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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Atom Nanolithography with Atom Pinhole Camera

V. I. Balykin, P. A. Borisov, Vladilen S Letokhov, Павел Николаевич Мелентьев, S. N. Rudnev, A. P. Cherkun, A. P. Akimenko, Pavel Yu. Apel, Vladimir A. Skuratov

Published Jun 1, 2007• 0 citations
Publisher

Abstract

The most efficient method for controlling the motion of material particles in experimental physics is the use of interaction potentials with electromagnetic fields. The static magnetic and electric interaction potentials are used in the optics of charged particles. In the optics of neutral atoms, the various interaction potentials of atoms with laser and electromagnetic fields are used in addition to the static fields. The effective methods of diffraction, interference, mirror reflection of atoms are implemented in atom optics. The most difficult problem in atom optics is the problem of high-resolution focusing of neutral atoms, which is promising for the nondestructive method for probing the surface at the atomic level, as well as for the creation of nanostructures on the surface. Although there are many proposals for focusing of atomic beams, this problem is experimentally unsolved. The main difficulty is the creation of the interaction potential of the atom with the electromagnetic field that is close to an ideal lens for atoms.In this paper, we experimentally implement another approach to the problem of focusing and construction of an image in atom optics, which is based on a well known idea of optical pinhole camera. The pinhole camera in optics is a camera without lens. Light forming an image passes through a small hole. In our experiment with the atom pinhole camera the Cr atomic beam passes through a set of holes in a metal mask and thereby forms, by analogy with optics, a glowing object of a given geometry. The atoms pass through the holes in the mask, propagate in vacuum along rectilinear trajectories, similar to light rays, and are incident on a thin film (thickness 5 mum) placed at a distance of 90 mm from the mask with a large number (n =107-108/cm2) of nanoholes (50 nm diameter). Each hole of the film is a pinhole camera for atoms, which forms its individual image of the object on the substrate surface placed at a distance of l = 5mum behind the film.

DOI
10.1109/cleoe-iqec.2007.4386700

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Atom “pinhole camera” with nanometer resolution

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An atom “pinhole camera” with nanometer resolution has been experimentally implemented for the first time. Owing to the use of this camera, an array of ∼10 6 identical nanostructures of Cr atoms with a characteristic size of the nanostructure of less than 50 nm has been created on a glass surface. Nanostructures of arbitrary shapes have been created.

Journal of Experimental and Theoretical Physics Letters•Dec 1, 2006•23 citations

Atom lithography:creating patterned magnetic layers

T. Meijer

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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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.

•Jan 1, 2012•0 citations

Atom lithography of iron

te E Edwin Sligte

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.

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Focusing of a neutral helium beam with a photon-sieve structure

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The manipulation of low-energy beams of neutral atoms and molecules via their de Broglie wavelength is a branch of atom optics often referred to as de Broglie matter wave optics. The application areas include fundamental quantum mechanics, atom interferometry, and the development of new microscopy instrumentation. The focusing of de Broglie matter waves with a Fresnel zone plate was used to demonstrate the first neutral helium microscopy imaging. The ultimate resolution of such a microscope is limited by the width of the outermost zone. Because a Fresnel zone plate for atoms cannot be fabricated on a substrate (the low-energy atom beams would not be able to penetrate the substrate material), this gives a fabrication determined limit for the first-order focus of around 30--50 nm. Therefore, it is important to search for alternative optical elements that enable higher resolution. Photon sieves consist of a large number of pinholes, arranged suitably relative to the Fresnel zones. The great advantages are that the width of the pinholes can be larger than the respective Fresnel zones and a free-standing pinhole is much easier to fabricate than a free-standing zone. Thus, with a photon-sieve structure applied for de Broglie matter wave manipulation, the fabrication limit for focusing is reduced to potentially around 3--5 nm. Here we present a realization of such an ``atom sieve,'' which we fabricated out of a silicon nitride (SiN) membrane, using electron-beam lithography and reactive ion etching. Our atom sieve is 178 $\ensuremath{\mu}\mathrm{m}$ in diameter and has 31 991 holes. The diameter of the holes varies from 1840 to 150 nm. Using a beam of neutral, ground-state helium atoms with an average wavelength of 0.055 nm, we demonstrate helium atom focusing down to a spot size of less than 4 $\ensuremath{\mu}\mathrm{m}$. The focus size is limited by the intrinsic velocity spread of the helium beam.

Physical Review A•Apr 8, 2015•22 citations
The most efficient method for controlling the motion of material particles in experimental physics is the use of interaction potentials with electromagnetic fields. The static magnetic and electric interaction potentials are used in the optics of charged particles. In the optics of neutral atoms, the various interaction potentials of atoms with laser and electromagnetic fields are used in addition to the static fields. The effective methods of diffraction, interference, mirror reflection of atoms are implemented in atom optics. The most difficult problem in atom optics is the problem of high-resolution focusing of neutral atoms, which is promising for the nondestructive method for probing the surface at the atomic level, as well as for the creation of nanostructures on the surface. Although there are many proposals for focusing of atomic beams, this problem is experimentally unsolved. The main difficulty is the creation of the interaction potential of the atom with the electromagnetic field that is close to an ideal lens for atoms.In this paper, we experimentally implement another approach to the problem of focusing and construction of an image in atom optics, which is based on a well known idea of optical pinhole camera. The pinhole camera in optics is a camera without lens. Light forming an image passes through a small hole. In our experiment with the atom pinhole camera the Cr atomic beam passes through a set of holes in a metal mask and thereby forms, by analogy with optics, a glowing object of a given geometry. The atoms pass through the holes in the mask, propagate in vacuum along rectilinear trajectories, similar to light rays, and are incident on a thin film (thickness 5 mum) placed at a distance of 90 mm from the mask with a large number (n =107-108/cm2) of nanoholes (50 nm diameter). Each hole of the film is a pinhole camera for atoms, which forms its individual image of the object on the substrate surface placed at a distance of l = 5mum behind the film.
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.
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.
In the field of atom optics, there has been much interests in developing components such as lenses, mirrors, beam splitters and waveguides to manipulate neutral atoms. Especially many clever methods to focus neutral atoms have been developed. The lenses have been constructed using the static electrical and magnetic fields, Fresnel zone plates, and radiation pressure forces. Before the laser cooling and trapping techniques were invented, the electrostatic fields could not be used for manipulating neutral atoms because of its weak interaction energy with atoms. After the cold atoms were available, it has been possible to manipulate atoms with weak electrostatic interaction energy. We developed an atom lens system using electrostatic fields.
We studied the focusing of atoms by multiple layers of standing light waves in the context of atom lithography. In particular, atomic localization by a double-layer light mask is examined using the optimal squeezing approach. Operation of the focusing setup is analyzed both in the paraxial approximation and in the regime of nonlinear spatial squeezing for the thin-thin, as well as thin-thick, atom lens combinations. It is shown that the optimized double light mask may considerably reduce the imaging problems, improve the quality of focusing, and enhance the contrast ratio of the deposited structures.
The manipulation of low-energy beams of neutral atoms and molecules via their de Broglie wavelength is a branch of atom optics often referred to as de Broglie matter wave optics. The application areas include fundamental quantum mechanics, atom interferometry, and the development of new microscopy instrumentation. The focusing of de Broglie matter waves with a Fresnel zone plate was used to demonstrate the first neutral helium microscopy imaging. The ultimate resolution of such a microscope is limited by the width of the outermost zone. Because a Fresnel zone plate for atoms cannot be fabricated on a substrate (the low-energy atom beams would not be able to penetrate the substrate material), this gives a fabrication determined limit for the first-order focus of around 30--50 nm. Therefore, it is important to search for alternative optical elements that enable higher resolution. Photon sieves consist of a large number of pinholes, arranged suitably relative to the Fresnel zones. The great advantages are that the width of the pinholes can be larger than the respective Fresnel zones and a free-standing pinhole is much easier to fabricate than a free-standing zone. Thus, with a photon-sieve structure applied for de Broglie matter wave manipulation, the fabrication limit for focusing is reduced to potentially around 3--5 nm. Here we present a realization of such an ``atom sieve,'' which we fabricated out of a silicon nitride (SiN) membrane, using electron-beam lithography and reactive ion etching. Our atom sieve is 178 $\ensuremath{\mu}\mathrm{m}$ in diameter and has 31 991 holes. The diameter of the holes varies from 1840 to 150 nm. Using a beam of neutral, ground-state helium atoms with an average wavelength of 0.055 nm, we demonstrate helium atom focusing down to a spot size of less than 4 $\ensuremath{\mu}\mathrm{m}$. The focus size is limited by the intrinsic velocity spread of the helium beam.