Direct-fabricating-patterns-using-stamping-transf.pdf

Direct fabricating patterns using stamping transfer process with PDMS mold of hydrophobic nanostructures on surface of micro-cavity Yi-Hao Huang, Jing-Tang Wu, Sen-Yeu Yang Department of Mechanical Engineering, National Taiwan University, Taipei 106, Taiwan a r t i c l ei n f o Article history: Received 7 July 2010 Accepted 9 August 2010 Available online 12 August 2010 Keywords: Transfer stamping process Hydrophobic nanostructures Anodic aluminum oxide (AAO) PDMS mold a b s t r a c t The transfer stamping process has been used to fabricate thin-fi lm pattern in recent years. Due to the characteristics of the materials of molds and inks, residual inks on the cavities of mold and residual layers on the substrate are still a problem. To solve the problem, we present a concept for fabrication of hydro- phobic nanostructures on the cavities of microstructures of the mold, which can effectively decrease the ink residing on the cavities of the mold during coating. First, the periodic nanopores are fabricated on the anodic aluminum oxide (AAO). Second, AAO membrane is employed as the template for fabricating nano- structures on the PC fi lm by embossing. And then, by partial protrusion of the nano-structured PC fi lm into the micro-holes of the mold, an array of protruded convex microstructures is formed. After that, polydimethylsiloxane (PDMS) mold is casted from the embossed PC fi lm. The contact angle of nanostruc- tures on the micro-cavities of PDMS mold is about 145?. Micro-patterns with no residual layers have been successfully transferred on the poly(ethylene terephthalate) (PET) substrate using a transfer stamping process with this PDMS mold. ? 2010 Elsevier B.V. All rights reserved. 1. Introduction Micro contact printing (lCP) 1 and nano contact printing (nCP) 2 have been developed in order to directly fabricate micro- structures with no residual layers. Ink patterns from the relief fea- tures of microstructures on polydimethylsiloxane (PDMS) mold can be transferred to substrates by the methods. A transfer stamp- ing process has also been proposed to fabricate thin-fi lm patterns on the PMMA substrate for application of organic thin-fi lm transis- tor 3,4. The process primarily relies on different adhesion forces of the transferring interfaces, ink/mold and ink/substrate, to trans- fer patterns from the relief features of the mold to the substrate. No complex pre-process is needed, and patterns can be transferred in one step. In ideal case, as shown in Fig. 1(a), the transfer stamping process can transfer micro-patterns directly from the stamp mold without residual layers. But in real case, as shown in Fig. 1(b), the inks often reside in cavities of the mold structure while spin coating, and causes lots of residual layers in the following step. This is because that pressure deforms mold in the step of stamping, leading on roof collapse (see Fig. 2(a). For solving roof collapse, many molds are designed and fabricated with high aspect ratio. However, PDMS molds with high aspect ratio have other defects, like buckling or lateral collapse 5, which are shown in Fig. 2(b) and (c), respectively, causing failure in the transfer stamping process. In order to improve the problems, we propose a novel method to fabricate mold with low aspect ratio but is able to transfer pat- terns without residual layers by controlling wetting phenomena on the surface of micro-cavity. Wetting phenomena on solid surfaces has been widely studied in recent years. Superhydrophobic mate- rials such as lotus leaves have been studied in both theoretical fi elds and experimentally 6,7, and they are useful in micro-fl uid- ics, self-cleaning coats, heat sink panels, and etc. Many methods have been developed to fabricate superhydrophobic materials such as lithography 8, template methods 9,10, ion bombardment 11,12, self-assembly of a monolayer 13, chemical deposition 14,15 and photocatalysis 16,17. However, these methods are highly costly, time consuming, complex or limited in size. On the other hand, due to its unique nanohoneycomb structure, anodic aluminum oxide (AAO) has been used as a nanostructured tem- plate for nanotechnology 18. The characteristic features of AAO could be easily controlled by adjusting the anodizing conditions 19,20, such as the anodic voltage, the temperature of the electro- lyte solution and the anodizing time. The structures of AAO have beenappliedinmany fi elds,including anti-refl ectionand hydrophobicity. In this paper, we present a novel method for the fabrication of hydrophobic nanostructures on the cavities of microstructures 0167-9317/$ - see front matter ? 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2010.08.006 Corresponding author. E-mail address: .tw (S.-Y. Yang). Microelectronic Engineering 88 (2011) 849854 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: using AAO template. We fi rst fabricate AAO template, and this tem- plate is then used to emboss a PC fi lm in the fi rst hot embossing process. Next, we operate the second hot embossing process with this embossed PC fi lm as a substrate and a stainless steel mold with micro-holes. After that, a PC fi lm of protruded convex micro- structures with AAO nanostructures is obtained. This PC fi lm with nano/micro structures is then applied as a master mold to cast its structures by PDMS. Finally, a PDMS mold of hydrophobic nano- structures on surface of micro-cavity is obtained. The complete fabrication process is shown in Fig. 3. We also demonstrate the hydrophobic property of AAO nanostructure by measuring the contact angle (CA) on the surface of PDMS molds with different structures. Lastly, the results of transfer stamping process are shown, providing that PDMS mold of hydrophobic nanostructures on surface of micro-cavity can improve the problems of residual inks and residual layers. 2. Fabrication of PDMS mold of hydrophobic nanostructures on surface of micro-cavity 2.1. Fabrication of AAO template with porous nanostructures and mold with micro-holes The material of the AAO template used is an industrial alumi- num sheet with 99.7% purity, and the fabrication process of AAO template is shown in Fig. 4. At the beginning, the sheet is electro- polished in a mixture solution of perchloric acid and ethanol (HClO4:C2H5OH = 1:4), and is applied as a cathode while a carbon rod is used as an anode. A constant voltage of 10 V is supplied for 15 min. Then, the polished sheet is anodized for the fi rst time in a solution of 0.1 M oxalic acid for 5 h; the temperature of the solution is maintained at 4 ?C, and a constant voltage of 80 V is ap- plied. Later, the sheet is immersed into a solution of 6 wt% phos- phoric acid at 90 rpm stirring speed the temperature of 32 ?C for 2 h to remove the oxygen layer on the sheet. The sheet is then anodized the second time under the same condition as the fi rst anodization but for only 10 min. Next, to enlarge the holes of AAO, the sheet is put into a solution of 6 wt% phosphoric acid at 90 rpm stirring speed and temperature of 32 ?C again for 15 min. Eventually, an AAO template with porous nanostructures is ob- tained. Fig. 5 shows the scanning electron microscope (SEM) image of AAO template with nano-holes. Due to the impurity of this aluminum material, the nanostructures on the AAO template are disordered, like lotus leaf which has surfaces mixed with micro and nano structures, creating high hydrophobic property. For fabricating microstructures, we fabricate a stainless steel mold with micro-holes. First of all, a 50-mm-thick stainless steel shadow mask with a micro-hole array is prepared, which supplied by Key Star Technology, Taiwan. The stainless steel shadow mask has an array of 300 ? 300 holes and is fabricated by double sided photolithography and wet etching. Fig. 6 is the optical microscope (OM) image of this stainless steel mold. The micro-holes are 145lm in diameter and 200lm in pitch. 2.2. Gas-assisted hot embossing process Fig. 7 is the scheme of gas-assisted hot embossing process. In this process, a mold and a polymeric substrate are fi rstly put onto a hot plate, and poly(ethylene terephthalate) (PET) fi lm is used as a seal fi lm to cover the mold and substrate. Next, the chamber is closed, and N2gas is blown into the chamber as pressure; in the meanwhile, the heater raises the temperature to or near to Tg(glass transition temperature) in order to soften the polymeric substrate. In this step, the mold is pressed to the softened substrate, emboss- ing the negative pattern on the substrate. After that, the chamber is cooled, the gas is released and the chamber is opened. Eventu- ally, this embossed substrate with structures is obtained. 2.3. Fabrication process The process to fabricate PDMS mold of hydrophobic nanostruc- tures on surface of micro-cavity is shown in Fig. 3. The fi rst step is to fabricate the nano-structured PC fi lm by protruding the bare PC fi lm into the pores of AAO template. The nano-structured PC fi lm is used as the substrate in the second step. The second step is the fabrication of protruded convex microstructures on this AAO Fig.1. Schemeoftransferstampingprocess:mold ? spincoating ? stam- ping ? de-molding: (a) ideal result of transfer stamping and (b) real result of transfer stamping. Fig. 2. Schematic showing the deformation of mold: (a) roof collapse, (b) buckling and (c) lateral collapse. 850Y.-H. Huang et al./Microelectronic Engineering 88 (2011) 849854 nano-structured PC fi lm. Protruded convex microstructures with AAO nanostructures are formed by partial protrusion of the soften fi lm into the micro-holes of the stainless steel mold under the ef- fects of the capillary and surface tension in the hot embossing pro- cess. After that, the following step is to replicate the structure of this embossed PC fi lm using PDMS mixture. This PC fi lm of pro- truded convex microstructures with nanostructures is applied as master mold; PDMS mixture, which consists of base agent and cur- ing agent at the ratio of 10:1, is then poured upon the master mold to cast the PDMS mold of hydrophobic nanostructures on surface of micro-cavity. After suitable curing time at room temperature, PDMSmoldofhydrophobicnanostructuresonsurfaceof micro-cavity is obtained. 2.4. Results and discussion of nanostructures on the cavities of microstructures Fig. 8(a) shows the SEM image of PC fi lm of protruded convex microstructures with nanostructures. This PC fi lm is fabricated by fi rst and second gas-assisted hot embossing processes in sequence. The parameters of the fi rst hot embossing are 155 ?C and 20 kgfcm2, and parameters are 140 ?C and 25 kgfcm2in the second hot embossing process. This fi gure shows that AAO nanostructures are fabricated successfully on the surface of the protruded convex microstructures on the embossed PC fi lm through the fi rst and sec- ond gas-assisted hot embossing processes. This PC fi lm is then used as a master mold, and replicated its structures by PDMS mixture. After curing PDMS mixture, the PDMS mold of hydrophobic nano- structures on surface of micro-cavity is obtained. Fig. 8(b) shows the SEM image of this PDMS mold of hydrophobic nanostructures on surface of micro-cavity. As shown, the surfaces of these micro-cavities are full of AAO nanostructures. This proves that the combination of AAO nanostructures with micro-cavity struc- tures is achieved through the second hot embossing process. The surface profi le of PDMS mold of hydrophobic nanostructures on surface of micro-cavity is shown in Fig. 9. This fi gure shows that the cavities are about 18lm in depth. In order to evaluate the hydrophobic ability of AAO nanostruc- tures, we measure the contact angle (CA) on different PDMS surface as shown in Fig. 10. The CA of the smooth PDMS surface without nanostructures is about 110? as shown in Fig. 10(a). On the contrary Fig. 10(b) shows that the CA of fl at PDMS with nanostructures. The Fig. 3. Schematic showing the fabrication process of PDMS mold with nanostructures on micro-cavities: (a) The nanostructures of AAO template is embossed to the PC fi lm by the fi rst gas-assisted hot embossing process, (b) protruded convex microstructures are formed by partially protruding the softened nano-structured PC fi lm into the micro- holes of the stainless steel mold in the second gas-assisted hot embossing process and (c) the PC fi lm with nano/micro structures is then applied as a master mold to cast its structures by PDMS, fabricating PDMS mold of hydrophobic nanostructures on surface of micro-cavity. Y.-H. Huang et al./Microelectronic Engineering 88 (2011) 849854851 CA is nearly 145?. It is found that the CA is higher than that on the smooth PDMS surface, which proves that nanostructures can im- prove the hydrophobic property effectively. Fig. 10(c) shows that the CA on the curved PDMS with nanostructures. Compared to Fig. 10(b), the result reveals that the hydrophobicity remains even after the second gas-assisted hot embossing process. 3. Application of PDMS mold of hydrophobic nanostructures on surface of micro-cavity 3.1. A transfer stamping process As mentioned above, Fig. 1 schematically shows the transfer stamping process: fi rst, the ink is spin coated onto the mold. Next, Fig. 4. Schematic of AAO process: (a) pure aluminum, (b) electropolishing, (c) 1st anodization, (d) anodic alumina removal, (e) 2nd anodization and (f) widening. Fig. 5. SEM image of AAO template with nano-holes. Fig. 6. Optical microscope (OM) image of stainless steel mold of micro-holes array. Fig. 7. Scheme of gas-assisted hot embossing process: (a) the mold-substrate stack, (b) covering the stack with seal fi lm, (c) enclosing the stack in the chamber, and then infusing gas and heating and (d) releasing gas, cooling, and de-molding. 852Y.-H. Huang et al./Microelectronic Engineering 88 (2011) 849854 this mold is pressed onto a substrate to transfer the ink pattern from the feature of this mold. The following step is de-molding. Finally, the transferred pattern is obtained. In our experiment, a polyethylene terephthalate (PET) fi lm (A-type, NAN YA, Taiwan) with a thickness of 180l m is used as the fl exibletransparent substrate, and the material of ink is positive photoresist EPG 510 (Everlight Chemical Inc., Taiwan). 3.2. Results and discussion of patterns transferred on the PET substrate In this study, the transfer stamping process is operated with PDMS mold of hydrophobic nanostructures on surface of micro- cavity, as well as PDMS mold simply with micro-cavities but with- out hydrophobic nanostructures. We operate transfer stamping process with these two molds individually. The material of ink we use is EPG510 (a sort of positive photoresist). At the beginning, proper quantity of EPG510 is dropped onto the mold; after 1 min, the spin coater is operated at 6000 rpm for 20 s. Then, the mold is pressed onto PET fi lm under 0.4 kgfcm2gas pressure for 1 min, transferring ink pattern from the mold feature to the PET fi lm. After that, the pressure is released and the next step is de-molding. Finally, the transferred pattern is obtained. Fig. 11 shows the results of transfer stamping process. Fig. 11(a) shows the transferred pattern of PDMS mold simply with micro- cavities but without hydrophobic nanostructures. It is obvious that there is residual EPG510 in these micro-cavities, which is then transferred onto the PET fi lm, creating a large quantity of residual layers. On the contrary, Fig. 11(b) shows the transferred pattern from PDMS mold of hydrophobic nanostructures on surface of micro-cavity to PET fi lm. The largest diameter of these circular shapes in the transferred pattern is 146.3lm, which is about 0.9% deviation from the mold structures. The smallest diameter of these circular shapes in the transferred pattern is 141.3lm, which is about 2.6% deviation. And the average diameter of these circular shapes in the transferred pattern is 144.3lm, resulting in an devi- ation of about 0.5%. Comparing Fig. 11(a) with Fig. 11(b), evidently, the AAO nanostructure helps solve the problems of residual inks and residual layers in transfer stamping process. 4. Conclusions Eliminating residual inks and residual layers in the transfer stamping process, in this paper, we propose and idea of creating hydrophobic nanostructures on the cavities of microstructures of Fig. 8. (a) SEM image of PC fi lm of protruded convex microstructures with nanostructures and (b) SEM image of PDMS mold of hydrophobic nanostructures on surface of micro-cavity. Fig. 9. The surface profi le of PDMS mold. Y.-H. Huang et al./Microelectronic Engineering 88 (2011) 849854853 the mold. We also report on a novel fabrication of nanostructures of anodic aluminum oxide (AAO) template on the protruded con- vex microstructures. The nanostructures and protruded convex microstructures are fabricated on the same polycarbonate (PC) fi lm by hot embossing in sequence. After that, PDMS mold is casted from the embossed PC fi lm, and eventually PDMS mold of hydro- phobic nanostructures on surface of micro-cavity is obtained. The contact angle on the fl at PDMS with nanostructures is nearly 145?, higher than that on the smooth PDMS surface without nanostructures, which is about 110?. The result proves that nanostructures can improve the hydrophobic property effectively. Besides, the contact angle on the curved PDMS with nanostruc- tures is also about 145?. Furthermore, micro-patterns with no residual layers have been successfully transferred on the PET substrate using a transfer stamping process with this PDMS mold. Obviously, the fabrication of PDMS mold of hydrophobic nano- structures on surface of micro-cavity helps solve the problems of residual inks and residual layers in transfer stamping process. References 1 A. Kumar, G.M. Whitesides, Appl. Phys. Lett.