Deliverables Resist assisted atom lithography Atom lithography has attracted much interest in the past decade for its potentials as a novel atomic nanofabrication (ANF) technique. Advantageous aspects of the method include parallel applications to a relatively large area exceeding 1mmxmm, negligible contribution of atomic diffraction to the minimal feature size because of the small de Broglie wavelength of the atoms, or low energy impact of the particles deposited on a substrate. Both mechanical and optical masks can be used to modulate the atomic beam density transferred to a substrate for structure generation. Specifically optical masks create periodic potentials that spatially segregate the atoms into an array of linear nanostructures spaced by a fraction of the laser wavelength. Due to its immaterial character, optical masks are defect-free, non-destructive and species-selective. ANF offers two main routes for applications: Resist assisted neutral atom lithography (NAL) and direct deposition (DD). For NAL an atomic beam induces chemical modification of a suitable surface, which is then transferred to the underlying substrate. One of the most attractive properties of ANF is the potential to pattern neutral atom beams to directly grow nanostructures on a substrate. Direct deposition experiments have been demonstrated for sodium, chromium, aluminium, ytterbium and more recently with iron. The investigation of direct deposition is, however, impaired by several aspects: DD in general calls for large doses (many monolayers) in order to generate significant structural heights of a few nanometre height; group III atoms, which are at the focus of this manuscript, are known to exhibit self aggregation into nanodroplets which deform the originally deposited atomic beam pattern. Resist-assisted neutral atom lithography, in contrast, offers a method to use low dosage atomic beams (few monolayers) to study the lateral distribution of atoms deposited onto substrates with nanometric resolution. The resist in our case is a self-assembled monolayer (SAM) of alkanethiols or organosilicon covering a gold or silicon surface. When a SAM is exposed to the atomic beam through a light or mechanical mask, the resist is chemically modified and wet etching transfers the pattern into the gold or silicon substrate. The chemical process governing the neutral atom - organic monolayer interaction that is very efficient is still not fully understood \cite we. Resist-assisted atom lithography was first demonstrated with metastable argon atoms and then extended to other noble gas atoms. Metastable noble gas atoms have a large internal energy (up to 20eV for He*) that can be released on surface impact and is considered to be responsible for chemical modification of the resist. With metastable noble gas atoms very low doses (even less than 1 atom per monolayer molecule) have been observed. It was later found that also caesium atoms cause a similar chemical modification of thiole-covered gold surfaces with threshold dosage of order 2 atoms per monolayer molecule. Very recently it was found that barium atoms offer yet another atom-thiole surface applicable for neutral atom lithography. This observation has initiated us to study the interaction of Ga and In atomic beams with thiole surfaces. The potential of atom nanofabrication and laser manipulation with these technologically relevant elements is currently studied. We have used Gallium and Indium atomic beams for resist-assisted atom lithography, showing the feasibility of the technique and estimating the threshold of the lithographic process. Our method sheds new light on the interaction of atomic beams with thiole covered gold surfaces, and it facilitates experimental investigations of the potential of group III elements for laser controlled atomic nanofabrication. Parallel mechanical masks Technological procedures were invented to fabricate serial as well as parallel working devices for shadow deposition of cold atom material on a sub-300nm scale. In case of serial deposition, cantilevers with integrated hollow tip carrying a 100nm aperture were used to deposit and write structures in a scanning vector oriented process. Technology for self-adjusted fabrication of apertures in the three-dimensional tip structure is based on the specific behaviour of carbon fluorine based silicon dioxide plasma etching. The detailed study of clogging behaviour of tips or apertures required parallel writing with arrays of aperture tips or apertures in plane membranes. In particular identifying written structures on plane substrates was substantially facilitated. In a first approach the same aperture tips were arrayed on a plane membrane. However, the geometry of tips (70.5 degree opening angle and 22µm base size) required 25µm spacing between tips to guarantee the mechanical stability of the membrane. Density of tips was substantially increased using conventional e-beam lithography followed by subtracting methods to obtain pierced membranes. In this way 3x3mm² membranes were realized carrying hundreds of apertures. Caesium nanostructures Nanostructures through direct deposition of cold Caesium atoms were grown onto a surface. This is a challenging and complex task for several reasons: Caesium is highly reactive in air, hence UHV operation of the whole experiment (manipulation, deposition, investigation) is required; the use of a laser cooled atom beam implies deposition in a sub-thermal regime, that, in principle, is remarkably different with respect to conventional experiments, where effusive beams are used. As a consequence, poor knowledge of the specific diffusion and growth phenomena expected in this regime can be retrieved in the literature. The cold atomic beam of the apparatus is produced continuously out of a pyramidal magneto-optical trap (MOT) for Caesium atoms, a specific configuration of mirrors mounted as a hollow pyramid, with a small apical aperture. Direct deposition of the cold Caesium atoms onto a surface was carried out, combined with in-situ microscopic diagnostics through UHV Scanning Tunnelling Microscopy. Direct deposition of cold Caesium atoms in the presence of standing wave optical mask was carried out by using HOPG substrates. It must be noted that, observation of a regular array of Caesium nanolines is made difficult due to the relatively small dimensions of the atomistically flat regions of the substrate. As a consequence, we concentrated our efforts so far in the investigation of single, isolated lines. Their alignment relative to the substrate is in perfect agreement with the expectations (i.e., structures are orthogonal to the standing wave direction), and their space extension appears in some images quite remarkable (almost continuous lines are observed with a length in the mm range). Besides continuous line, also structures composed of isolated mutually aligned atoms, as well as zig- zag lines have been imaged, with a typical height corresponding to that of a few atoms. Further work will be devoted to clarify the role of process parameters and of substrate features in determining the shape of the deposits. The final assessment of the lateral resolution requires further work, including the use of different substrates and a comprehensive investigation of the role of the process parameters, yet the present preliminary data indicate a lateral confinement of the deposited atoms within a transverse size of the order of 10 nm, and a negligible contribution from background (unfocused) atoms, at least for the process parameters explored so far. Searching for OpenAIRE data... There was an error trying to search data from OpenAIRE No results available
