interaction of the beam with the sample and the consequent modification of the sample's geometry, even for beams of light ions. For example, in the case of Sn nanospheres under a He+ beam, the authors found a minimum theoretical detectable particle size limit of ∼1 nm and an experimental limit of ∼5 nm. © 2009 American Vacuum Society.

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On the influence of the sputtering in determining the resolution of a scanning ion microscope

For the introduction of EUV lithography, development of high performance EUV resists is of key importance. This development involves studies into resist sensitivity, resolving power and pattern uniformity. We have used a sub-nanometer-sized 30 keV helium ion beam to expose chemically amplified (CAR) EUV resists. There are similarities in the response of resists to He+ ions and EUV photons: both excite Secondary Electrons with similar energy distributions. The weak backscattering of the He+ ions results in ultra-low proximity effects. This fact enables the exposure of dense and detailed patterns by focused He+ ion beams without the need for proximity correction. This paper presents contact holes and lines at 40-nm pitch in an EUV CAR resist. We have used resist sensitivity, contrast, resolution (CD) and pattern fidelity (LCDU, LWR and dose-to-print) as metrics for a comparison of SHIBL with EUVL. We show that Scanning Helium Ion Beam Lithography (SHIBL) can be a useful and economically attractive technology to (pre-)screen novel EUV resists prior to their final performance evaluation in an EUV scanner. © 2014 SPIE.

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Evaluation of EUV resist performance below 20nm CD using helium ion lithography

The authors report the direct-write growth of hammerhead atomic force microscope (AFM) probes by He+ beam induced deposition of platinum-carbon. In order to grow a thin nanoneedle on top of a conventional AFM probe, the authors move a focused He+ beam during exposure to a PtC precursor gas. In the final growth stage, a perpendicular movement of the beam results in the required three-dimensional (hammerhead) shape. The diameter of the needle depends on the ion beam dose, beam dwell time, and speed of the beam movement. A nanoneedle radius below 10 nm and a hammerhead smaller than 35 nm have been achieved. This fabrication process is robust and enables precise control over the three-dimensions of the hammerhead AFM probe. Finally, the authors test the capabilities of the fabricated AFM probes for two-dimensional metrology of sidewall angles and line-edge roughness of trenches and shark-fins in silicon.

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Helium ion beam induced growth of hammerhead AFM probes

To explore the possibilities of the Orion plus helium ion microscope (HIM) as a nanofabrication tool, the HIM at the TNO VLL is equipped with a pattern generator and a gas injection system. The nanofabrication research focuses on three areas: lithography, direct write and Helium Ion Beam Induced Processing (HIBIP) Lithography of helium ions with negative and positive resists is being studied to obtain ultimate resolution combined with dense patterns. The helium ions interact very locally with the resist. As a result, we observe (almost) no proximity effects, not even at pattern densities of 50% and line widths below 10 nm. For a 5 nm thin HSQ resist layer, 6 nm lines with 15 nm pitch have been achieved as shown in Figure 1 [1]. One of the most simple direct write options is to sputter directly material with helium ions [2]. Auand Pt-rods have been sliced to obtain small incisions. Figure 2 shows an 8 nm incision in Au-rods imaged with the HIM and TEM. The incision is slightly curved on the outside of the Au due to the surface tension. A limitation of direct write is the bubble formation in the substrate ate extremely high ion dose, as investigated by Livengood et al. [3]. HIBIP is performed using the following precursors: Pt, TEOS and XeF2. The Pt precursor is used to illustrate the possibilities of creating conducting 3D nanostructures. Figure 3 shows lines with dimensions of approximately 200, 15 and 30 nm (length, width and height). The same objectives are being investigated for TEOS precursors to fabricate non-conducting objects. XeF2 has been tested for etching materials with high precision. In conclusion, the helium ion microscope equipped with a gas injection system and a pattern generator is a new nanofabrication tool. It allows the user to modify and inspect samples on a nanometer scale with the same instrument.

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E. van Veldhoven

Papers

Model for nanopillar growth by focused helium ion-beam-induced deposition

On the influence of the sputtering in determining the resolution of a scanning ion microscope

Evaluation of EUV resist performance below 20nm CD using helium ion lithography

Helium ion beam induced growth of hammerhead AFM probes

Nanofabrication with a High Resolution Helium Ion Beam