Surface percolation and growth (SPAG), an innovative approach to far field optical nano-lithography.

DARÍO KUNIK; LIA I PIETRASANTA; OSCAR E. MARTÍNEZ

Zurich Switzerland

Conferencia; CLEO (Conference on Lasers and Electro-Optics)/Europe-EQEC 2009.; 2009

A nanolithography scheme is presented by which the structure size can be controlled in the tens of nanometer range by means of the initiation of the polymerization mediated by a very inefficient energy transfer from a fluorescent dye molecule after single photon absorption. The mechanism relies in the following distinctive steps: 1- Mixture of the dye molecules and polymer in adequate proportions. 2- Deposition of the mixture on a transparent substrate. 3- Adsorption of a fraction of the dye molecules on the substrate material. 4- Illumination of the mixture with a focused beam through the substrate at the absorption band of the dye. 5- Initiation of the polymerization by due to a very low quantum yield energy transfer from the dye to the polymer. 6- Percolation of the structure growing at the substrate surface. 7- Deposition of fresh dye molecules from the bulk onto the growing surface. 8- Photoinitiation of the fresh molecules as the structures grows.We have modelled the different steps of the process and it can be shown that the percolation step is critically dependent on the bulk and surface dye concentration and that after the percolation threshold is reached the growth mechanism can be modelled as a sequence of dynamic deposition of fresh dye and subsequent triggering of the polymerization. The model was tested experimentally and in the figure one example is given where for one particular dye concentration the height of the structure is plotted as a function of the exposure time (determined by the scan speed and beam size). The height was determined for each point by AFM topographies. Three typical situations are illustrated in the topographies of lines scans shown in the figure. One is below the percolation threshold, a second one just at the threshold and a third one well above such threshold.The dye-polymer mixture used was the same that was used for two photon litography before [1,2], but in that case the polymerization was initiated by another UV sensitive molecule. As in this case a single photon process is used with a laser dye as initiator, STED [3] schemes should be applicable for lateral dimension reduction, hence having the technique a potential 3D 20nm resolution.