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Atom Optics and Helium Atom Microscopy

Building on the success of experiments completed recently at the Cavendish, the goal of this exciting project is to develop the world's first atom- beam microscope.

Background - Helium Atom Scattering

The use of helium as a structural probe in surface science has long been appreciated [2]. De Broglie's relation shows that the wavelength of a particle varies inversely as the particle's mass. Thus, even at thermal energies, an helium beam has the subnanometre wavelengths required for atomic resolution. As the helium beam is also uncharged and inert, it allows an entirely non-destructive and non-invasive means of studying the most delicate surface structures. In contrast to other structural probes, such as electrons or x-rays, helium atom scattering (HAS) is entirely surface sensitive and is dominated by scattering which involves no energy transfer between probe and sample. As a result, HAS is of fundamental importance in observing, in-situ, the growth mechanism of thin metal films and molecular adsorbates, without interacting with or disrupting the specimen itself (for example, [2-7]). Due to an unusually large interaction cross-section with hydrogen, HAS is also uniquely placed to observe the interactions of hydrogen and other light adsorbates with surfaces [7]. Despite the profound impact that hydrogen can have on surface physics and chemistry, particularly catalysis and metallurgy, it is not readily observed directly by other techniques.

Figure 1: Focused beam cross-section, showing normalised intensity vs direction (in mm). Optimum results at this stage produced a final spot diameter of 210 microns [1].

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 [8]. 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 [2]. 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 [9].

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 into shape.

Mirror Mount

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 [10]. We have demonstrated that improved mirror design can reduce the aberrations of previous experiments [1] 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 [11]. The current mirror is shown in figure 2.

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 [12], 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.

AFM of Si(111)

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.

Microscope Schematic

Figure 4: An idealised schematic of a possible scanning helium microscope.

D.A. MacLaren and W Allison (Cambridge)
B Holst (Gottingen)

Acknowledgements

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.

References

More details on some of these references may be found in our publications page.
[1] B. Holst and W. Allison, "An atom-focusing mirror", Nature (London) 390, (1997), 244.
[2] D. Farías and K. Rieder, "Atom beam diffraction from solid surfaces", Rep. Prog. Phys. 61, (1998), 1575.
[3] 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.
[4] 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.
[5] 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.
[6] 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.
[7] B. Poelsema and G. Comsa, "Scattering of thermal energy atoms from disordered surfaces", Springer Tracts in Modern Physics 115, (1989), Springer, Berlin.
[8] C.S. Adams, M. Sigel and J. Mlynek, "Atom Optics", Physics Reports 240, (1994), 143.
[9] 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.
[10] D.A. MacLaren, W. Allison and B. Holst, "Single crystal optic elements for Helium atom microscopy", Rev. Sci. Instrum. 71, (2000), 2625. [11] 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.
[12] 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.

DAM