Cold atom deposition for nanoscale structuring of surfaces

Atom lithography [1] has attracted a great interest in the scientific community as a technique for the fabrication of nanometer-scale ordered structures with a relatively simple apparatus. The atom lithography approach is similar to conventional optical litography, the light beam and the mechanical mask being replaced by an atomic beam and an immaterial light mask, respectively. Nanostructures are produced by space segregation of the atomic beam, due to its interaction with a quasi-resonant standing wave, that allows to realize ordered structures with interferometric precision and a space resolution well below 100 nm through either direct deposition or resist-assisted processes. The ultimate limits of the technique and the role of the interactions between the deposited atoms and the substrate are under recent debate. In order to better understand the relevant growth processes and the limits of atom lithography in terms of space definition, we have built an apparatus where a continuous beam produced out of a pyramidal-MOT funnel is used. The funnel is followed by a collimation stage, based on 2D optical molasses along the transverse directions. The main advantage of this choice is the lower translational velocity of the atoms composing the beam with respect to conventional effusive sources (10 m/s vs hundreds of m/s typical for thermal beams). This leads to a larger interaction time during the collimating and immaterial mask (focusing) stages, which is expected to give rise to sharper deposited structures thanks to a significant reduction of aberration effects and of the amount of uncollimated/unfocused atoms arriving onto the substrate. The experimental set-up exploits a pyramidal funnel, made of two prisms and two mirrors arranged in the shape of an inverted pyramid with a small hole (1mm x 2 mm) at its apex, mounted on a stainless-steel holder inside an ultra high vacuum system. The light for manipulating cesium atoms is produced by diode lasers operating around 850 nm, mounted in the external cavity configuration. The quadrupolar magnetic field for the pyramidal-MOT operation is produced by two coils in anti-Helmholtz configuration. We have characterized the atomic beam leaving the funnel [2], i.e., we have measured its density, divergence, longitudinal velocity and temperature. Induced fluorescence images, acquired using a CCD camera, and absorption measurements show a beam divergence of 25 mrad, a longitudinal velocity in the range 10-15 m/s (depending on the trapping parameters) and a flux up to 4 x 109 atoms/s. The atomic beam diameter (FWHM) is 1 cm measured 30 cm downward the funnel apex. The collimation stage is based on 2D optical molasses. The collimating laser beam is elliptical in shape (13 mm x 6 mm), with the longer axis along the longitudinal direction, which leads to an interaction time of 1 ms. We have explored both the "lin perp lin" and the σ+σ - polarization configuration. With the "lin perp lin" configuration we found a minimum divergence of 8 mrad at a collimating laser detuning of -9Γ (Γ being the cesium natural linewith, 5.13 MHz), limited by the presence in the collimation region of stray magnetic fields produced by the MOT coils. We were able to reach less than 2 mrad (corresponding to the measurement uncertainty) in beam divergence with the σ+σ - polarization configuration. The collimation stage allows us also to reduce the beam diameter to 4 mm at 30 cm from the funnel apex, with an increase of the atomic beam intensity relevant for our applicative purposes. The next step of the experiment will be deposition of cesium through a light mask (a one-dimensional laser standing wave) on several kinds of substrates. Currently we are working on the deposition of nanostructured cesium beam on Self Assembled Monolayer (a nonanethiol layer grown on a gold substrate). Our preliminary results demonstrate that the SAM is efficiently impressed by the arrival of the laser-cooled cesium atoms. By the time of the Conference we expect to obtain nanostructures (arrays of parallel planes, spaced by half the laser wavelength) through resist-assisted atom lithography. The work is supported by EC through RTD-IST "NANOCOLD", and by CNR through Progetto Applicativo "Nanotecnologie". [1] J.J.McClelland, "Nanofabrication via Atom Optics", in Handbook of Nanostructured Materials and Technology, Academic Press, Cambridge, (1999). [2] A. Camposeo, A. Piombini, F. Cervelli, F. Tantussi, F. Fuso, and E. Arimondo, Optics Comm. 200 231 (2001