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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.

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Bonn Universitaet, Institut fur Angewandte Physik
53115 Bonn
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