Publisher's Note: “A simple approach to neutral atom microscopy” [Rev. Sci. Instrum. 82, 103705 (2011)]

Witham, Philip; Sánchez, Erik J.

doi:10.1063/1.3668130

Cited by 7 publications

(13 citation statements)

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“…They were transmission, shadow im-ages of porous structures, obtained by scanning a beam focussed down to 2 µm using a Fresnel zone plate. Since then, two other research groups have managed to obtain helium microscope images in reflection with micron range resolution using pinhole microscopes [3][4][5][6]. The first reflection images were published in 2011 by Withman and Sanchez using a setup with just a pinhole and no skimmer [3].…”

Section: Introductionmentioning

confidence: 99%

Theoretical model of the helium zone plate microscope

2017

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Neutral helium microscopy is a new technique currently under development. Its advantages are the low energy, charge neutrality, and inertness of the helium atoms, a potential large depth of field, and the fact that at thermal energies the helium atoms do not penetrate into any solid material. This opens the possibility, among others, for the creation of an instrument that can measure surface topology on the nanoscale, even on surfaces with high aspect ratios. One of the most promising designs for helium microscopy is the zone plate microscope. It consists of a supersonic expansion helium beam collimated by an aperture (skimmer) focused by a Fresnel zone plate onto a sample. The resolution is determined by the focal spot size, which depends on the size of the skimmer, the optics of the system, and the velocity spread of the beam through the chromatic aberrations of the zone plate. An important factor for the optics of the zone plate is the width of the outermost zone, corresponding to the smallest opening in the zone plate. The width of the outermost zone is fabrication limited to around 10 nm with present-day state-of-the-art technology. Due to the high ionization potential of neutral helium atoms, it is difficult to build efficient helium detectors. Therefore, it is crucial to optimize the microscope design to maximize the intensity for a given resolution and width of the outermost zone. Here we present an optimization model for the helium zone plate microscope. Assuming constant resolution and width of the outermost zone, we are able to reduce the problem to a two-variable problem (zone plate radius and object distance) and we show that for a given beam temperature and pressure, there is always a single intensity maximum. We compare our model with the highest-resolution zone plate focusing images published and show that the intensity can be increased seven times. Reducing the width of the outermost zone to 10 nm leads to an increase in intensity of more than 8000 times. Finally, we show that with present-day state-of-the-art detector technology (ionization efficiency 1×10-3), a resolution of the order of 10 nm is possible. In order to make this quantification, we have assumed a Lambertian reflecting surface and calculated the beam spot size that gives a signal 100 cts/s within a solid angle of 0.02 sr, following an existing helium microscope design

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Section: Introductionmentioning

confidence: 99%

Theoretical model of the helium zone plate microscope

2017

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“…The first neutral helium microscopy images were published in 2008 1 . Several groups have worked on the technique [2][3][4][5][6] . In a neutral helium microscope (NEMI), a beam of neutral helium atoms with narrow velocity (wavelength) distribution is used to image a sample.…”

Section: Introductionmentioning

confidence: 99%

Atom sieve for nanometer resolution neutral helium microscopy

Flatabø

Greve

Eder

et al. 2017

Journal of Vacuum Science &Amp; Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phen

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Neutral helium microscopy is a new tool for imaging fragile and/or insulating structures as well as structures with large aspect ratios. In one configuration of the microscope, neutral helium atoms are focused as de Broglie matter waves using a Fresnel zone plate. The ultimate resolution is determined by the width of the outermost zone. Due to the low-energy beam (typically less than 0.1 eV), the neutral helium atoms do not penetrate solid materials and the Fresnel zone plate therefore has to be a free-standing structure. This creates particular fabrication challenges. The so-called Fresnel photon sieve structure is especially attractive in this context, as it consists merely of holes. Holes are easier to fabricate than the free-standing rings required in a standard Fresnel zone plate for helium microscopy, and the diameter of the outermost holes can be larger than the width of the zone that they cover. Recently, a photon sieve structure was used for the first time, as an atom sieve, to focus a beam of helium atoms down to a few micrometers. The holes were randomly distributed along the Fresnel zones to suppress higher order foci and side lobes. Here, the authors present a new atom sieve design with holes distributed along the Fresnel zones with a fixed gap. This design gives higher transmission and higher intensity in the first order focus. The authors present an alternative electron beam lithography fabrication procedure that can be used for making high transmission atom sieves with a very high resolution, potentially smaller than 10 nm. The atom sieves were patterned on a 35 nm or a 50 nm thick silicon nitride membrane. The smallest hole is 35 nm, and the largest hole is 376 nm. In a separate experiment, patterning micrometer-scale areas with hole sizes down to 15 nm is demonstrated. The smallest gap between neighboring holes in the atom sieves is 40 nm. They have 47011 holes each and are 23.58 μm in diameter. The opening ratio is 22.60%, and the Fresnel zone coverage of the innermost zones is as high as 0.68. This high-density pattern comes with certain fabrication challenges, which the authors discuss.

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“…Early focussing experiments using neutral helium atoms were carried out by O.Carnal et al, Holst and Allison and Doak et al in the 1990's [5][6][7]. Since then two other research groups have managed to obtain helium microscope images in reflection with micron range resolution using pinhole microscopes [8][9][10][11]. The first reflection images were published in 2011 by Withman and Sanchez using a setup with just a pinhole and no skimmer [8].…”

Section: Introductionmentioning

confidence: 99%

“…Since then two other research groups have managed to obtain helium microscope images in reflection with micron range resolution using pinhole microscopes [8][9][10][11]. The first reflection images were published in 2011 by Withman and Sanchez using a setup with just a pinhole and no skimmer [8]. This setup still claims the best resolution achieved so far with a helium microscope: 350 nm [12].…”

Section: Introductionmentioning

confidence: 99%

Theoretical model of the helium pinhole microscope

2016

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In recent years, the development of neutral helium microscopes has gained increasing interest. The low energy, charge neutrality, and inertness of the helium atoms makes helium microscopy an attractive candidate for the imaging of a range of samples. The simplest neutral helium microscope is the so-called pinhole microscope. It consists of a supersonic expansion helium beam collimated by two consecutive apertures (skimmer and pinhole), which together determine the beam spot size and hence the resolution at a given working distance to the sample. Due to the high ionization potential of neutral helium atoms, it is difficult to build efficient helium detectors. Therefore, it is crucial to optimize the microscope design to maximize the intensity for a given resolution and working distance. Here we present an optimization model for the helium pinhole microscope system. We show that for a given resolution and working distance, there is a single intensity maximum. Further we show that with present-day state-of-the-art detector technology (ionization efficiency 1×10-3), a resolution of the order of 600 nm at a working distance of 3 mm is possible. In order to make this quantification, we have assumed a Lambertian reflecting surface and calculated the beam spot size that gives a signal 100 cts/s within a solid angle of 0.02π sr, following an existing design. Reducing the working distance to the micron range leads to an improved resolution of around 40 nm

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Publisher's Note: “A simple approach to neutral atom microscopy” [Rev. Sci. Instrum. 82, 103705 (2011)]

Cited by 7 publications

References 0 publications

Theoretical model of the helium zone plate microscope

Theoretical model of the helium zone plate microscope

Atom sieve for nanometer resolution neutral helium microscopy

Theoretical model of the helium pinhole microscope

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