Multiple Scattering In Scanning Helium Microscopy

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. Scanning helium microscopy (SHeM) is a nascent technology that scans a narrow beam of low energy neutral helium atoms over a surface, to produce images of materials without any possibility of beam damage.1–4 The technique can be applied widely and has particular applications in imaging delicate samples, which are difficult to measure using existing techniques.5 Examples include the imaging of insulators, polymers, and biological materials, all of which can be done without coatings or other preparation. As the technique becomes used more broadly, it is crucial to have a good understanding of the image formation process. Here, we report on the significant role that multiple scattering plays in the contrast observed in SHeM images. We use a test sample with simple, well defined, topography. By comparing experimental images with quantitative contrast modeling, we obtain a clear understanding of the process. We also show how quantitative topographic information can be extracted without making any assumptions about the atom-surface interaction. These insights are then used to understand and interpret helium images of a bio-technologically relevant sample in its native state. Contrast in scanning helium microscopy has similarities with the origins of contrast in scanning electron microscopy, both of which involve rastering a focused or collimated beam across the sample and the collection of a fraction of the backscattered signal. In the case of helium atoms, a narrow spot can be generated via simple pinhole collimation, as used in the current work;3 via diffractive focusing with a Fresnel zone plate or similar;6–8 or through the use of atom mirrors.9–12 Since the local surface position and orientation affect the resulting distribution of scattered particles, topographic contrast is evident in both cases. The scattering geometries mean that images appear as if they are illuminated from the direction of the detector, and when a point on the sample is occluded from the detector, typically by a convex region of sample structure, “masked” regions are formed in the image. However, there are also significant differences between the angular distributions of scattered electrons and helium atoms, due to the underlying differences in interaction with surfaces,13 and the different relative detector sizes; helium detectors only cover a relatively small fraction of solid angle. These issues have a significant effect on contrast formation and image interpretation. In current SHeM instruments, the incident beam is typically at [Math Processing Error] to the sample normal and illuminates a small region on the surface, corresponding to a particular pixel in the image. Atoms scattered through a total angle of approximately [Math Processing Error] reach the detector and are counted to give the pixel intensity. There are three primary contributions to topographic contrast, which have been discussed in the literature and are useful to distinguish here. First, height contrast arises primarily from a change in the proportion of the scattered signal that is detected using a fixed position detector;14,15 however, such contrast is weak and only appears over large changes in the height.14–16 Second, angular orientation contrast14,16 occurs when the local orientation of the sample changes the portion of the scattering distribution that enters the detector aperture. For the largely diffuse scattering that occurs from unprepared surfaces, higher intensity is expected when the local surface is orientated toward the detector. Finally, masking, due to the detector being occluded from the illuminated spot on the sample, gives very strong contrast as primary scattered atoms cannot be detected. Masking is independent of the atom-surface interaction14–17 but is related to the underlying surface topography, thus enabling quantitative topographic information to be extracted. A particularly important feature of SHeM is that since there is essentially no possibility of sample penetration by, or adsorption of, the probe particles, incident helium atoms can undergo multiple scattering. Given atoms travel along straight line paths, there is a limited probability of them reaching the detector entrance aperture after a single scattering event ([Math Processing Error] in our current arrangement). However, by scattering from the sample multiple times, atoms can reach the detector indirectly, and thus, multiple scattering makes a further contribution to topographic contrast. We also note that multiple scattering can also provide weak diffuse illumination, a process that has been previously noted15,17 but not examined in detail. A SHeM image of our test sample is shown in Fig. 1, along with its corresponding surface profile. The sample consists of a set of trenches (manufactured by plasma focused ion beam milling of a silicon wafer), each with the same area but different depths. Measured depths of some of the trenches are given in Table I. The helium image clearly shows a distinction between the “shallow” trenches on the right hand side of the image, exhibiting dark regions that widen with the depth, and “deep” trenches on the left hand side of the image, showing very similar, almost uniform, contrast without distinct dark regions. The transition from shallow to deep trenches occurs at a depth/width ratio of about 1. The deep trenches appear significantly brighter than the dark regions in the shallow trenches. Looking closely, the intensity inside the deep trenches increases from left to right, and on the inside right, it even exceeds the intensity of the flat substrate.

FIG. 1. SHeM image of the test sample with the corresponding surface profile. The helium beam is incident from the right as shown, while the detector is located to the left. As a result of re-deposition during ion beam milling, the sides of the trenches are not vertical and edges of the trenches are rounded. The scale bar length is [Math Processing Error]. See the supplementary material for further details.

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