Plasma-Based Surface Modification of Polydimethylsiloxane for PDMS-PDMS Molding

We present and compare two processes for plasma-based surface modification of Polydimethylsiloxane (PDMS) to achieve the antisticking behavior needed for PDMS-PDMS molding. The studied processes were oxygen plasma activation for vapor phase silanization and plasma polymerization with tetrafluoromethane/hydrogen mixtures under different processing conditions. We analyzed topography changes of the treated surfaces by atomic force microscopy and contact angle measurements. Plasma treatment were conducted in a parallel plate reactive ion etching reactor at a pressure of 300？mTorr, 30？Watts of RF power and a total flow rate of 30？sccm of a gas mixture. We found for both processes that short, low power, treatments are better to create long-term modifications of the chemistry of the polymer surface while longer processes or thicker films tend to degrade faster with the use leaving rough surfaces with higher adherence to the molded material. 1. Introduction As reviewed by Macdonald and Whitesides [1], PDMS (polydimethylsiloxane) has proven to be a suitable material for microfluidic device fabrication. It is optically transparent and can be molded to reproduce structures down to nanoscale resolution [2, 3], usually by direct casting on photoresist features over silicon wafers. SU8 is the most commonly used photoresist for master fabrication; single- or multilevel SU8 lines and structures have been extendedly used for PDMS microchannel definition, however, reutilization of resist masters is limited due to degradation by thermal stress during the curing process and by demolding forces. Some strategies such as coatings with diamond-like carbon (DLC) and DLC-polymer hybrids can be applied to increase the durability of the molds [4]. In this paper we present an improved method to replace the use of conventional photoresist masters with more durable positive copies in PDMS. Due to its elasticity, flexible masters in PDMS are not affected by thermal stress during the heating-cooling cycles of the curing process and it is easier to achieve a conformal contact between mold and countermold in closed molding configurations for the production of parts with through holes. Similar methods have been presented to mold complex 3D structures with through holes: Lucas et al. in 2008 [5] presented the use of flexible molds made with freestanding SU8 layers for double-side molding of PDMS, Jo et al. in 2000 [6] used a plastic sheet as countermold for SU8 molds and Janelle et al. in 2000 [7] presented the use of slabs of PDMS treated with