Unlocking new contrast in a scanning helium microscope

ARTICLE Received 13 Aug 2015 | Accepted 13 Nov 2015 | Published 4 Jan 2016 DOI: 10.1038/ncomms10189 OPEN Unlocking new contrast in a scanning helium microscope M. Barr1,*, A. Fahy1,*, J. Martens1, A.P. Jardine2, D.J. Ward2, J. Ellis2, W. Allison2 & P.C. Dastoor1 Delicate structures (such as biological samples, organic films for polymer electronics and adsorbate layers) suffer degradation under the energetic probes of traditional microscopies. Furthermore, the charged nature of these probes presents difficulties when imaging with electric or magnetic fields, or for insulating materials where the addition of a conductive coating is not desirable. Scanning helium microscopy is able to image such structures completely non-destructively by taking advantage of a neutral helium beam as a chemically, electrically and magnetically inert probe of the sample surface. Here we present scanning helium micrographs demonstrating image contrast arising from a range of mechanisms including, for the first time, chemical contrast observed from a series of metal–semiconductor interfaces. The ability of scanning helium microscopy to distinguish between materials without the risk of damage makes it ideal for investigating a wide range of systems. 1 Centre for Organic Electronics, University of Newcastle, Callaghan, New South Wales 2308, Australia. 2 Cavendish Laboratory, University of Cambridge, Madingley Road, Cambridge CB3 0HE, UK. * These authors contributed equally to this work. Correspondence and requests for materials should be addressed to P.C.D. (email: ). NATURE COMMUNICATIONS | 7:10189 | DOI: 10.1038/ncomms10189 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms10189 M icroscopy is an essential tool for the discovery, application and fabrication of new materials, structures and devices. Moreover, with modern fabrication taking advantage of an ever-broader library of new materials, microscopy techniques need to be applicable to a wide range of organic and inorganic samples1,2. However, there exists a range of systems that remain challenging to image, such as transparent, fragile, weakly bonded, insulating, very rough and magnetic samples3,4. Neutral helium atoms are the ideal probe of such systems owing to their low energy, lack of net charge or spin and short de Broglie wavelength. Indeed, the favourable properties of neutral helium atoms as a surface probe have already been exploited for many years in the diffraction-based technique of helium atom scattering (HAS)4,5. Scanning helium microscopy (SHeM) is a spatially resolved variant of HAS that operates analogously to a scanning electron microscope, with the electron beam replaced with a beam of neutral helium atoms6,7. Based on extensive work with HAS, the nature of the probe– sample interaction is well understood at a qualitative level, and research into better quantitative analyses is an active topic in the field of surface scattering5,8–13. The neutral helium atoms backscatter from the outer electronic corrugation of the sample, thus giving the technique its absolute surface sensitivity and nondestructive qualities. The possible scattering pathways give rise to contrast in the collected image; a critical consideration since useful imaging depends not only on resolution, but on the contrast available. Although the field of atom optics is well established4,14–17 and SHeM is already exceeding the resolution limits of traditional optical microscopy18, the specific scattering mechanisms by which SHeM image contrast arises is a new area of research. To date, predictions of possible SHeM contrast modes have been purely speculative with no direct comparisons of experimental contrast with theory yet available4. Here we present the first observation of chemical contrast originating from inelastic effects in neutral atom microscopy. SHeM images of different ultrathin patterned metal films on silicon substrates show strong chemical (but weak topological) contrast. Altering the mean energy of the helium beam results in a significant reduction of image contrast, thus providing an unambiguous observation of an inelastic scattering-based process. Finally, we show that current theory is not yet capable of fully explaining the observed contrast. Results Topological contrast. Topological contrast is the dominant mechanism in SHeM images of microscopically rough specimens. Deviations from a perfect plane scatter the helium away from the specular channel (‘diffuse scattering’), yielding Michelson contrast C: C ¼ tanðyÞtanðdÞ; ð1Þ where y is the detector angle and d is the angular mismatch of two scattering planes4. As such, changes in the surface morphology will influence the intensity recorded at each pixel of the image. Further topological contrast in the form of shadowing and masking is possible if either the beam or detector is completely occluded due to a surface asperity. Figure 1 shows a comparison of an optical and SHeM micrograph (taken with an instrument detailed previously7) of a section of a wing from the honey bee species Apis mellifera. The complex folds of the membrane of the wing are the almost indiscernible in the optical image due to the transparency of the membrane material and the range of sample plane heights (in Fig. 1a, the distance from the sample slide to wing top is B1.5 mm). However, all of these features are readily apparent in the SHeM image. The absolute surface sensitivity of the helium 2 a b c d Figure 1 | Topological contrast in SHeM. Comparison of reflection optical (Leica M205 C) (a,c) and SHeM (b,d) micrographs of a honey bee wing (Apis mellifera) as an example of topological contrast. Bottom images taken from the square region are indicated in a. Scale bars, 500 and 50 mm, respectively. atoms means that only features on the top side of the wing are observable. For example, masking of the incident helium beam is visible from both the hairs on the wing surface (Fig. 1b) and where the wing rests on the substrate. Thus, SHeM produces intuitive images of biological samples with no sample preparation required and no risk of beam damage to the substrate. Chemical contrast. While topological contrast is readily observed, the properties of the helium probe particle have been predicted to yield weaker, more exotic mechanisms4. For example, the composition and local atomic character of a sample surface should also give rise to variations in the helium reflectivity through the structure factor of the scattering centres. The mean energy and momentum of a neutral helium beam with a de Broglie wavelength of the order Ångstroms is well-matched to those of phonon-induced surface charge–density oscillations, making it capable of interacting with dynamic surface processes. Indeed, an atomic beam of helium is ideal to probe such processes since its small size and mass (as compared to heavier noble species) minimizes the lattice displacement and hence helium atoms will excite or de-e (...truncated)