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Annual Meeting Optical Society of America

Submicrometer direct-write optical lithography with cold atoms

Mara Goff Prentiss, Karl K. Berggren, Gregory L. Timp, Robert E. Behringer, J. E. Cunningham

Published Jan 1, 1992• 0 citations
Publisher

Abstract

We passed a transversely cooled sodium atomic beam through an optical standing wave, which channeled the atoms into the intensity minima (maxima) for light tuned below (above) the sodium 32S1/2 → 32P3/2 resonance frequency. The channeled atoms retained their spatial distribution when they were deposited onto the silicon surface. The surface was thus covered by a permanent periodic sodium grating with an area of 1 x 0.1 cm2 and a period of d = 294.3 ±0.3 nm, which is half the wavelength of the standing wave field. The entire grating was completed in 10 min. We measured the period of the grating by backscattering weak probe light. The probe light had a wavelength λ, which we varied between 5580 and 5800 Å, and was incident at an angle ϕ relative to the normal. There is constructive interference when the angle θ with respect to the normal to the grating is such that λ = d(sin θ + sin ϕ). The dependence of θ on ϕ allowed us to measure d. Semiclassical theory suggests the width of the lines in the grating could be of the order of 100 Å.

DOI
10.1364/oam.1992.thuu1

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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. 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•Jan 1, 2012•0 citations
We passed a transversely cooled sodium atomic beam through an optical standing wave, which channeled the atoms into the intensity minima (maxima) for light tuned below (above) the sodium 32S1/2 → 32P3/2 resonance frequency. The channeled atoms retained their spatial distribution when they were deposited onto the silicon surface. The surface was thus covered by a permanent periodic sodium grating with an area of 1 x 0.1 cm2 and a period of d = 294.3 ±0.3 nm, which is half the wavelength of the standing wave field. The entire grating was completed in 10 min. We measured the period of the grating by backscattering weak probe light. The probe light had a wavelength λ, which we varied between 5580 and 5800 Å, and was incident at an angle ϕ relative to the normal. There is constructive interference when the angle θ with respect to the normal to the grating is such that λ = d(sin θ + sin ϕ). The dependence of θ on ϕ allowed us to measure d. Semiclassical theory suggests the width of the lines in the grating could be of the order of 100 Å.
We show that light can be used as a lens to focus a collimated neutral atomic beam to submicron dimensions during deposition onto a substrate. We have used an optical standing wave at 589 nm as an array of cylindrical lenses to focus a perpendicular sodium beam into a grating on a substrate, with a periodicity of 294.3\ifmmode\pm\else\textpm\fi{}0.3 nm. This result is the first direct evidence of submicron focusing of atoms by light, and represents a fundamentally new scheme for submicron lithography.
Direct-write atom lithography is a technique in which highly periodic nanostructures are directly deposited on a substrate by patterning an atomic beam with a standing light wave, tuned near a resonance frequency of the corresponding atom. In this thesis an extension of this technique for the production of ferromagnetic nano- structures is investigated. Of all ferromagnetic elements Fe is the most suitable to per- form atom lithography. Light with a wavelength of 372 nm is needed to produce Fe nanostructures by atom lithography. Also, the atomic beam should be highly collimated, for which laser cooling is commonly used. For this purpose, a frequency doubled Ti:S laser tuned to 744 nm is constructed, capable of producing more than 100 mW of light at a wavelength of 372 nm. Laser cooling requires a laser with a frequency stability better than the natural linewidth of the atom. To meet this requirement, the laser is locked on the atomic transition by means of polarization spectroscopy on a hollow cathode discharge cell. The discharge cell produces a sufficient density of ground state Fe atoms at a temperature which is more than a factor of two lower than the temperature necessary to produce that density in a thermal cell. With this technique the laser is locked stable within 0.2 MHz for hours. A laser cooling simulation based on the semiclassical approach is presented. Although the intention was to simulate laser cooling of Fe with this code, for the commonly used pxpy polarization gradient configuration an analysis of the simulation results shows that the semiclassical approach is only valid for transitions with recoil parameters ¿r on the order of 10¡4 or less. For the standard laser cooling transitions only the transitions in Rb and Cs satisfy this condition. A drastic reduction in calculation time compared to quantum Monte-Carlo calculations is to be expected for these transitions by implementing an analytical approach to the long-term contribution of the diffusion coefficient. For Fe, full quantum Monte-Carlo calculations are necessary. Laser cooling of Fe on the 372 nm transition has been studied experimentally. This transition, although the only one realistically available, is far from perfect for laser cooling, as it is not a fully closed transition: it has a leak ratio of approximately 1/240 to other states. Nevertheless, a collimation of aRMS = 0.17 mrad could be achieved by laser cooling. However, contrary to the normally obtained central beam flux increase, the ground state atom flux decreased to approximately 70 % of the non-cooled value. Because of both the complexity of the laser cooling setup and the restrictions in perfor- mance, we have also attempted to produce Fe nanostructures without using laser cooling techniques, but simply relying on geometrical collimation of the atomic beam. In this way, Fe nanolines are deposited with a pitch of 186 nm, a FWHM width of 50 nm, and a height of up to 6 nm. This opens the way for applying direct-write atom lithography to a wide variety of elements. In a MOKE (Magneto-Optical Kerr effect)-setup we are able to verify that the deposited Fe-layer is ferromagnetic. However, the magnetic anisotropy due to the presence of the nanolines can not be detected with a MOKE-setup or a MFM (Magnetic Force Microscope). In contrast, the magnetization of background-free nano- lines, deposited through a mechanical mask with a pitch of 744 nm and a FWHM of
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.