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.

/pdf/helium-ion-microscopy-20lveyia0z.pdf

Helium ion microscopy

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.

Spin orbit torque driven switching is a favourable way to manipulate nanoscale magnetic objects for both memory and wireless communication devices. The critical current required to switch from one magnetic state to another depends on the geometry and the intrinsic properties of the materials used, which are difficult to control locally. Here we demonstrate how focused helium ion beam irradiation can modulate the local magnetic anisotropy of a Co thin film at the microscopic scale. Real-time in-situ characterisation using the anomalous Hall effect showed up to an order of magnitude reduction of the magnetic anisotropy under irradiation, and using this, multi-level switching is demonstrated. The result is that spin-switching current densities, down to 800 kA cm-2, can be achieved on predetermined areas of the film, without the need for lithography. The ability to vary critical currents spatially has implications not only for storage elements, but also neuromorphic and probabilistic computing.

Helium Ion Microscopy for Reduced Spin Orbit Torque Switching Currents

Tailoring of Magnetic Anisotropy by Ion Irradiation for Magnetic Tunnel Junction Sensors

We present the results of magnetic force microscopy investigations of domain structures in multilayer [Co (0.5 nm)/Pt (1 nm)]5 thin film structures (denoted hereafter as Co/Pt) modified by additional Co capping layers and by ion irradiation. It is demonstrated that a Co capping layer essentially changes the domain structure and decreases the threshold of magnetization reversal, due to the formation of noncollinear magnetization in Co/Pt. It is shown that local irradiation with a focused He+ ion beam enables the formation of regions with decreased easy-axis anisotropy (magnetic bubbles) that have the inverse magnetization direction in the demagnetized state of Co/Pt. The experimental results demonstrate that the magnetic bubbles can be switched using a probe of a magnetic force microscope. The possible application of these effects for the development of magnetic logic and data storage systems is discussed.

Magnetic Force Microscopy of Nanostructured Co/Pt Multilayer Films with Perpendicular Magnetization

We use a focused helium ion beam microscope to controllably remove material from Pt-capped Co/Pt thin film multilayers with perpendicular magnetic anisotropy. The dose required to remove several nm of the cap layer is similar to that required to induce a spin reorientation transition. Milling depth for the Pt overlayer is found to scale with the logarithm of ion dose, with very high removal yields of hundreds of atoms per He+ ion at low doses. While the deposited energy is of order of previous broad beam helium irradiations, the current density impacting on the surface of the sample is several tens of A cm−2, orders of magnitude higher. Patterning of the platinum capping layer with 20 nm resolution is demonstrated.

https://iopscience.iop.org/article/10.1088/0022-3727/46/19/195501/pdf

Local modification of magnetic anisotropy and ion milling of Co/Pt multilayers using a He + ion beam microscope

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Local modification of magnetic anisotropy and ion milling of Co/Pt multilayers using a He + ion beam microscope