A desktop supersonic free-jet beam source for a scanning helium microscope (SHeM)

Barr, Matthew G.; O’Donnell, Kane M.; Fahy, Adam; Allison, W.; Dastoor, Paul C.

doi:10.1088/0957-0233/23/10/105901

Cited by 17 publications

(8 citation statements)

References 21 publications

(28 reference statements)

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“…See Fig. 3 Around the same time as Witham and Sánchez, Barr et al had worked on a pinhole microscope with a slightly different design, using a skimmer in combination with a collimating pinhole aperture [34]. For a detailed discussion of different helium microscope designs see Section 6.…”

Section: Some Background Historymentioning

confidence: 99%

“…In a Neutral Helium Microscope, atoms start their journey in the source. A typical SHeM source follows the design established for helium atom scattering (HAS) [36,34,37]: helium is accelerated in a supersonic expansion from a high pressure reservoir through a de-Laval nozzle 2 , into a vacuum chamber, known as the expansion chamber [38]. There, the central part of the beam is selected by a conically shaped aperture, called the skimmer 3 .…”

Section: The Helium Sourcementioning

confidence: 99%

“…However, note that for low background pressure the shock structure and Mach disc are less pronounced and disappear. Figure reproduced from [34] the beam) [36,42]. The absolute differences between a cold (liquid nitrogen cooled) and a warm source at the same pressure can easily be on the order of 1 • 10 13 counts/s • m 2 [42].…”

Section: The Helium Sourcementioning

confidence: 99%

See 2 more Smart Citations

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: Some Background Historymentioning

confidence: 99%

Section: The Helium Sourcementioning

confidence: 99%

See 1 more Smart Citation

Neutral Helium Microscopy (SHeM): A Review

Palau¹,

Eder²,

Bracco³

et al. 2021

Preprint

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show abstract

“…[10] and fig. 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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“…The source operates in an intermediate pressure regime [10] and consists of a nozzle (10 μm aperture) based on the design of Buckland et al [11] and a Beam Dynamics skimmer (100 μm aperture) that is used to sample the centerline atoms (producing the collimated beam) and separate the source chamber from the remainder of the apparatus. The nozzle can be moved with respect to the skimmer with an x-y-z manipulator (UHV Designs) and the pressure behind the nozzle, known as the stagnation pressure, can be controlled with a resolution of 1 bar.…”

Section: Design Of a Pinhole Shemmentioning

confidence: 99%

A design for a pinhole scanning helium microscope

Barr

Fahy

Jardine

et al. 2014

Nuclear Instruments and Methods in Physics Research Section B:

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We present a simplified design for a scanning helium microscope (SHeM) which utilises almost entirely off the shelf components. The SHeM produces images by detecting scattered neutral helium atoms from a surface, forming an entirely surface sensitive and non-destructive imaging technique. This particular prototype instrument avoids the complexities of existing neutral atom optics by replacing them with an aperture in the form of an ion beam milled pinhole, resulting in a resolution of around 5 microns. Using the images so far produced, an initial investigation of topological contrast has been performed.

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A desktop supersonic free-jet beam source for a scanning helium microscope (SHeM)

Cited by 17 publications

References 21 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

A design for a pinhole scanning helium microscope

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