Fast resolution change in neutral helium atom microscopy

Flatabø, Ranveig; Eder, Sabrina; Ravn, A K; Samelin, B.; Greve, Martin; Reisinger, T.; Holst, Bodil

doi:10.1063/1.5029385

Cited by 6 publications

(4 citation statements)

References 28 publications

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“…That is to say O 2 S is the image of the source at the sample. For the zone plate microscope it is the demagnified image of the skimmer or the source limiting aperture [77], σ A is the Airy disk contribution from edge diffraction from the zone plate or pinhole and σ cm is a chromatic aberration term that appears for the case of the zone plate. The first equation holds under the assumption that the Fresnel number is smaller than 1, which is the case for the limit of small pinholes.…”

Section: Helium Optics / Resolution Limitsmentioning

confidence: 99%

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Neutral Helium Microscopy (SHeM): A Review

Palau¹,

Eder²,

Bracco³

et al. 2021

Preprint

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Neutral helium microscopy, also referred to as scanning helium atom microscopy and commonly abbreviated SHeM, is a novel imaging technique that uses a beam of neutral helium atoms as an imaging probe. The technique offers a number of advantages such as the very low energy of the incident probing atoms (less than 0.1 eV), unsurpassed surface sensitivity (no penetration into the sample bulk), a charge neutral, inert probe and a high depth of field. This means that fragile and/or non-conducting samples can be imaged as well as samples with high aspect ratio, with the potential to obtain true to scale height information of 3D surface topography with nanometer resolution. However, for a full exploitation of the technique, a range of experimental and theoretical issues still needs to be resolved. In this paper we review the current state of research in the field. We do this by following the trajectory of the helium atoms step by step through the microscope. From the initial acceleration in the supersonic expansion used to generate the probing beam over the interaction of the helium atoms with the sample (contrast properties) to the final detection and post-processing. We also review recent advances in scanning helium microscope design and we present an overview of the samples that have been investigated with SHeM up till now.

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Section: Helium Optics / Resolution Limitsmentioning

confidence: 99%

“…2.2.3. This design has the additional advantage that it enables fast resolution change by changing in-situ between different collimating apertures [77].…”

Section: Microscopes With Micro-skimmersmentioning

confidence: 99%

Neutral Helium Microscopy (SHeM): A Review

Palau¹,

Eder²,

Bracco³

et al. 2021

Preprint

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show abstract

“…1 by Barr et al [14] small skimmers (often known as 'microskimmers' [15]) can be used to reduce the source size, but their narrow geometry means atoms backscattered into the beam from either the leading edge, or the internal skimmer walls, risks affecting the beam intensity significantly. An alternative is to introduce a second aperture behind the skimmer that, provided the volume is adequately pumped, will set the source size of the instrument whilst minimising additional beam interference [16].…”

Section: Free Jet Expansionmentioning

confidence: 99%

A method for constrained optimisation of the design of a scanning helium microscope

et al. 2019

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We describe a method for obtaining the optimal design of a normal incidence Scanning Helium Microscope (SHeM). Scanning helium microscopy is a recently developed technique that uses low energy neutral helium atoms as a probe to image the surface of a sample without causing damage. After estimating the variation of source brightness with nozzle size and pressure, we perform a constrained optimisation to determine the optimal geometry of the instrument (i.e. the geometry that maximises intensity) for a given target resolution. For an instrument using a pinhole to form the helium microprobe, the source and atom optics are separable and Lagrange multipliers are used to obtain an analytic expression for the optimal parameters. For an instrument using a zone plate as the focal element, the whole optical system must be considered and a numerical approach has been applied. Unlike previous numerical methods for optimisation, our approach provides insight into the effect and significance of each instrumental parameter, enabling an intuitive understanding of effect of the SHeM geometry. We show that for an instrument with a working distance of 1 mm, a zone plate with a minimum feature size of 25 nm becomes the advantageous focussing element if the desired beam standard deviation is below about 300 nm.

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“…Contrast in scanning helium microscopy has similarities with the origins of contrast in scanning electron microscopy, both of which involve rastering a focused or collimated beam across the sample and the collection of a fraction of the backscattered signal. In the case of helium atoms, a narrow spot can be generated via simple pinhole collimation, as used in the current work; 3 via diffractive focusing with a Fresnel zone plate or similar; [6][7][8] or through the use of atom mirrors. [9][10][11][12] Since the local surface position and orientation affect the resulting distribution of scattered particles, topographic contrast is evident in both cases.…”

mentioning

confidence: 99%

Multiple scattering in scanning helium microscopy

Lambrick

Vozdecky

Bergin

et al. 2020

Applied Physics Letters

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Using atom beams to image the surface of samples in real space is an emerging technique that delivers unique contrast from delicate samples. Here, we explore the contrast that arises from multiple scattering of helium atoms, a specific process that plays an important role in forming topographic contrast in scanning helium microscopy (SHeM) images. A test sample consisting of a series of trenches of varying depths was prepared by ion beam milling. SHeM images of shallow trenches (depth/width < 1) exhibited the established contrast associated with masking of the illuminating atom beam. The size of the masks was used to estimate the trench depths and showed good agreement with the known values. In contrast, deep trenches (depth/width > 1) exhibited an enhanced intensity. The scattered helium signal was modeled analytically and simulated numerically using Monte Carlo ray tracing. Both approaches gave excellent agreement with the experimental data and confirmed that the enhancement was due to localization of scattered helium atoms due to multiple scattering. The results were used to interpret SHeM images of a bio-technologically relevant sample with a deep porous structure, highlighting the relevance of multiple scattering in SHeM image interpretation.

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Fast resolution change in neutral helium atom microscopy

Cited by 6 publications

References 28 publications

Neutral Helium Microscopy (SHeM): A Review

Neutral Helium Microscopy (SHeM): A Review

A method for constrained optimisation of the design of a scanning helium microscope

Multiple scattering in scanning helium microscopy

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