Image formation in the scanning helium microscope

• Published in: Ultramicroscopy Vol. 192; pp. 7 - 13
• Main Authors: Fahy, A., Eder, S.D., Barr, M., Martens, J., Myles, T.A., Dastoor, P.C.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.09.2018
• ISSN: 0304-3991; 1879-2723
• DOI: 10.1016/j.ultramic.2018.05.004

•Scanning helium micrographs may exhibit instances of occlusion, namely shadowing and masking.•The scattering geometry results in a projection distortion along one axis of the produced micrographs.•The minimum resolvable step height for the current instrument is approximately 67 µm.•The minimum resolvable planar angle for the current instrument is approximately 4.3°. The scanning helium microscope (SHeM) is a new addition to the array of available microscopies, particularly for delicate materials that may suffer damage under techniques utilising light or charged particles. As with all other microscopies, the specifics of image formation within the instrument are required to gain a full understanding of the produced micrographs. We present work detailing the basics of the subject for the SHeM, including the specific nature of the projection distortions that arise due to the scattering geometry. Extension of these concepts allowed for an iterative ray tracing Monte Carlo model replicating diffuse scattering from a sample surface to be constructed. Comparisons between experimental data and simulations yielded a minimum resolvable step height of (67 ± 5) µm and a minimum resolvable planar angle of (4.3 ± 0.3)° for the instrument in question.