Microcontact Printing Using Flexible Flat PDMS Stamps with Metal Embedment

*, 1 Ikjoo Byun, 2Jongho Park, and 1Beomjoon Kim 1CIRMM, Institute of Industrial Science, The University of Tokyo, Japan

2JST ERATO Higashiyama Live-Holonics Project, Nagoya University, Japan *ikjbyun@iis.u-tokyo.ac.jp

Abstract - This paper reports a microcontact printing (��CP) using flexible flat polydimethylsiloxane (PDMS) stamps with metal embedment. In conventional �CP process, self-assembled monolayer (SAM) ink can be transferred by conformal contact between astructural PDMS stamp and a substrate. In our research we utilized that hexadecanethiol (HDT), one of SAM ink molecules for �CP, can be soaked to the PDMS, but not to the Cr layer. Based on this fact, the PDMS with Cr embedment was used as a stamp for �CP even though there is 'no structural tip' (i.e. flat) in PDMS stamps.The new stamps for �CP have no mechanical deformation of stamps' tip which is crucial problem of conventional PDMS stamps during �CP. Moreover, there are several advantages compared to other flat PDMS stamps, such as no limitation of lifetime and no contamination problem during fabrication process.

Keywords - Microcontact printing (�CP), Dry lift-off process, Surface modification, Self-assembled monolayer (SAM)

I. INTRODUCTION

Microcontact printing (�CP) has developed rapidly into a robust printing tool to generate micro- or nano-patterns over the past few decays because of its intuitional simple and convenient process with low cost [1]. In typical �CP scheme, a soft elastomeric stamp is coated with an alkanethiol solution and brought into contact with a substrate to form patterns of self-assembled monolayers (SAM) on the surfaces of substrate [2]. With this simple “stamping” process, many applications, from printing proteins patterns [3] to micro-electrode arrays [4] and organic semiconductors [5], have been reported.

A polydimethylsiloxane (PDMS, typically Sylgard 184 from Dow Corning, Young’s modulus: ~1.6 MPa) is the most widely employed as a stamp material for �CP because its low elasticity allows conformal contact between the stamp and the substrate. However, when the aspect ratio, the height of the features divided by their lateral dimensions, is high, the PDMS

structures tend to bend, merge or laterally collapse due to the forces generated by peeling the stamps off and capillary force during inking a stamp as shown in Fig. 1 (a, b) [6]. On the other hand, when the aspect ratio is low, recessed surfaces of the stamps, along with the raised plates, can be deformed into contact with the substrate (i.e. roof collapse) as shown in Fig. 1 (c) [7]. This mechanical deformation of stamps' tip during contact printing can produce undesirable effects, which limits the precision of �CP.

It was reported that submicron patterning was achieved by h-PDMS (hard PDMS, Young’s modulus: ~9 MPa) [8]. Although h-PDMS stamps show smaller mechanical deformation than PDMS stamps do, �CP by h-PDMS stamps is still difficult due to its high brittleness and high coefficient of thermal expansion.The composite stamps with h-PDMS and PDMS can improve the pattern accuracy and the contact in conformal, but there are still many difficulties that preparation of two-layer stamps, cracks at h-PDMS layer, high thermal instability and occasional delamination at the interface of two layers [9].

Another approach to avoid the mechanical deformation of stamps is to use flat PDMS stamps instead of physically structured conventional PDMS stamps. The key of flat stamps is that SAM inks are selectively soaked to the parts defined by UV or chemicals [10, 11]. However, some limitations obstruct the use of these flat PDMS stamps as shown in Fig 2. For example, UV defined flat PDMS stamps return to its original hydrophobic state within a few days [10]. Meanwhile, chemical patterning on PDMS stamps by nano-imprinting and silanization should be carried out for every stamping [11].

Fig. 1. Schematic images of (a)-(c) the �CP failure by mechanical deformation of conventional PDMS stamps. (d) No mechanical deformation of flexible flat PDMS stamps with metal embedment during �CP.

Fig. 2. Schematic images of some limitations of conventional flat PDMS stamps for �CP. (a) UV or chemically treated flat PDMS stamps, (b) flat PDMS stamps with metallic stencil mask.

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Proceedings of the 2012 IEEENanotechnology Material and Devices Conference

October 16-19, 2012, Hawaii, USA

Recently, �CP with metallic nanostencil mask was suggested [12]. This nanostencil mask can act as a vapor-transport barrier without mechanical deformation of PDMS stamps. In this case, however, there is a risk of contamination of PDMS stamps caused by metal etchant during lift-off process, and it is difficult to make patterns on curved substrates due to a glass backing used as a supporter on the PDMS.

In this research, we introduce �CP using metal embedded flat PDMS stamps. SAM inks are selectively soaked to the PDMS, but not to an embedded Cr layer. This Cr layer acts as a transport barrier of SAM. Thus, the PDMS with metal embedment can be used as stamps for �CP. This technique has several advantages. First, there is no mechanical deformation of stamps' tip (e.g., buckling, lateral collapse and roof collapse) during �CP as shown in Fig. 1. Second, the proposed method not only solves both lifetime and contamination issues, but also enables to create patterns even on curved substrates as the PDMS stamps becomes much flexible. Third, the stamps are easy to handle compared to h-PDMS which is too brittle to be cracked.

The key point of fabricating the new stamps is that well embedment of metal layers into the PDMS without contamination during fabrication process. For this purpose the dry lift-off process was carried out that modifying the surface properties of the substrate and metal patterns through SAM treatment and manually peeling off the PDMS with embedded metal layers.

II. EXPERIMENT

A. Fabrication of Metal Embedded Flat PDMS Stamps A fabrication process of our new PDMS stamps is shown in

Fig. 3, briefly. A Si substrate was cleaned with piranha solution (3:1 mixture of sulfuric acid and hydrogen peroxide) for 10 min. The Si substrate was then treated with a solution of 10 mM of octadecyltrichlorosilane (OTS, CH3(CH2)17SiCl3) in hexane by immersing the substrate for 30 min, followed by rinsing sequentially with hexane, ethanol and deionized water (DI water). A self-assembled OTS monolayer on silicon substrate with thin native oxide functioned as an anti-adhesive between Si and metal layers. Cr and Au were thermally

deposited on the OTS treated substrate with thickness of 10 nm and 50 nm, respectively, and then it was patterned by conventional lithography. This substrate with metal patterns was treated with a solution of 10 mM of 11-mercaptoundecanoic acid (11-MUA, HO2C(CH2)10SH) in ethanol by immersing the substrate for 30 min, followed by rinsing sequentially with ethanol and DI water. 11-MUA acted as an adhesive between the Au and the PDMS.

PDMS prepolymer solution was prepared by mixing a silicone elastomer base and a curing agent (Sylgard 184, Dow Corning) with a weight ratio of 10:1. Then, PDMS mixture was degassed in vacuum in order to remove air bubbles formed during the mixing process. After that, this mixture was poured into a Petri dish containing the Si substrate with metal patterns, degassed again, and cured on a hot plate at 75 oC for 3 hours. Finally, this cured PDMS was manually peeled off from Si substrate with Cr and Au layers.

B. Microcontact Printing (�CP) Fig. 4 shows �CP process using flexible flat PDMS stamps.

Au was deposited on Si substrate as a substrate for �CP (50 nm of thermally deposited Au on 10 nm adhesion layer of thermally deposited Cr). Before use, the substrate was cleaned with acetone, ethanol and DI water.

The fabricated PDMS stamps with metal embedment were cleaned with ethanol, then inked with a few drops of 10 mM of hexadecanethiol (HDT, CH3(CH2)15SH) and dried in a stream of air for 1 min. After drying, the stamps were placed in conformal contact with Au substrate. Relatively short contact time (10 sec) and low pressure (2.5 gcm-2) were applied to reduce the diffusion of HDT molecules. Finally, the Au layer was selectively etched to confirm the result of �CP. A freshly prepared etching solution consisting of 1 mM K4Fe(CN)6 3H2O(Wako, 99.5% purity), 10 mM K3Fe(CN)6 (Wako, 99% purity), 100 mM Na2S2O3 5H2O (Wako, 99% purity) and 1 M KOH (Wako, 85% purity) was used to etch the gold area which was not protected by SAM ink.

Fig. 4. Schematic images of �CP process using flat PDMS stamps with metal embedment. (a) Inking the SAM ink to PDMS stamps. The SAM ink can be soaked to the PDMS parts except the metal parts, (b) Stamping the PDMS stamps to the substrate with conformal contact, (c) Releasing the PDMS stamps from the substrate, (d) Selective Au etching for confirming the �CP.

Fig. 3. Schematic images of �CP using flexible flat PDMS stamps with metal embedment: (a) Au/Cr deposition on OTS treated substrate, (b) lithography, metal etching and 11-MUA treatment on Au surface, (c) PDMS formation on the substrate with metal layers, (d) releasing the PDMS stamps from the substrate, (e) optical image of fabricated stamps.

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III. RESULT AND DISCUSSION

A. Surface Modification The key point of fabrication of this new stamp is well-

embedment of metal layers into PDMS without contamination during fabrication process. For this purpose dry lift-off process was carried out that embedding the metal patterns into PDMS by peeling off the PDMS from substrate manually. The large difference of adhesion between PDMS/metal and metal/Si substrate made successful fabrication results.

For better releasing of metal layers from the Si substrate, the Si substrate was treated by OTS (water contact angle: 110o,surface energy: 17 mJ/m2) shown in Fig. 5. After OTS treatment, the surface energy of Si substrate became low enough to release the metal layers (Cr/Au) by peeling off the PDMS which was cured on Au surface, but high enough to adhere the metal layers (Cr/Au) with the substrate during lithography and metal etching process.

Another surface modification by 11-MUA was carried out on Au surface to promote chemical binding with PDMS (water contact angle: 25o, surface energy: 67 mJ/m2) shown in Fig 6. The bonding mechanism is less certain, but clearly the carboxylate of carboxyl linkages in 11-MUA and subsequent bond with alkoxy group of PDMS is likely.

B. Fabrication Result of Metal Embedded PDMS Stamps Fig. 7 shows the flexible flat PDMS stamps that were

fabricated through the process above. All metal layers were transferred from the Si substrate into the PDMS layer without any residues. Because of mechanical stress during the peeling off process, one-dimensional ‘wavy’ geometries were obtained on metal layers.

C. Result of �CPThe scanning-electron-microscopy (SEM) images show a

detail dimension of final �CPed-patterns (Fig. 8). The selectively soaked HDT molecules were successfully printed on the Au layer, and they acted as an etch mask during the Au etching process. The linewidth of PDMS stamps was 13 �m, but the linewidth of �CPed-Au-patterns was 15 �m. According to this result, it can be estimated that there was a diffusion of HDT molecules during �CP. Diffusion was not critical in this experiment, however, the diffusion issue can be reduced by optimizing variables of �CP (e.g. contact time and applied pressure). It was observed that the ‘wavy’ geometries of metals, which were embedded in the PDMS, did not affect the quality of �CP critically.

IV. CONCLUSION

The flexible flat PDMS stamps with metal embedment were successfully fabricated by dry lift-off process. Metal patterns could be embedded by surface modification through SAMs, OTS and 11-MUA, used as an anti-adhesive and an adhesive,respectively.

HDT ink was selectively soaked to the PDMS, but not to the Cr layer embedded to the PDMS. Thus, this soaking selectivity of HDT ink enabled �CP using this new flat PDMS stamp. The mechanical deformation of PDMS stamps is a crucial problem

Fig. 7. Optical images of metal patterns which were embedded to PDMS. Bright or yellow area is Cr/Au and dark area is PDMS.

Fig. 8. SEM images of Au patterns after �CP of HDT and Au etching process. Bright area is Au with HDT, and dark area is Cr/Si substrate.

Fig. 6. 11-MUA treatment on Au patterns. (a) Schematic images of 11-MUA formation on Au, (b) water contact angle changed from 90o (before 11-MUA treatment) to 25o (after 11-MUA treatment).

Fig. 5. OTS treatment on Si substrate with thin native oxide. (a) Schematic images of OTS formation on Si with thin native oxide, (b) water contact angle changed from 3o (before OTS treatment) to 110o (after OTS treatment).

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of conventional structured PDMS. During contact printing, we could not observe this issue. Metal embedded flat PDMS stamps reduce or eliminate the limitation of conventional PDMS stamps, and also enable the high-resolution patterning. Furthermore, it is expected that �CP can be possible on non-planer surface with higher resolution accompanied by no mechanical deformation of stamps' tip.

ACKNOWLEDGMENT

One of the authors (I. Byun) was supported through the Global COE Program "Global Center of Excellence for Mechanical Systems Innovation" by the Ministry of Education, Culture, Sport, Science and Technology.

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