Fabrication of micro/nano fluidic channels by nanoimprint lithography and bonding using SU-8 Rong Yang a , Bing-Rui Lu a , Jing Wan a , Shen-Qi Xie a , Yifang Chen b , Ejaz. Huq b , Xin-Ping Qu a , Ran Liu a, * a State Key Lab of ASIC and System, Department of Microelectronics, Fudan University, Street: 220, Handan Road, Shanghai, 200433, China b Micro and Nanotechnology Centre Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, UK article info Article history: Received 29 September 2008 Received in revised form 31 January 2009 Accepted 2 February 2009 Available online 7 February 2009 Keywords: Micro/nano fluidics Nanoimprint lithography SU-8 Bonding abstract Polymer-based micro/nano fluidic devices are becoming increasingly important for biological applica- tions and fluidic control. In this article, we report and demonstrate a novel fabrication process for micro/nano fluidic channels based on nanoimprint lithography (NIL) and bonding using SU-8, which is an excellent material for micro/nano fluidics application due to its good chemical stability, bio-compat- ibility and optical properties. Bonding pressure and temperature were found to be essential to the bond- ing strength and channel profile control. This fabrication technique has great potential for low-cost mass production of SU-8 based micro/nano fluidic devices. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction There is a wide range of applications for three-dimensional (3D) micro/nano fluidic devices such as separation of protein molecules [1], DNA stretching [2], and control and detection of chemical reac- tions [3]. Conventional fabrication processes for 3D micro/nano channels, which are mostly made with silicon or related materials, involve complex and time-consuming steps such as lithography, etching and bonding [4]. Recently, polymer fabrication techniques, such as e-beam lithography [5], interferometric lithography [6], proton beam writing [7], stacked reversal UV nanoimprint [8], have attracted great interest and have been applied to produce polymer- based micro/nano fluidic devices. Polymer-based micro/nano flu- idic channels have many advantages such as easy fabrication, low processing temperature, biocompatible and recyclable. Due to its excellent chemical resistance, high optical transpar- ency and easy fabrication of high-aspect-ratio features, SU-8 has been recognized to be an ideal material for micro/nano fluidic channels [9,10] and even fully integrated lab-on-chip microsys- tems [11,12]. Several methods have been developed to achieve a highly hydrophilic surface of SU-8 which will make the filling of micro/nano channels with fluidics by capillary action possible [13–15]. In this paper, we present a novel fabrication process for micro/ nano fluidic channels based on nanoimprint lithography and soft- bonding on SU-8. Sealed SU-8 micro/nano channels with good bonding strength and channel profile control are achieved using optimized bonding pressure and temperature. Our technique exhibits many advantages such as low-cost, low bonding tempera- ture, few process steps and good channel profile, and is promising for high throughput production. 2. Experiment details Fig. 1 schematically presents the process flow to fabricate SU-8 micro/nano fluidic channels. UV curable SU-8 is first spin coated on a clean and dehydrated surface of a Pyrex substrate and then im- printed by silicon grating templates, as shown in Fig. 1a. Based on our earlier work [16], the imprint is carried out by an in-house made press at 120 °C under a pressure of 150 bar for 10 min, fol- lowed by UV exposure and post exposure bake (PEB) to cure the SU-8. The silicon templates are pre-treated with DC20 to reduce surface energy for easy separation from SU-8 after imprinting. In Fig. 1b, two flat SU-8 layers with thicknesses of about 5 and 60 nm, respectively, are spin coated consecutively on a thick PDMS layer and then thermally bonded with the patterned SU-8 layer, as shown in Fig. 1c. After bonding, the PDMS layer is removed and the SU-8 layers are transferred onto the patterned SU-8 layer, forming sealed micro/nano fluidic channels. The bonding pressure and tem- perature are optimized to ensure good bonding strength and chan- nel profile. In order to form uniform SU-8 layer on a hydrophobic surface of PDMS, oxygen plasma treatment is carried out in a RIE chamber be- fore spin-coating SU-8. PDMS is exposed to oxygen plasma at an oxygen flow rate of 60 sccm and RF power of 50 W for 1 min, 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.02.002 * Corresponding author. Tel./fax: +86 21 55664548. E-mail address: rliu@fudan.edu.cn (R. Liu). Microelectronic Engineering 86 (2009) 1379–1381 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee