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Photorefractive Effects, Materials, and Devices

Atom lithography with reflection holograms in lithium-niobate crystals

Konrad Peithmann, M. Flaspöhler, K. Buse, Markus Mützel, S. Tandler, D. Haubrich, Dieter Meschede

Published Jan 1, 2003• 0 citations
Publisher

Abstract

Atom lithography uses neutral atoms, e. g. cesium, to produce small structures with nanometer resolution on wafers by transforming a light intensity pattern into a density pattern of an atomic beam. In order to achieve a 2D complex structure, two elementary holographic gratings are written in an iron-doped lithium- niobate crystal, using light with the wavelength 852 nm. Illuminating the crystal with the reference wave simultaneously reconstructs two signal waves, and these three beams interfere and create a light pattern with two different grating periods. This light pattern is imprinted onto a beam of cesium atoms. A characterization of the ’’holographic mirror” as well as the results of the atom lithography experiment are presented, and some further extensions and perspectives of this new method are discussed.

DOI
10.1364/pemd.2003.697

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Atom lithography uses neutral atoms, e. g. cesium, to produce small structures with nanometer resolution on wafers by transforming a light intensity pattern into a density pattern of an atomic beam. In order to achieve a 2D complex structure, two elementary holographic gratings are written in an iron-doped lithium- niobate crystal, using light with the wavelength 852 nm. Illuminating the crystal with the reference wave simultaneously reconstructs two signal waves, and these three beams interfere and create a light pattern with two different grating periods. This light pattern is imprinted onto a beam of cesium atoms. A characterization of the ’’holographic mirror” as well as the results of the atom lithography experiment are presented, and some further extensions and perspectives of this new method are discussed.
Atom lithography is a technique to structure layers of atoms during deposition, using interactions of near-resonant light fields with neutral atoms. The basic scheme uses a standing wave light field, aligned just above a substrate, while a beam of atoms impinges perpendicularly on the standing wave light field. The interaction of the light field and the atoms focuses the atoms towards the crests or troughs of the standing wave intensity, dependent on the tuning of the light field. The focused atoms are subsequently deposited on the substrate, resulting in an array of lines, whose periodicity is determined by the wavelength of the light field. The standing wave light field can therefore be seen as an array of lenses, focusing the atom beam into an array of lines. The first experiments on atom lithography were performed in the 1990s with sodium and chromium. In this thesis experiments with iron are described, which is a magnetic element and thus allows for the creation of magnetic nanostructures. To focus atom beams into structures using atom lithography, well-colimated atom beams are required. The most common technique to create collimated atom beams is transverse laser cooling, but iron has no closed optical transition suitable for laser cooling. Fortunately Smeets and Te Sligte found that even then atom lithography of iron is possible. If the standing wave light field is considered to be an array of lenses, the focusing of the atom beam on a single lens is determined by the local collimation of the atom beam at that position. This local collimation is determined by the geometry of the beam source and the distance of the beam source to the standing wave lens. Using a small beam source (e.g. 1 mm diameter) positioned far from the standing wave (e.g. 1 m), the geometrically collimated atom beam is suitable for atom lithography. Chapter 3 shows what the implications of geometric collimation are on the structure formation in atom lithography. The geometric effects that locally collimate the atom beam on the sample also introduce local offset angles of the atom beam with respect to the focusing light field. Structure formation has been observed with lines of up to 7 nm high on a background layer of 15 nm average height, over areas of up to 400 µm × 6 mm. This indicates that the offset angles can vary over an 8 mrad range, which is more than an order of magnitude larger than the local angle of collimation. The offset angle influence the local geometry of the deposited structures: an increasing offset angle decreases the local line height, increases the width and creates a skewness in the local line shape. The local geometry of the lines has been modelled with a Monte Carlo particle tracking model and the results were compared to experiments. It was found that the highest and narrowest lines suffer from significant broadening due to surface diffusion in experiments, limiting the achievable structure width of iron on SiOx samples to about 80 nm. We have also found that in atom lithography without laser cooling, each standing wave lens is effectively imaging the source geometry onto the substrate. We therefore propose using structured atom beam sources to image more complex patterns on sub-wavelength scales in a parallel way. The reason to use iron in atom lithography, is the option to create magnetic nanostructures. However, in previous experiments no clear magnetic signature of the atom lithographic structuring of iron was found. Therefore in Chapter 4 firstly the magnetic properties of unstructured layers of iron (Fe), nickel (Ni) and FexNi1-x are investigated. These layers are all ferromagnetic, but with a reduced magnetic moment compared to bulk values due to contaminations and crystal growth effects. As a reference for periodic atom lithography structures, periodic line structures are created with interference lithography in a polymer layer and onto these structures thin layers of FexNi1-x are shadow deposited, thereby creating a layer periodically modulated in both height and elemental composition. This periodic modulation in composition has been observed with SEM-EDX on sub-micron scales. The structures showed a clear magnetic anisotropy with an easy axis along the direction of the lines, in accordance with expectations. In Chapter 5, the magnetic properties of atom lithographic iron line structures are presented. Using MOKE microscopy, direct comparison of structured and unstructured parts of a sample is possible, which allows for the observation of clear magnetic signatures due to the atom lithographic structuring. In layers of an average thickness of about 15 nm, no anisotropy is induced by the line structures, while a magnetic easy axis along the direction of the lines is expected. The line structures introduce an isotropic increase in the coercivity of the layers, indicating that the line structures can be considered as isotropic corrugations instead of directional line structures. For layers of 30 nm average thickness and thus higher line structures, a clear magnetic anisotropy is observed for the highest line structures. This anisotropy can be seen as a magnetic easy axis along the direction of the lines, as expected for line structures. However, we also observe a sharp increase in coercivity for applied fields perpendicular to the lines. This phenomenon is intriguing and may be explained by a pinning of head-to-head domain walls along the direction of the lines. In Chapter 5, we also report on the first results of co-deposition of FexNi1-x, where the Fe is structured into lines using atom lithography, while the Ni is deposited uniformly, thereby creating an alloy of modulating composition. Magnetic analysis of these structures indicates that these structures are anisotropic, both the coercivity and the shape of domain walls is dependent on the angle of the applied field relative to the line structures. Finally in Chapter 6, we investigate the possibilities to focus a thermal beam of atoms into a single 100 nm spot using light fields, thereby creating a nanopencil suitable for deposition of nanostructures. A Monte Carlo particle tracking program was developed to model the all optical focusing or funneling of 10 µm sized atom beams to a 100 nm spot. This model included effects of initial beam divergence, magnetic substructure, laser cooling, and spontaneous as well as stimulated diffusion and it was applied to a number of promising light field configurations. The best results obtained for experimentally realistic settings are a beam focused to a full width at half maximum of FWHM = 0.55 µm in a blue detuned hollow beam where atoms are focused towards the dark center, or a FWHM = 0.31 µm in a red detuned axicon light field, where atoms are focused towards the maximum intensity at the heart line. However, in the latter case the focused beam has a flux only 25 times larger than the background flux, which is unpractical for applications. The limiting factors of all schemes are heating due to stimulated diffusion and the limited interaction time available for a thermal beam of atoms. A nanopencil could be possible if it were based on the focussing of a slow and / or monochromatic atom beam, but this would seriously complicate the practical application of such a device.
Atomic beam holography is a unique technique by which to fabricate a nanoscale object by using the interference of matter waves of ultracold atoms. The atomic de Broglie wave coming through a hologram interferes with and reconstructs a desired structure on a substrate. We first demonstrated holographic manipulation of metastable Ne atoms of 7 nm wavelength, where we showed the reconstruction of an “F” pattern and improvement of it. In this article, we propose and show techniques to manipulate three-dimensionally cooled Ne atoms. Atomic beam holography has the potential to make atomic-size objects, and the freedom in designing the hologram enables us to make not only a binary pattern but also an analog (gray scale) pattern. The resolution demonstrated in this experiment was about 60μ, but the minimum feature size expected is theoretically the same as the wavelength, i.e., several nanometers.
Slow neutral beams of metal atoms can be manipulated using the intensity gradient of near-resonant light-fields enabling the deposition of atoms onto a substrate in a processes often referred to an atom lithography. A suitably shaped light-field gradient is used to control the path of metal atoms using the dipole force created by the interaction between atoms and the strong, near-resonant optical intensity gradient. Relatively simple patterns such as lines and dots have been created using optical standing waves while more complex light-fields might be created using computer generated optical elements to manipulate the laser beam.
Atom lithography is a technique in which a light field is used to pattern an atomic beam. This patterned flux is then deposited onto a substrate, resulting in a nanostructured thin film. The smallest structures that have been made thus far using this technique are around 30 nm wide. This thesis investigates the technique, expanding its possibilities. The work-horse for the development of atom lithography has been Cr, as this transition metal atom has a closed transition in a wavelength range that is accessible to dye lasers. We extend the technique to Fe, the first ferromagnetic element to be used for atom lithography. The setup that was used to do this experiment is described, along with its critical design parameters. We present nanostructures that are typically 50 nm wide, and up to 4 nm high. The spacing between the nanolines is 186.05 nm. The nanostructure profile is compared to that of a simulated deposition process, and found to match. To the authors’ knowledge, this is the first demonstration of direct write atom lithography without laser cooling. A preliminary incursion into the magnetic properties of the nanostructures deposited is presented. In addition to giving an overview of the general ferromagnetic properties that might be expected, a deeper investigation of the magnetic anisotropy of the nanostructures deposited in this experiment is given. Novel resonant light masks are used in an experiment performed at the University of Konstanz (Germany). These light masks, using exactly instead of nearly resonant light, reveal some intriguing quantum mechanical effects. The most important of these features is the possibility to place structures closer together – at quarter wavelength spacings rather than half-wavelength intervals. Finally, the influence of surface diffusion on the structures obtained in an atom lithography experiment is investigated using kinetic Monte Carlo simulations. Several diffusion limiting effects are investigated; the influence of small amounts of residual reactive background gas is found to describe the experimental observations.
Atom lithography uses standing wave light fields as arrays of lenses to focus neutral atom beams into line patterns on a substrate. Laser cooled atom beams are commonly used, but an atom beam source with a small opening placed at a large distance from a substrate creates atom beams which are locally geometrically collimated on the substrate. These beams have local offset angles with respect to the substrate. We show that this affects the height, width, shape, and position of the created structures. We find that simulated effects are partially obscured in experiments by substrate-dependent diffusion of atoms, while scattering and interference just above the substrate limit the quality of the standing wave lens. We find that in atom lithography without laser cooling the atom beam source geometry is imaged onto the substrate by the standing wave lens. We therefore propose using structured atom beam sources to image more complex patterns on subwavelength scales in a massively parallel way.