Soft Lithography for Surface Micropatterning

Abstract

In the first part of this work, a literature review of soft lithographic techniques is presented. Soft lithography includes a number of techniques for the fabrication of two- and three dimensional microand nanostructures based on molding, embossing and printing. In the experimental part of this work, two soft lithographic methods of surface micropatterning are used in order to produce an array of gold dots onto a substrate. The methods used are microcontact printing and transfer printing. Different master designs and their fabrication is presented, followed by stamp fabrication. The masters are fabricated using conventional microfabrication techniques and photolithography. Master designs tested are inverted pyramid shape fabricated by anisotropic etching of silicon, a high aspect ratio master and a low aspect ratio photoresist master. The elastomeric stamps are negative replicas of the masters and they are fabricated of poly(dimethylsiloxane) (PDMS). The stamps have a regular array of protruding features, in order to make a contact to the substrate during printing. Stamp designs chosen for microcontact printing are the pyramid stamp and the low aspect ratio stamp. In microcontact printing experiments eicosanethiol solution is used as the ink with which the stamp is wetted and the pattern of the stamp printed onto a gold-coated substrate. Both wet and contact inking methods are tested. Eicosanethiol forms a self assembling monolayer (SAM) onto the gold surface. The printed pattern is used as an etch mask in the subsequent wet of etching gold, which is performed in a solution of thiourea and ferric(III)nitrate. After the etching, an array of gold dots remains on the substrate. Stamp design chosen for transfer printing is the low aspect ratio stamp. In the version of transfer printing used in this work, eicosanethiol forms a layer on the silicon dioxide film, without the formation of a SAM. A gold-coated PDMS stamp is used to transfer a gold pattern onto the pre-treated silicon substrate. The results of microcontact printing show that the features are smaller at the edges of the printed area, possibly due to uneven pressure during printing or unevenness of the stamp itself. The mask used in the fabrication of the low aspect ratio stamps was not of high enough quality to produce homogenous stamps, thus leading to a periodical size variation in the prints made with the low aspect ratio stamps. Therefore, pyramid stamp is better suited for microcontact printing of gold dots. Diffusion of the thiols causes pattern distortion in microcontact printing; contact inking diminishes diffusion and produces smaller patterns than wet inking. In transfer printing the pattern is overall regular and smooth and the periodical size variation is not visible. The low aspect ratio stamp is well suited for transfer printing. Both methods are suited for the gold dot array fabrication. The diameter of the dots created with these methods is in the range of 1 – 2 µm.

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Soft Lithography for Surface Micropatterning