On the design of Neutral Scanning Helium Atom Microscopes (SHeM) : Optimal configurations and evaluation of experimental findings
Abstract
Scanning Helium Microscopes (SHeMs) are novel microscopy tools using neutral helium atoms as the imaging probe. Helium atoms have several advantages compared to other probes such as electrons or helium ions. Helium atoms are neutral and inert and when compared to electrons their higher mass leads to a smaller de-Broglie wavelength for a given energy. Furthermore, helium atoms are strictly surface sensitive, scattering off the electron density distribution off the surface. These combined properties allow for non-destructive mapping of the surface of virtually any vacuum-compatible solid sample. Helium ions have a similar mass but they interact more strongly with the sample because they are not inert and require much higher energies to achieve electrostatic focusing. Charge neutrality makes helium a great imaging corpuscle, but also means that designing SHeMs is very difficult. Neutral helium atoms are very hard to manipulate, as electromagnetic fields cannot be used to focus and redirect the beam - instead, one needs to use diffraction optics and apertures. They are also hard to detect because helium has the highest ionisation potential of all atoms - hindering the task of ionisation based detectors. Therefore, to have a functioning microscope, one needs to form a highly intense atom beam. This thesis presents the work done over the last years to optimise the intensity of SHeMs, and more generally their atom-optics configuration. Amongst the papers included here are the first ones to show that SHeM optics have well-defined intensity maxima that give optimal designs. These papers show that existing designs were suboptimal and that the intensity could be increased several orders of magnitude. This thesis also features the first paper to present a design for a 3D imaging SHeM. A true nano-scale stereo microscope based on Heliometric stereo, a technique adapted from light. Besides these theoretical papers, two papers are included that focus on understanding the helium beam using experimental data. These papers are important as they provide the experimental foundations for the theoretical models used. Amongst other findings, the papers explore the importance of the Knudsen number at the skimmer, the validity of different intensity models, and the top-hat profile of the beam. The research presented here happened in parallel to a two order of magnitude improvement in detector efficiency. I believe that now we are in the position to build high-resolution SHeMs that have the potential to become an important tool for science and industry.. . .
Has parts
Paper 1. Adrià Salvador Palau, Gianangelo Bracco, Bodil Holst, Theoretical model of the helium pinhole microscope, Physical Review A 94, 6, 2016. The article is available in the thesis. The article is also available at: https://doi.org/10.1103/PhysRevA.94.063624Paper 2. Adrià Salvador Palau, Gianangelo Bracco, Bodil Holst, Theoretical model of the helium zone plate microscope , Physical Review A 95, 1, 2017. The article is available in the thesis. The article is also available at: https://doi.org/10.1103/PhysRevA.95.013611
Paper 3. SD Eder, Adrià Salvador Palau, T. Kaltenbacher, Gianangelo Bracco, Bodil Holst, Velocity distributions in microskimmer supersonic expansion helium beams: High precision measurements and modeling, Review of Scientific Instruments 89, 11, 2018. The article is available at: https://hdl.handle.net/1956/21809
Paper 4. Adrià Salvador Palau, SD Eder, Truls Andersen, Anders Komr Ravn, Gianangelo Bracco, Bodil Holst, Center-line intensity of a supersonic helium beam, Physical Review A 98, 6, 2018. The article is available in the thesis. The article is also available at: https://doi.org/10.1103/PhysRevA.98.063611
Paper 5. Sam MLambrick, Adrià Salvador Palau, Poul Erik Hansen, Gianangelo Bracco, John Ellis, Andrew P Jardine, Bodil Holst, True-to-size surface mapping with neutral helium atoms, Physical Review A 103, 5, 2021. The article is available at: https://hdl.handle.net/11250/2982372
Paper 6. SD Eder, AK Ravn, B Samelin, G Bracco, Adrià Salvador Palau, T Reisinger, Erik Bergback Knudsen, K Lefmann, B Holst, Zero-order filter for diffractive focusing of de Broglie matter waves, Physical Review A 95, 2, 2017. The article is available in the thesis. The article is also available at: https://doi.org/10.1103/PhysRevA.95.023618
Paper 7. R. Flatabo, M. M Greve, SD Eder, M. Kallane, Adrià Salvador Palau, K. Berggren, B. Holst, Atom sieve for nanometer resolution neutral helium microscopy, Journal of Vacuum Science and Technology B 35, 6, 2017. The article is available in the thesis. The article is also available at: https://doi.org/10.1116/1.4994330