2003 ME Graduate Student Conference

The goal of the present study was to fabricate perforated membrane structures in polymers with the pore diameter ranging from 10 μm down to the sub μm scale. The perforated membrane structure plays an important role in many biological systems. Thus the ability to reproduce such architecture would help with the study of biological systems by mimicking biological cell membrane-like structures and open new vistas in the study of transport behavior in cell biology and the separation of biological vesicles. Such structures also have potential uses as components in polymer optics and modular micro/nanofluidic devices.[1-3]. This study presents fabrication of mechanically stable, free-standing membrane structures in polymers down to 500 nm pore diameter by using all parallel patterning processes and their applications in lipid bilayer study by selective patterning of the membrane surface using Poly(L-Lysine)poly(ethylene glycol) (PLL-PEG). Fluorescence dye stained lipid solution was used to preferentially pattern the membrane surface. Images collected with an in house evanescentfluorescence microscope showed patterned fluorescence when PLL-PEG was used (Fig. 1). Imprint lithography using Si masters was used to define microscale patterns, which was combined with a sacrificial layer technique where double resist layers were spin-coated on Si substrates. A lift-off resist (LOR, Microchem) and a low molecular weight poly(methyl methacrylate) (PMMA, Microchem) were used as the lower sacrificial layer and a UV-curable SU-8 was used for the active membrane layer. The mechanical stability for the thin SU-8 membrane layers was achieved by employing a modified imprinting process which combined thermal imprinting with a post UV-curing. The Si stamps were fabricated using a combination of photolithography and semiconductor micromachining techniques. Si wafers were initially spin-coated with a thin layer of hexa-methyl-di-silazane (HMDS, Silicon resources) used as an adhesion promoter [4]. After drying for 5 min, a 2 μm layer of S1813 (Shipley) photoresist was applied by spin-coating and subsequently a pre-exposure bake was performed at 95°C for 90 s. Photolithography was performed with a custom designed photomask in a ‘Quintel’ UV exposure station. The exposed wafer was then developed in MF 319 developer solution (Microchem) for 120 s, which was followed by washing in DI water bath for 2 min and drying with a N2 gun. The pattern transfer down to the Si substrate was achieved using a deep reactive ion etching (DRIE) process. The Si etching was performed using an inductively coupled plasma (ICP) machine from STS systems at Micro Electronics Research Center (Georgia Institute of Technology). The gas chemistry and the power used were SF6:O2:C4F8 =130:13:100 sccm and 600 W, respectively. The DRIE process involved alternate etch and passivation cycles to etch down and subsequently passivate the sidewall so as to maintain a high degree of anisotropy, thus resulting in almost vertical sidewall profiles (Fig. 2). The Si masters were subsequently coated with fluorinated silane [5] in vapor phase to have a hydrophobic surface for better imprint quality. Si wafers were diced to 22 cm 2 and subsequently cleaned with acetone, isopropanol, and de-ionized (DI) water followed by blow-drying with clean dry air. The cleaned wafer pieces were first spin-coated either with a liftoff resist (LOR 3B from Microchem) or a low molecular weight poly(methyl methacrylate) (PMMA, Mw=25 kg/mol) at 3000 rpm for 60 s resulting in 300 nm in thickness. A post-baking was performed at 150°C for 3 min and 100°C for 3 min for LOR and PMMA, respectively. The samples were then spin-coated with SU-8 (Microchem) at 2000 rpm for 60 s, which resulted in a 5.5 μm thick SU-8 layer. A post bake was performed at 95°C for 5 min. Imprinting was performed with Si stamps under various conditions using a 6” nanoimprinter (Obducat). Best imprint results for SU-8 were achieved at the imprint temperature of 135°C and 40 bar pressure. The imprinted samples were subject to O2 plasma etch at 250 mTorr and 150 W for 30 s to remove the residual SU-8 layer from the imprinted trenches, in order to expose the LOR sacrificial layer to the developer during liftoff. The etched samples were then cured with a UV lamp for