Annual Report 2007 - The Australian Nanotechnology Network

Mr. Kane O’Donnell (Uni of Newcastle) – visit to the University of
Cambridge, UK.
Research Report
Kane O’Donnell
School of Mathematical and Physical Sciences
University of Newcastle
kane.odonnell@newcastle.edu.au
August 30, 2007
1 Introduction
There has long been a desire for a spatially-resolved neutral atom scattering technique.
Helium atom scattering (HAS) is a well-established surface science technique combining
true surface sensitivity with a sub-Angstrom de Broglie wavelength at an energy that is low
enough not to perturb even very delicate surface adsorbates. The primary advantage of
using such a surface probe for spatially resolved analysis is that the energy and surface
sensitivity of a helium beam lends itself well to several contrast mechanisms (geometric,
chemical and thermal) simultaneously[2]. For samples with spatially-varying properties
that are of interest, for example, metal-semiconductor interfaces or glass blends, a helium
atom microscopy technique would be an invaluable surface science technique. Indeed, for
delicate but microscopically rough, ultra-thin or insulating surfaces, helium atom
microscopy may prove to be the ideal technique where traditional scanning electron
microscopy (SEM) or scanning probe microscopy (SPM) cannot be used. The scanning
helium microscope (SHeM) at the Cavendish laboratory is intended to be the first such
device and is nearing completion as of the end of the Fellowship receipient’s research visit.
Like most other ‘optical’ (as opposed to scanning probe) microscopy techniques, a
helium atom microscope consists of four elements -a source, a sample,
focussing/defocussing optics, and a detector. The two possible configurations for these
components are illustrated schematically in Figures 1 and 2. The source for the helium
atom microscope consists of a skimmed, supersonic free jet helium source of a similar kind
as used in a standard helium atom scattering apparatus. The SHeM, in particular, uses a
source with a 10 micron nozzle that can be cooled to cryogenic temperatures, allowing the
beam energy to be controlled. The supersonic free jet produced by expanding high
pressure ( 100 Bar) helium through such a small nozzle is skimmed with a 2-50 micron
skimmer to produce a beam with an energy variation of the order of 2%, a reasonably
monochromatic beam. The optical brightness of such a beam is comparable to the
brightness of the electron source in an SEM[2]. In a scanning configuration the skimmer is
made as small as possible to reduce the optical source size and hence the spot size on the
sample, whereas in the imaging configuration broad sample illumination is desirable and
hence a larger skimmer is used.
As mentioned in Figures 1 and 2, the focussing element for the SHeM consists of a
50 micron thick single-crystal silicon wafer bent to an ellipsoidal section electrostatically
using a parallel-plate capacitor configuration. The macroscopic profile of the mirror is
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