AVS2018 Session HI-ThA: Emerging Ion Sources, Optics, and Applications

Practical use of a gas field ionization ion source (GFIS) has been tried for decades, but it was known difficult to realize. The GFIS has an extremely high brightness and a small source size, so that the convergence performance is excellent. As the atomic structure formation technology of the emitter tip has matured, the He - GFIS has been used in the real world scanning ion microscopes (SIMs). At first, their application was focused on the observation of the surface of samples because of its surface sensitivity, long focal depth, and a high resolution. Recently, however, Ne - GFIS has also been used for applications on fine direct fabrication which was difficult with a gallium liquid metal ion source.

Some of the major insights in the development of modern materials have come from scanning probe, electron, and ion microscopy, with advances in resolution and sensitivity enabling new material science. While charged particle beam techniques are widely used they have the serious drawback of causing damage to sample surfaces, while scanning probe techniques are limited to relatively flat surfaces and suffer from limited scan speeds. Instead, we are pursuing a different approach, using neutral atom beams. Here we will report on recent advances and development of the scanning helium microscopy (SHeM) technique.

Since SHeM uses a neutral beam of helium atoms at very low energy (<100 meV), the technique is suitable for measuring a variety of samples including insulators, semiconductors, organic and biological species. It is particularly attractive as the approach does not require any complicated post processing techniques. We will report on recent studies on range of materials and potential new applications, including measurements performed in collaboration with colleagues at the University of Newcastle (Australia) [1,2]. A particular focus will be on describing the underlying mechanisms of contrast formation.

Many of the technological challenges associated with SHeM have now been addressed, including helium focusing, sample preparation and nanoscale manipulation, thus enabling preliminary instruments to be developed[1,2,3]. One of the remaining challenges is adequate detection of neutral atom beams, which is a particular problem due to helium’s high ionization energy[3]. Applications that require time-sensitive measurements require a small ionization volume; however, when very high temporal resolution is not required, as with SHeM, very large ionizers with high detection efficiencies can be used. We will also report a recently developed detector, based on the approach recently applied to surface spin-echo experiments [4,5,6], and having the highest yet reported sensitivity for helium atoms.

[1] Nucl. Instr. Meth. Phys. Res B 340 76-80, 2014.

[2] Nature Communications, 7, 10189, 2016.

[3] “Atom, molecule and cluster beams”; Springer: Berlin, (2000).

[4] Phys Rev Lett. 105, 136101, 2010.

[5] Phys. Rev. Lett. 117, 196001, 2016.

[6] Phys. Rev. Lett., 52 (19), pp. 5085‐5088, 2016.

The application of the helium ion microscope (HIM) has expanded in various fields, such as nano-patterning, material science, and biology, due to its high spatial resolution for imaging and high-precision machining [1-3]. HIM realized sub-nm resolution with gas field ion sources (GFIS) which generate s ion beams from one or three topmost atoms of tips to obtain high beam current density. However, it is difficult to fabricate atomically sharp tips, such as trimer/single atom tip (TSAT), in an ultra-high vacuum (UHV) condition. TSAT can be typically fabricated by either a build-up method or field-assisted reactive gas etching method with oxygen and nitrogen [4-7]. However, these methods usually adopt resistive tip heating at about 1000 K as pre-cleaning of tip surface before the tip sharpening process. This heating leads to complex system because of a power supply circuit to provide or flow a current through a heating loop, where the tip was welded. In our study, we show that TSAT can be fabricated by field evaporation effect with an oxide layer which remains on the tip surface owing to the absence of tip heating.

As the result of this study, we could get a single crystalline field ion microscopy (FIM) image of W(111) with fabricating a TSAT by field evaporation phenomenon without tip surface cleaning by high temperature heating process. The oxide layer which remained after electrochemical etching process induces etch-like phenomenon in UHV condition without any additional gas injection. In order to analyze verify the proposed etching process, the analytical techniques of transmission electron microscope (TEM), energy filtered transmission electron microscope (EFTEM), and electron energy loss spectroscopy (EELS) were used. To compare the etching results whether the insulating layer present or not, we did additional experiment for tip heating. It was found that tungsten oxides contained in the insulating layer of the tip surface causes the etching. This method is much simpler than conventional methods because it uses only field evaporation phenomenon for fabricating TSAT. Therefore, we can simplify the equipment configuration since there is no need to heat the tip.

[1] N.Economou et al., Scanning 34(2): 83-89, 2012

[2] M.Postek et al., Scanning Microscopy 2009 (Vol. 7378, p. 737808)

[3] Joy, D. C. (2013). Helium Ion Microscopy: Principles and Applications (pp. 1-64). New York: Springer.

[4] M. Rezeq et al., The Journal of chemical physics, 124(20), 204716, 2006

[5] F.Rahman et al., Surface Science, 602(12), 2128-2134, 2008

[6] VT Binh et al., Surface Science, 202(1-2), L539-L549, 1988

[7] TY Fu et al., Physical Review B, 64(11), 113401, 2001

Among focused ion beam (FIB) users there is a constant demand for improved performance. Improved performance is defined as more beam current in the same spot size or same beam current in a smaller spot size. Another common driver is the desire for different ion species; specifically a non-reactive or inert species such as a noble gas. The ideal FIB system would be able to alternate between a heavy mass ion for efficient milling and a light mass ion for minimal sample damage imaging. Furthermore, it is desired to have a wide range of currents available for the different milling applications; high currents (100 nA) for high volume milling and low currents (1 pA) for nano-milling and currents in-between for precise small volume milling applications. The most versatile FIB systems are currently based on the Ga liquid metal ion source (LMIS) and hence are the most used. Specialized high volume plasma ion source based FIBs and gas field ion source (GFIS) FIBs for imaging are fulfilling specialized applications. To date, there has not been a good alternative specie ion source replacement for the Ga LMI.

The Nano Aperture Ion Source (NAIS) is based on electron impact gas ionization inside a sub-micron gas chamber. In a realistic configuration, the simulations predict a source brightness in the range of 1-3 x 10⁶ A/m²sr V with a corresponding maximum energy spread of 1 eV. Recent efforts have been devoted to measuring the brightness and to understanding how the relevant physical process determine the ion beam performance. Processes investigated include the initial velocity and position distributions of the neutral gas particles, ionization cross sections, electron current density, and ion-neutral scattering¹. The coulomb interactions and electric fields and their effect on beam performance was determined using analytical models, numerical calculations, and ray tracing. The particle interactions was found to limit the achievable brightness.

The brightness was measured using a small ion focusing column in combination with a knife edge ion transmission detector inside of a conventional SEM. A brightness of 1 x 10⁵ A/m²sr V was experimentally measured. The experimental value compares favorably to the calculation brightness value for the given experimental conditions, providing optimism that the theoretical B of 1-3 x 10⁶ will be achieved if NAIS is properly implemented. Limiting the measured brightness was in the test setup were poor electron beam current density and a weak accelerating field.

1. Leon van Kouwen, The Nano Aperture Ion Source, PhD Thesis TU Delft 2017

2. David S. Jun, Development of the Nano-Aperture Ion Source, PhD Tehsis Tu Delft (2014)

We present an approach to mitigate the ion beam induced damage inflicted on semiconductor nano-structures during ion beam irradiation. Nanopillars (with diameter a of 35 nm and height of 70 nm) have been irradiated with either a 50 keV Si⁺ broad beam from an implanter or a 25 keV focused Ne⁺ beam from a helium ion microscope (HIM) . Upon irradiation of the nanopillars at room temperature with a medium fluence (2e16 ions/cm²), strong plastic deformation has been observed which hinders further device integration. This differs from predictions made by the simulations using TRI3DYN. However, irradiation at elevated temperatures with the same fluence would preserve the shape of the nanopillars.

This work is supported by the European Union’s H-2020 research project ‘IONS4SET’ under Grant Agreement No. 688072.