Improving Photoresist Spray Coating on 3D Structures for Microfluidic Devices
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Improving Photoresist Spray Coating on 3D Structures for Microfluidic Devices Shinya Kumagai1, Naoya Fukuda1, Hisayoshi Tajima1, and Minoru Sasaki1 1 Toyota Technological Institute, 2-12-1 Hisakata, Tenpaku, Nagoya, 468-8511, Japan ABSTRACT Spray coating of a photoresist onto three-dimensional (3D) structure was investigated. To improve the uniformity of photoresist deposition onto the 3D structure, a shield plate with an aperture was used. The shield plate set over a sample permitted the resist deposition on the sample surface located in the aperture area while the plate blocked the deposition for the other area. The spray flow which is suitable for the resist deposition can be used effectively. Numerical analysis revealed that the vertical velocity component of gas flow was enhanced in the aperture area. In the experiments of the spray coating, the difference between the resist film thicknesses deposited on top and bottom trench surfaces was decreased. On the trench sidewall, resist bump formation, which was frequently observed in spray coating, was suppressed. The uniform resist deposition is necessary to realize 3D microdevices by lithography. In the microfluidic devices of dielectrophoresis, aside from the top and botttom trench surfaces, the trench sidewall can be used to fabricate device structures such as electrode for dielectrophoresis. INTRODUCTION Dielectrophoresis (DEP) is a useful technique for controlling the transport of micro/nano-scale objects (e.g., latex beads, cells, proteins, and DNA) in a liquid using trench-type microdevices[1-7]. A pair of electrodes is used to collect the object. However, conventional planar lithography limited the positions of the electrodes. The electrodes were located at the top or bottom of trench structures. Once the fabrication of a three-dimensional (3D) structure can be realized, variations of the electrode design are expanded [8,9]. An example of a DEP microtrench device is schematically shown in Fig. 1. Combining the collecting electrodes with branch structures of the micro trenches, separation of the objects is achieved. Particle
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Figure 1. Schematic drawing of trench-type device with electrodes for DEP. The electrode structures are located at bottom and side wall of the trench. Fabrication of 3D metal electrodes requires a uniformly coated photoresist film over the trench structure. The spray coating of a photoresist is one of the promising methods for the uniform deposition [10-15]. The spray coating is schematically shown in figure 2(a). Photoresist liquid is mixed with N2 gas to form a spray of photoresist particles. The photoresist particles are 1
transported by the gas flow and deposited on a sample with 3D structures. However, there remains a challenge to realize uniform coating. One of the typical profiles of the coating is shown in Fig. 2(b). The resist film is thicker at the top surface and thinner at the bottom surface. On the sidewall, thicker bumps are observed around the convex corners of the trench structures. When the spray flo
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