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The formation of contrast in scanning helium microscopy



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The formation of contrast in scanning helium microscopy Over the last decade a new microscopy technique has emerged that uses neutral helium atoms as the probe particles. It has been termed scanning helium microscopy (SHeM), also known as neutral atom microscopy (NAM). SHeM produces helium atom micrographs by scanning the sample beneath a focused or collimated helium microprobe. As the technique is maturing, research efforts are moving on from the development of proof of concept instruments to the exploring of applications and optimising designs for the second generation of machine. In particular the mechanisms of contrast formation in SHeM are an active area of investigation.

The work presented in this thesis explores contrast formation in SHeM and how that knowledge might be used to interpret helium micrographs. Specifically a geometric model of contrast is developed which is implemented numerically in the form of both a ray-tracing framework and integral approaches. Work is also presented looking at design considerations for helium microscopes, informed heavily by simulations. A particular aspect of instrumentation, the `pinhole-plate', is shown to allow the Cambridge A-SHeM (a first generation pinhole SHeM using a 90° total scattering angle) to operate in a modular manner with improved spatial resolution, improved angular resolution, or making changes in image perspective possible. Using the high spatial resolution mode the first helium micrographs with a large working distance and a beam width below 1μm are presented.

Measurements presented demonstrate that a `cosine-like' diffuse model of scattering is the default atom surface scattering in SHeM for unprepared technological surfaces. Further measurements highlight the importance of multiple scattering as a key feature of topographic contrast in SHeM and how a proper understanding of multiple scattering is necessary for the interpretation of samples of technological interest. It is shown that multiple scattering is understood both quantitatively and qualitatively. Using a specialised arrangement of the Cambridge A-SHeM the first diffraction patterns measured from a microscopic spot size are presented. Diffraction contrast as an alternative to diffuse topographic contrast is then discussed. Finally a technique, coined heliometric stereo, that makes use of the observation of cosine-like scattering and multiple detection directions to perform 3D reconstructs is presented. The method is explored in detail using simulated data in order to demonstrate where it may most effectively be applied, and a proof of principle experimental reconstruction is performed on the A-SHeM.

The thesis finishes with a discussion of the overall conclusions that can be drawn from the current work, and the outlook for scanning helium microscopy in the near term.





Jardine, Andrew


Scanning helium microscopy, Neutral atom microscopy, Helium atom scattering, Atomic microscopy


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge