Unfortunately, HAS is not at present a spatially resolved technique and so
cannot currently be used to study surface phenomena on a localised scale.
As a result, the benefits of HAS cannot be employed in many 'real-life'
systems, where spatial inhomogeneities are of critical importance in
determining the surface chemistry. This stems from an inability to
produce an intense, yet highly focused, helium beam, which in turn derives
from previous difficulties in manipulating helium atoms.
Atom Optics and the Helium atom mirror
As technology continues towards successively smaller regimes, the need for
manipulation of atoms is becoming increasingly important. In the past,
advances in atom optics have demonstrated the manipulation of reactive,
ionised or excited atoms via interactions with magnetic or optical fields
. However, these techniques are not applicable to the focusing of
groundstate helium, and so cannot be used in the development of Helium Atom
Microscopy. Instead, we have developed an ultra-smooth mirror capable of
focusing an impinging beam of helium atoms into a nano-probe (figure 1).
A successful helium atom mirror must provide both a high reflectivity and
low aberrations. The mirror must exhibit perfection on both atomic
(less than nm) and macroscopic (greater than mm) length-scales and
will, in essence, be the most perfect mirror known to man. High
reflectivity depends upon microscopic considerations, where the mirror
surface must be smooth, highly crystalline and free from surface defects
or contamination; any deviation from a smooth surface results in diffuse
scattering away from the specular direction . On a macroscopic scale,
the mirror must be shaped to the Cartesian 'ideal' reflector surface, as
derived from geometric optics, if aberrations in the helium image are to
be avoided .
Our approach has been to separate the problems of macroscopic and
microscopic control. We use electrostatic actuation of a chemically
prepared silicon wafer to form the mirror surface. Whilst the
atomic-level perfection is governed by the wafer's chemical treatment, the
macroscopic shape is dependant upon the way in which the crystal is bent
Figure 2: The completed helium atom mirror and exploded schematic. A
thin Si(111)-(1x1)H crystal mirror is clamped between two thin annular
alumina discs and suspended above an electrode array. Application of a
potential difference between electrodes and crystal cause the crystal to
deflect into the lower alumina aperture. Careful control over the boundary
conditions set by the aperture and electrode size and dimensions afford
control over the macroscopic mirror shape. Control over the silicon
chemical preparation allows for atomic-scale perfection.
Geometric optics define the ideal shape of a mirror surface. In the
laboratory, we can approximate this shape by sucking a thin crystal
through a carefully defined aperture. The first key stage of this project
was therefore a detailed and systematic study of the ideal boundary
conditions required to optimise the mirror: this has been completed .
We have demonstrated that improved mirror design can reduce the
aberrations of previous experiments  by three orders of magnitude.
Phase stepping interferometry profilometry of a prototype have also been
made: these studies have demonstrated that our optimised design produces,
for the first time, the asymmetric, biaxial crystal profiles required for
off-axis, asymmetric imaging . The current mirror is shown in figure
Meanwhile, we have also researched the ultra-smooth crystal
surfaces required for microscopic control. We have chosen a hydrogen
passivated silicon surface, Si(111)-(1x1)H, for its stability in vacuum,
its ease of preparation and its unique surface perfection. The surfaces
are prepared in the cleanrooms of the Semiconductor Physics Group at the
Cavendish. Significant research into improved preparation and storage of
clean silicon mirrors has been completed. The wet-chemical preparation
that we have developed is an extension of that in the literature , but
we have been able to improve the surface quality of the Si(111)-(1x1)H
crystals used for atom reflection studies. We have been successful in
reducing the crystal's surface defect density and contamination levels,
thereby increasing the specular reflectivity of the mirror surface. An
image of a cleaned silicon surface, taken by Atomic Force Microscopy, is
given in figure 3.
Figure 3: Atomic Force Micrograph of the ultra-smooth Si(111)-(1x1)H
surface. Note that the vertical scale is 2 orders of magnitude smaller
than the xy scale. Several Si bilayer steps, each 0.38 nm high, are
evident. A few isolated surface defect particles are also evident.
Construction of a Scanning Helium Microscope (SHeM) Having
perfected the helium atom mirror, we are now a postion to contruct the
world's first scanning helium microscope. Such a device will combine the
unique advantages of HAS with spatial resolution, offering unprecedented
insight on a host of delicate and organic systems.
A schematic of a
possible SHeM is given in figure 4. The basic design is similar to that
of scanning electron microscopy, where an electron beam is rastered across
the sample and the backscattered electrons are collected to form an image.
For a SHeM, the electron source is swapped for an atomic beam source, to
form a beam of approximately 2 mm diameter. The beam is focused using the
helium atom mirror above, and directed onto the sample of interest. By
rastering the sample through the microprobe, detection of the
backscattered helium atoms will be used to produce an image. Several
different contrast mechanisms exist, including surface roughness contrast,
diffraction contrast and Debye-Waller (temperature) contrast. As the
helium atom beam interacts with only the outermost electronic
corrugations, the difference in reflectivity between the delocalised
electron sea of a metal and the corrugated surface of a semiconductor can
be an order of magnitude, offering a further contrast mechanism.
Figure 4: An idealised schematic of a possible scanning helium
D.A. MacLaren and W Allison (Cambridge)
This work is supported predominantly by the
EPSRC. Some collaborative
research was supported by The British
Council, under the British-German Academic Research Collaboration
(ARC) Programme, Project No. 1117.00. Our partners in Germany were the Max Planck Institut fur Stromungsforschung, Gottingen.
We have also recently embarked upon a new European Union funding
initiative (Framework 6 Programme) with several collaborators across the
European Union. Our role in this project is a more refined study of the
wet-chemistry outlined above.
More details on some of these references may be found in our publications page.
 B. Holst and W. Allison, "An atom-focusing mirror", Nature (London) 390, (1997), 244.
 D. Farías and K. Rieder, "Atom beam diffraction from solid surfaces", Rep. Prog. Phys. 61, (1998), 1575.
 S. Foulias, N.J. Curson, M.C. Cowen and W. Allison, "Growth and metallisation in potassium adsorption on the Si(100) surface", Surf. Sci. 331 - 333, (1995), 522.
 N. Camillone III, C.E.D. Chidsey, G-Y.Liu and G. Scoles, "Superlattic structure at the surface of a monolayer of octadecanethiol self-assembled on Au(111)", J. Chem. Phys. 98, (1993), 3503.
 K.H. Rieder, M. Baumberger and W. Stocker, "Selective transition of chemisorbed hydrogen to subsurface sites on Pd(111)", Phys. Rev. Lett. 51, (1983), 1799.
 N.J. Curson, H.G. Bullman, J.R. Buckland and W. Allison, "Molecular chemisorbed phases and 2-D silicide structures in the CVD of silane on Cu(111)", Phys. Rev. B 55, (1997), 10819.
 B. Poelsema and G. Comsa, "Scattering of thermal energy atoms from disordered surfaces", Springer Tracts in Modern Physics 115, (1989), Springer, Berlin.
 C.S. Adams, M. Sigel and J. Mlynek, "Atom Optics", Physics Reports 240, (1994), 143.
 R. J. Wilson, B. Holst and W. Allison, "Optical properties of mirrors for focusing of non-normal incidence atom beams", Rev. Sci. Instrum. 70, (1999), 2960.
 D.A. MacLaren, W. Allison and B. Holst, "Single crystal optic elements for Helium atom microscopy", Rev. Sci. Instrum. 71, (2000), 2625.
 H.T. Goldrein, D.A. MacLaren, P.J. Rae and W. Allison, "Phase-stepping interferometric profilometry of atom-optical mirrors", Oral presentation, Applied Optics and Optoelectronics Conference, (Loughborough), Sept. 2000.
 For example, G.J. Pietsch, U. Köhler and M. Henzler, "Anisotropic etching versus interaction of atomic steps: Scanning Tunneling microscopy observations on HF/NH4F-treated Si(111)", J. Appl. Phys. 73, (1993), 4797 and references therein.