Atom sieve for nanometer resolution neutral helium microscopy
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 determin...
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Veröffentlicht in: | Journal of vacuum science and technology. B, Nanotechnology & microelectronics Nanotechnology & microelectronics, 2017-11, Vol.35 (6) |
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Sprache: | eng |
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Zusammenfassung: | 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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ISSN: | 2166-2746 2166-2754 |
DOI: | 10.1116/1.4994330 |