Structure and Stability of Patterned Self-Assembled Films of Octadecyltrichlorosilane Formed by Contact Printing

Abstract

The structures of thin films of octadecyltrichlorosilane (OTS) formed by contact printing and adsorption from solution on Al and SiO 2 /Si surfaces have been investigated by X-ray photoelectron spectroscopy (XPS), ellipsometry, reflection−absorption infrared spectroscopy (RAIRS), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The structures of the OTS films were found to be strongly influenced by the method of preparation used. The films formed by contact printing for 30 s with an OTS “ink” are composed of close-packed, predominantly all-trans alkyl chains aligned nearly perpendicular to the surface, while films formed by adsorption of OTS for 30 s from a solution of comparable concentration are made up of sparsely adsorbed and more randomly oriented chains. Large changes in the film structure (from conformationally disordered, poorly oriented to well-packed, highly oriented hydrocarbon chains) were observed as the reaction time was increased for samples prepared by immersion in an OTS-containing solution (e.g. 15−30 min in a 1 mM solution). On the other hand, films prepared by contact printing reached or exceeded full monolayer mass coverages after only 30 s of substrate contact with an elastomeric stamp inked with a 10 mM solution of OTS in hexane. The OTS films formed by contact printing are stable at the high temperatures and aggressive reaction conditions necessary for their use as molecular resists in directing the selective deposition of metal and ceramic thin films by MOCVD and sol-gel methods, respectively. In this paper we show how such factors as the OTS ink concentration and stamp contact time influence the patterning of OTS thin films by microcontact printing. We find that reactive spreading and island formation of OTS domains in regions of the pattern not in contact with the stamp limit the fidelity of the patterning carried out at dimensions less than a few microns. We discuss several aspects of the physical processing which mediate these effects and propose possible methods for eliminating or greatly reducing them.