Large area ordered lateral patterns in confined polymer thin films Hao-Li Zhang, Toshiki Okayasu, David G. Bucknall * Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK Received 12 July 2003; received in revised form 20 January 2004; accepted 21 January 2004 Abstract The thermal stability of a model system consisting of a polymer film on a Si substrate capped by a thin metal layer has been investigated. When the model system is heated to a sufficiently high temperature characteristic surface wrinkling structures are formed with well defined periodicity over large areas. It is suggested that this wave-like surface morphology is driven by the thermal expansion coefficient mismatch of the different layers. A mechanism based on bending of a thin stiff surface on a thin elastic medium has been adopted to predict the pattern periodicity which gives satisfactory results with the experimental values. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Polymers; Topography 1. Introduction During the past few decades, there has been a dra- matic increase in the research on fabrication of polymer based opto-electronic devices, such as light emitting diodes [1] and thin film transistors [2]. Patterning poly- mer structures into a predetermined morphology is of great importance for these kinds of applications [3]. Conventional photolithography has been very successful in fabricating silicon based devices, but it is not always compatible with processing organic materials such as conductive polymers. One of the biggest problems with photolithography is that the solution used for develop- ing photo-resist may cause damage or contamination to the conducting polymer layers. Therefore, efforts have been made to develop other methods more suitable for patterning polymer materials. Many approaches have been studied, among which nanoimprinting [4] and self- assembly (SA) [5,6] are probably the most widely investigated methods for patterning polymer thin films. The nanoimprinting approach replicates the topo- graphic pattern from a prefabricated rigid mold into polymer films via an embossing process. In SA ap- proaches, the polymer thin film structure is influenced by the interactions with for example electrical fields [7], thermal gradients [8] and surface interactions [9], to produce the desired patterns. Most of the above wet-chemistry-free approaches require the polymer layers to be soften by heating to a high temperature, normally above the glass transition temperature (T g ), in order to allow the thin film to de- form into the designed morphology. From a physics point of view, the structure of most of the polymer based electronic devices consists of a thin polymer layer con- fined between two conductive surfaces. When the poly- mer becomes soft due to heating, the system becomes unstable and may deform spontaneously to reduce the free energy of the system. Furthermore, the confinement of polymer molecules on length scales comparable to the size of the molecules can affect their physical properties, such as chain conformation and mobility. The interac- tions between the polymer molecules and the confining materials also affect the stability of the polymer mor- phology. The understanding of the thermal stability of * Corresponding author. Tel.: +44-1865-273763; fax: +44- 1865-273764. E-mail address: david.bucknall@materials.ox.ac.uk (D.G. Bucknall). 0014-3057/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.01.043 European Polymer Journal 40 (2004) 981–986 www.elsevier.com/locate/europolj