A design for a pinhole scanning helium microscope

Abstract

We present a simplified design for a scanning helium microscope (SHeM) which utilises almost entirely off the shelf components. The SHeM produces images by detecting scattered neutral helium atoms from a surface, forming an entirely surface sensitive and non-destructive imaging technique. This particular prototype instrument avoids the complexities of existing neutral atom optics by replacing them with an aperture in the form of an ion beam milled pinhole, resulting in a resolution of around 5 microns. Using the images so far produced, an initial investigation of topological contrast has been performed.

A highly contrasting scanning helium microscope

“…Increasing the nozzle-to-skimmer distance, even slightly, results in a much sharper peak that is more appropriate for imaging. Comparisons to similar scans for the SHeM I instrument 10 show a significant reduction in the effusive beam intensity for all nozzle-to-skimmer positions. Campargue 16 gave that the ideal (i.e., the highest downstream supersonic beam intensity) is achieved at the nozzle-to-skimmer distance z ns given by…”

“…The detector functions in stagnation mode, meaning that based on a constant pump rate the incoming gas will give rise to a stable population which may then be sampled to yield the intensity at that point on the sample surface. 10 The detector chamber itself is pumped by two Edwards EXT75DX turbo pumps mounted in series, maximising the performance for light gases. A pair of butterfly valves on the detector chamber provide control of the pump rate, allowing for the peak helium signal and the time taken for the pressure to equilibrate to be varied.…”

“…10 The model first calculates the centreline intensity of the helium beam as a function of the stagnation conditions including pressure, temperature, and nozzle diameter. The throughput into the differential chamber due to the conductance of the skimmer is then found, with any helium atoms that do not pass through the skimmer becoming part of the background pressure in the source chamber.…”

“…The method used to calculate the signal-to-noise for these images differs slightly from that presented previously with regards to SHeM I. 10 In this instance, signal-to-noise is calculated by comparing the mean signal to the RMS noise for the same region, namely, the flat silicon substrate. Equivalent analysis on TEM grid images as produced by SHeM I (such as Figure 5(a) in Ref.…”

“…[2][3][4][5][6][7][8] Recent progress in SHeM development has involved an optical arrangement using a simple pinhole. 9,10 The authors recently reported on a SHeM geometry (henceforth referred to as "SHeM I") designed to optimise the number of possible contrast mechanisms available to the technique through having the helium beam interact with the sample at 45 • . By monitoring the specular position, we maximise the chances of detecting a difference between the specularly and diffusely scattered helium.…”