Precision Patterning of PDMS Thin Films: A New ... - Semantic Scholar
Precision Patterning of PDMS Thin Films: A New Fabrication Method and Its Applications Kee Suk Ryu+ and Chang Liu University of Illinois, 208 North Wright Street, Urbana, IL 61801, USA. + Corresponding author e-mail: [email protected] Abstract We present a novel fabrication method for patterning PDMS thin films on solid surfaces. The method allows microstructures with precise lateral dimensions and heights to be formed using PDMS. We have applied this technology to form microfluidic channel units, reaction chambers, and micro elastic gaskets. This new fabrication method could potentially give rise to an efficient and reliable approach of constructing microfluidic systems. Keywords: PDMS, microfluidic 1. Introduction Polydimethylsiloxane (PDMS) elastomer is widely used in microfluidic applications to form components such as channels, valves, and diaphragms [1,2]. The PDMS material offers many advantages. It is transparent and biocompatible. It can be easily processed by molding and acquired for low costs. It is elastic and can form fluid seals effectively. PDMS is commonly used as a bulk material [3]. The predominant fabrication process associated with PDMS is bulk molding. It is previously impossible to form fine features made of PDMS with controlled lateral dimensions and heights (e.g., less than 10 µm) on solid surfaces (e.g., silicon or glass). There are two major causes to this deficiency. First, PDMS is not photodefinable and cannot be photolithographically patterned like photoresist. Secondly, PDMS prepolymer is viscous. It is impossible to form thin films of PDMS using spin coating, or any other method we know of. Earlier work showed that even when spinning wafers at 8,000 rpm, the resultant PDMS thickness is greater than
Figure 1: Schematic diagram illustrating the principle of forming/patterning PDMS thin film.
40 µm [4]. 2. Method for PDMS Patterning The principle of the PDMS patterning process is discussed in the following (see Fig. 1). A photoresist layer is first deposited on top of a solid substrate (e.g., glass or silicon) and patterned by using conventional lithography process. We pour a PDMS prepolymer solution (in the form of a viscous liquid) over the substrate surface. A flat and smooth blade is used to traverse the substrate surface while maintaining contact with the top surface of the photoresist layer. Excessive PDMS pre-polymer is removed, leaving PDMS only in recessed regions between protruding photoresist molds. After the remaining PDMS is thermally cured, the photoresist mold is removed selectively by using acetone. The height of the resultant PDMS pattern corresponds to the thickness of the photoresist. Figure 2 shows an SEM micrograph of parallel PDMS lines that are 50 µm wide and 5 µm tall. 3. Applications Based on this PDMS patterning technique, we developed a method to form microfluidic channels (Fig. 3). Patterned PDMS form protruding ridges that define boundaries (banks) of fluid channels on one substrate piece (labeled bottom substrate in Fig. 3a). A top substrate piece is then pressed against the bottom substrate, slightly deforming the PDMS ridges and forming enclosed flow channels (Fig. 3b). PDMS patterns for fluid channels are shown in Fig.4. Pressure-driven channel flow was demonstrated (Fig. 5). We have found out that the formed PDMS channel can flow water-based solutions without treating the PDMS surface to be hydrophilic. The bulk PDMS channel that is being widely used has to be treated with such as oxygen plasma to
Figure 2: SEM micrograph of parallel lines made of PDMS to demonstrate the proof-of-concept.
Figure 3: Schematic diagram illustrating the principle of forming microfluid channels using PDMS micro patterning and soft bonding of two substrates. (a) PDMS patterns that define fluid channels are formed on the bottom substrate and then (b) sandwiched between two plates. The cutout in the top substrate is intentional to show the interior of enclosed channels.
Figure 4: SEM micrograph showing PDMS patterns used to form a 100-µm-wide microfluidic channel.
let the water based solutions to flow. Since the bottom substrate and top substrate, which account much of the surface area the flow is being exposed to, there is no need of treating the surface. This micro-channel formation method is rapid, low cost, reliable, and reconfigurable. 4. Conclusions We have demonstrated a novel fabrication method to pattern PDMS material with controlled dimensions employing a novel fabrication method that employs the conventional photolithography technique. A simple microfluidic channel was fabricated and fluid-dispensing experiment has been demonstrated successfully. It is also possible to form thin PDMS O-ring gaskets (registered with through holes) to establish fluid inlet/outlet for microfluidic chips (Fig. 6). These methods can potentially be applied to other elastomer materials. Acknowledgements
Figure 5: Optical micrograph of two parallel fluid channels formed by using process shown in Fig. 3. Colored IPA solution runs in parallel fluid channels.
Figure 6: SEM micrograph of micro gaskets that are registered with through-holes in silicon substrate.
This work is supported by the Defense University Research initiative in Nanotechnology program (NAVY CL 2468 ANTIC), the Nano Science and Engineering Center (NSEC), and initiative of NSF under NSF award number EEC118025. References 1. Jo, B.-H.; Van Lerberghe, L.M.; Motsegood, K.M.; Beebe, D.J., Three dimensional micro-channel fabrication in PDMS elastomer, J. MEMS, Vol. 9, pp. 76-81, 2000 2. M.A. Unger, H.-P. Chou, T. Thorsen, A. Scherer, and S.R. Quake, Monolithic Microfabricated Valves and Pumps by Multilayer Soft Lithography", Science 288: 113-116, 2000. 3. D. J. Campbell, K. J. Beckman, C. E. Calderon, P. W. Doolan, R. H. Moore, A. B. Ellis, G. C. Lisensky, Replication and Compression of Bulk Surface Structures with Polydimethylsiloxane Elastomer, J. Chem. Educ. Vol. 76 , 537, 1999 4. M. Khoo, C. Liu, Micro magnetic silicone elastomer membrane actuator, Sensors and Actuators, 89(3), 2001
Figure 1: Schematic diagram illustrating the principle of forming/patterning PDMS thin film.
40 µm [4]. 2. Method for PDMS Patterning The principle of the PDMS patterning process is discussed in the following (see Fig. 1). A photoresist layer is first deposited on top of a solid substrate (e.g., glass or silicon) and patterned by using conventional lithography process. We pour a PDMS prepolymer solution (in the form of a viscous liquid) over the substrate surface. A flat and smooth blade is used to traverse the substrate surface while maintaining contact with the top surface of the photoresist layer. Excessive PDMS pre-polymer is removed, leaving PDMS only in recessed regions between protruding photoresist molds. After the remaining PDMS is thermally cured, the photoresist mold is removed selectively by using acetone. The height of the resultant PDMS pattern corresponds to the thickness of the photoresist. Figure 2 shows an SEM micrograph of parallel PDMS lines that are 50 µm wide and 5 µm tall. 3. Applications Based on this PDMS patterning technique, we developed a method to form microfluidic channels (Fig. 3). Patterned PDMS form protruding ridges that define boundaries (banks) of fluid channels on one substrate piece (labeled bottom substrate in Fig. 3a). A top substrate piece is then pressed against the bottom substrate, slightly deforming the PDMS ridges and forming enclosed flow channels (Fig. 3b). PDMS patterns for fluid channels are shown in Fig.4. Pressure-driven channel flow was demonstrated (Fig. 5). We have found out that the formed PDMS channel can flow water-based solutions without treating the PDMS surface to be hydrophilic. The bulk PDMS channel that is being widely used has to be treated with such as oxygen plasma to
Figure 2: SEM micrograph of parallel lines made of PDMS to demonstrate the proof-of-concept.
Figure 3: Schematic diagram illustrating the principle of forming microfluid channels using PDMS micro patterning and soft bonding of two substrates. (a) PDMS patterns that define fluid channels are formed on the bottom substrate and then (b) sandwiched between two plates. The cutout in the top substrate is intentional to show the interior of enclosed channels.
Figure 4: SEM micrograph showing PDMS patterns used to form a 100-µm-wide microfluidic channel.
let the water based solutions to flow. Since the bottom substrate and top substrate, which account much of the surface area the flow is being exposed to, there is no need of treating the surface. This micro-channel formation method is rapid, low cost, reliable, and reconfigurable. 4. Conclusions We have demonstrated a novel fabrication method to pattern PDMS material with controlled dimensions employing a novel fabrication method that employs the conventional photolithography technique. A simple microfluidic channel was fabricated and fluid-dispensing experiment has been demonstrated successfully. It is also possible to form thin PDMS O-ring gaskets (registered with through holes) to establish fluid inlet/outlet for microfluidic chips (Fig. 6). These methods can potentially be applied to other elastomer materials. Acknowledgements
Figure 5: Optical micrograph of two parallel fluid channels formed by using process shown in Fig. 3. Colored IPA solution runs in parallel fluid channels.
Figure 6: SEM micrograph of micro gaskets that are registered with through-holes in silicon substrate.
This work is supported by the Defense University Research initiative in Nanotechnology program (NAVY CL 2468 ANTIC), the Nano Science and Engineering Center (NSEC), and initiative of NSF under NSF award number EEC118025. References 1. Jo, B.-H.; Van Lerberghe, L.M.; Motsegood, K.M.; Beebe, D.J., Three dimensional micro-channel fabrication in PDMS elastomer, J. MEMS, Vol. 9, pp. 76-81, 2000 2. M.A. Unger, H.-P. Chou, T. Thorsen, A. Scherer, and S.R. Quake, Monolithic Microfabricated Valves and Pumps by Multilayer Soft Lithography", Science 288: 113-116, 2000. 3. D. J. Campbell, K. J. Beckman, C. E. Calderon, P. W. Doolan, R. H. Moore, A. B. Ellis, G. C. Lisensky, Replication and Compression of Bulk Surface Structures with Polydimethylsiloxane Elastomer, J. Chem. Educ. Vol. 76 , 537, 1999 4. M. Khoo, C. Liu, Micro magnetic silicone elastomer membrane actuator, Sensors and Actuators, 89(3), 2001
