Multiple scattering in scanning helium microscopy

Abstract

Using atom beams to image the surface of samples in real space is an emerging technique that delivers unique contrast from delicate samples. Here, we explore the contrast that arises from multiple scattering of helium atoms, a specific process that plays an important role in forming topographic contrast in scanning helium microscopy (SHeM) images. A test sample consisting of a series of trenches of varying depths was prepared by ion beam milling. SHeM images of shallow trenches (depth/width < 1) exhibited the established contrast associated with masking of the illuminating atom beam. The size of the masks was used to estimate the trench depths and showed good agreement with the known values. In contrast, deep trenches (depth/width > 1) exhibited an enhanced intensity. The scattered helium signal was modeled analytically and simulated numerically using Monte Carlo ray tracing. Both approaches gave excellent agreement with the experimental data and confirmed that the enhancement was due to localization of scattered helium atoms due to multiple scattering. The results were used to interpret SHeM images of a bio-technologically relevant sample with a deep porous structure, highlighting the relevance of multiple scattering in SHeM image interpretation.

True-to-size surface mapping with neutral helium atoms

“…We are in the fortunate situation that cosine distributed scattering can also be used to model the scattering of neutral helium atoms from many surfaces, where it is known as Knudsen's cosine law [18]. Although helium scattering from highly prepared 'pristine' atomic surfaces shows complex scattering distributions [6,8], many 'normal', 'unprepared' or 'technological' surfaces studied to date are consistent with an approximate cosine distribution [15][16][17][18] and recent SHeM image modelling shows excellent agreement with simulations that use a cosine model of scattering [12][13][14]. Under Knudsen's scattering law, the photometric stereo method may therefore be applied to helium microscopy and in fact requires very little modification.…”

“…1 in Lambrick et. al [12] was used as a representative experimental SHeM image. The darkest pixel in the image was assumed to be representative of the background signal and was subtracted, then the standard deviation and mean intensity from pixels on a flat region of the sample were taken to be the noise and the signal level respectively, giving an SNR of ∼ 30.…”

“…mean the situation when the helium atoms are 'bouncing' from one distinct area on the sample to another [12,13,26]. This can cause regions in images to appear brighter, in particular where there is significant topography in the form of deep or tall features (high aspect ratios).…”

“…An alternative approach to 3d imaging with atoms has been made possible by the recent discovery that unlike highly prepared 'pristine' atomic surfaces, which scatter with an angular distribution containing strong specular and diffracted components [6,8], many 'unprepared' technological surfaces scatter diffusely with an approximate cosine like distribution [11][12][13][14]. The cosine-like dis-tribution appears and is centred on the surface normal, even when the sample is illuminated on a microscopic level.…”

Neutral Helium Microscopy (SHeM): A Review

“…Here the atoms may undergo more than one (elastic or inelastic) collision with the surface, which gives shadowing effects. Multiple scattering contrast is described in [89], see also [73]. In the extreme case, when the atoms are thermally equibrilated with the surface through the multiple scattering.…”