Direct fabrication of micro/nano fluidic channels by electron beam lithography Daniel M. Koller * , Nicole Galler, Harald Ditlbacher, Andreas Hohenau, Alfred Leitner, Franz R. Aussenegg, Joachim R. Kren Institute of Physics, Karl-Franzens-University, A-8010 Graz, Austria and Erwin Schrödinger Institute for Nanoscale Research, Karl-Franzens-University, A-8010 Graz, Austria article info Article history: Received 22 September 2008 Received in revised form 13 November 2008 Accepted 14 November 2008 Available online 28 November 2008 Keywords: e-beam lithography Nano fluidic Micro fluidic 3D Mask fabrication SU–8 Micro fluidic channel abstract We utilize the strongly energy dependent electron penetration properties of the negative tone electron resist SU–8 for direct fabrication of micro/nano fluidic channels. Electron beam lithography is thereby applied in a two step process. First, the SU–8 is exposed down to the substrate forming supporting struc- tures, a second exposure step with accordingly modified exposure parameters results in elevated struc- tures. As we demonstrate, this process allows the fabrication of precisely aligned nanoscopic fluidic channels over lengths of several millimeters. In addition, an application as microscale shadow masks for evaporation based deposition processes is discussed. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Micro- and nanoscale technologies put a great demand on flex- ible high resolution fabrication techniques. Applications including organic electronics, micro-electromechanical systems, micro-opti- cal components or microfluidic devices [1,2] require the ability of reproducible fabrication processes. There is a need for new con- cepts and processes which are easily transferred from a develop- ment level to a high throughput industrial level. A promising opportunity to overcome spatial restrictions makes in this context use of three-dimensional (3D) sub-micrometer SU–8 structures. The main idea is to put the complexity of the fabrication on the lithographic process in order to simplify the following processing steps. Several lithographic techniques have intrinsic 3D structuring capabilities [3–5]. In this particular case we focus on electron beam lithography (EBL) [6], because of its potential to create sub- micrometer patterns [7–9]. This fabrication process shows poten- tial for high throughput processes, as besides electron sensitivity SU–8 also shows selective sensitivity to UV radiation [10]. Here, we aim at self-supporting 3D structures as fluidic channels and evaporation masks in the nano/micrometer size range. 2. Fabrication process We used a SU–8–3010 negative resist and a diluting solution (SU–8 2000 thinner) from MicroChem Corp. to achieve defined lay- ers of 1 lm thickness. Electron beam exposure was carried out on a RAITH 100–2 system based on a ZEISS Gemini field emission scan- ning electron microscope. The fabrication process introduced in ref. [11] was performed in the following steps as illustrated in Fig. 1: (a) High vacuum vapor deposition of a conductive layer (e.g. gold, ITO,...) onto a cleaned glass substrate. Since SU–8 is a very sensitive resist the applied doses are relatively low, this al- lows electron beam exposure on non-conductive substrates with- out charging effects. Hence, this first step is optional. (b) The SU– 8 was diluted in a mixing ratio of 1:1 which gave 1 lm thick layers in a spin-coat process at 4000 rpm for 40 s. A soft bake of the SU–8 coated substrate was performed on a heating plate at 95 °C for 1 min. Then, electron beam exposure was performed at a dose of 1 l C/cm 2 in a two step process with different electron energies: (c) First, the supporting structures were exposed at an electron acceleration voltage of 10 kV. Under this condition the energy of electrons is high enough to penetrate through the whole thickness of the resist layer. (d) In the following exposure step with a de- creased electron energy of 5 keV the cover structure is patterned. In this case the electron energy is too low to penetrate through the whole resist. Electron exposure is rather restricted to a layer of about 500 nm thickness. The alignment for the double exposure process plays a minor role, since, there is no sample handling be- tween the two exposure steps. A slight misalignment depends 0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.11.025 * Corresponding author. E-mail address: daniel.koller@uni-graz.at (D.M. Koller). Microelectronic Engineering 86 (2009) 1314–1316 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee