Direct laser patterning of self-assembled monolayer using elliptical laser beams: A theoretical parametric study Martin Y. Zhang 1 , Mohammad Reza Shadnam 2 , A. Amirfazli n Department of Mechanical Engineering, University of Alberta, Edmonton, AB, Canada T6G 2G8 article info Article history: Received 8 December 2010 Received in revised form 30 March 2011 Accepted 30 March 2011 Available online 6 May 2011 Keywords: Self-assembled monolayer Elliptical beam Patterning abstract A theoretical quantitative analysis of processing parameters for application of an elliptical laser beam to achieve maximum patterning area is the focus of this study. Direct laser patterning (DLP) of self- assembled monolayers (SAM) is achieved by localized heating of the sample above the SAM desorption temperature. Through use of elliptical laser beams in the present work, three goals are achieved by analyzing the heat diffusion model and related thermo-kinetics model: (1) optimal working conditions (combination of laser power, scanning velocity and aspect ratio) for DLP to produce maximum feature size, or highest processing velocity at a given power; (2) identification of conditions that reduces the potential thermal damage to the substrate; (3) shedding light on issues related to uniformity or homogeneity of heating a substrate using an elliptical laser beam. A heat diffusion model is employed to provide the resulting surface temperature caused by elliptical laser beams, and the coupled thermo- kinetics model is used to determine the final SAM coverage generated by DLP. Parametric analysis revealed that 70–150 mW can be used to pattern feature sizes in the range of 2–10 times of equivalent circular beam size. It is also found that each elliptical laser beam has a unique optimal aspect ratio to result in the widest feature size for a given laser power and scanning velocity. The edge transition width increases with an increase of the aspect ratio. Keeping the aspect ratio of elliptical laser beam small (i.e. b o20), a sharp edge definition could be obtained; if an aspect ratio larger than 30 is used, a surface with gradual edge definition could be obtained. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction A self-assembled monolayer (SAM) is a two-dimensional, one- molecule-thick film; it spontaneously forms when certain organic molecules, e.g. thiols and silanes, adsorb on a surface, i.e. noble metals and silica, respectively. SAMs are important for scientific and technological purposes, and they have a variety of applica- tions in bio-technological devices, MEMS, and micro-fluidics [1–3]. Patterned SAM surfaces, i.e. surfaces having different wettability, charge, or biocompatibility in neighboring regions are useful for producing wall-less micro-fluidic channels, and chemical gradients, which is of interest for controlled liquid delivery on surface [4–5] and cell separation [6] applications. One of the most studied and used SAM systems is alkaline thiols placed on a gold film that is supported by a glass or silicon substrate, here we also consider such system. In the context of direct laser patterning (DLP) methodology for SAM patterning using circular beams [7–14], patterning with elliptical beams (by using cylindrical lenses, for instance), is considered in this study. In DLP, an initially formed homogeneous monolayer, e.g. 1-hexadecanethiol (HDT) SAM (hydrophobic) placed on gold film is irradiated by a laser beam, e.g. 488 nm CW argon ion laser beam [7,8] to form a bare region (because of thermal desorption of SAMs); then the bare region is backfilled by a second monolayer species, e.g. 16-mercaptohexadecanoic (MHA) acid SAM (hydrophilic) through solution deposition (details of SAM preparation procedure and the experimental set-up can be found elsewhere [8]). The procedure creates hydrophilic patterns on hydrophobic background that can control liquid spreading on a surface. In this way, DLP can be used to manipulate the surface properties. Note that the manipulation of surface properties is not limited to surface wettability; depending on the SAM’s tail group, surface charge or surface biocompat- ibility can be changed as well. Previously, patterns with feature sizes of 4–170 mm have been produced using laser beams with circular cross sections (various sizes and powers) in a single pass mode [7,8]. However, making features larger than 170 mm is of interest in many fields. For instance, a potential application is to enhance the efficiency of creating chemical Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/optlastec Optics & Laser Technology 0030-3992/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2011.03.034 n Corresponding author. Tel.: þ1 780 492 6711; fax: þ1 780 492 2200. E-mail address: a.amirfazli@ualberta.ca (A. Amirfazli). 1 Current address: School of Industrial Engineering, Purdue University, West Lafayette, IN 47907, USA. 2 Current address: R&D Incentive Practice, KPMG LLP, 777 Dunsmuir Street, Vancouver, BC, Canada V7Y 1K3. Optics & Laser Technology 43 (2011) 1377–1384