Communications to the Editor Bull. Korean Chem. Soc. 2011, Vol. 32, No. 1 33 DOI 10.5012/bkcs.2011.32.1.33 Roof-Collapsed PDMS Mask for Nanochannel Fabrication Jinyong Lee, Young-keu Yoon, Jungwook Kim, Yoori Kim, and Kyubong Jo * Department of Chemistry and Interdisciplinary Program of Integrated Biotechnology, Sogang University, Seoul 121-742, Korea. * E-mail: jokyubong@sogang.ac.kr Received October 6, 2010, Accepted October 26, 2010 Key Words: Unconventional lithography, Roof-collapsed PDMS mask, Nanochannels Si Substrate Si Substrate AZ-GXR(Positive PR) PDMS Mask Si Substrate AZ-GXR(Positive PR) PDMS Mask Photolithography Roof collapse Figure 1. A scheme of nanochannel fabrication with roof collapsed mask. PDMS mask has a nanoslit patterns with the height ranged from 350 to 700 nm and the widths from 40 μm to 80 μm. PDMS nanoslits are placed on the thin positive photoresis layer of thickness range from 100 nm to 1 μm. Then PDMS nanoslits spontaneously collapse. Conventional photolithography aligner transfer roof-collapsed nano- channel patterns to the photoresist layer. Here we present a radically simple approach for nanochannel fabrication without the use of any expensive equipment or any instrument modifications. Our approach consists of consecutive simple photolithographic steps: first, we prepare sub-micron high but very wide PDMS nanoslits by using soft lithography. Second, as a mask we utilize PDMS nanoslits collapsed on the thinly coated photoresist layer. Standard photolithography trans- fers roof-collapsed PDMS nanochannels to a sub-micron thick photoresist layer. Then, we add a micropattern overlay by using micromoulding in capillary method. Consequently, we fabricate nanochannels embedded in microchannels by combination of simple processes: soft lithography, roof-collapse, and standard photolithography. Nanochannel is an important component in a microfluidic device for biochemical analytical systems. Dimensions com- parable to biomolecules allow nanochannels to confine, mani- pulate, and visualize single biomolecules. 1-2 Recently, consider- able attention has been directed to nanochannel confined DNA elongation to determine DNA size without separation. 3-4 In addition, elongated DNA molecules can be utilized as platforms for biochemical studies such as RNA polymerase on DNA back- bones. Accordingly, nanochannel promises more comprehen- sive biochemical analysis platform. Conventionally, nanochannel fabrication requires expensive equipment such as electron beam (e-beam) and focused ion beam (FIB) lithography which are prohibitively expensive for a single academic lab. Also, these serial techniques are much slower than the parallel approach of projection lithography such as X-ray lithography, which however requires a huge synchro- tron facility to generate nanometer wavelengths. Alternatively, nanoimprint lithography emerged as a simple process with high throughput and high resolution over large area. 5 However, nano- meter resolution template patterns are being sold at a very high price. Thus, contemporary conventional nanofabrication tech- nologies are not appropriate to simply test a variety of creative or premature ideas in an academic lab. As alternative solutions, a number of unconventional approaches have been developed for avoiding these financial limitations. 6 Recently, a simple unconventional approach for the formation of nanochannels has been reported that short and wide elasto- meric nanoslits of polydimethylsiloxane (PDMS) collapse to form gutters of sub-micron for nanofluidic device, called roof- collapsed nanochannels. 7-8 Paradoxically, roof collapse has been an issue for micro-contact printing 9 and the formation of PDMS nanoslits. 2 Several papers have been reported for new theories and experimental findings to avoid elastomeric deform- ation. 10-11 Comparing to other conventional or unconventional nanochannel fabrication, roof collapse is far easier, but this approach has intrinsic limitations to add more sophisticated and complex structures such as multiple layers. 7 Here we design a novel approach of transferring roof collapsed PDMS nanochannels to solid nanostructures on the photoresist. Figure 1 illustrates our scheme how to use roof collapsed nano- channel pattern as a photolithographic mask. Briefly, we place PDMS mask (40 μm × 700 nm) on contact with a thin layer of photoresist, and then roof collapsing occurs on a photoresist layer. As standard lithography procedure, ultraviolet light is exposed onto the PDMS mask to transfer roof collapsed nano- channels to a thin photoresist layer ranged from 100 nm to 1 μm: the height is controlled by mixing ratio of photoresist with thinner. Figure 2a shows a scanning electron microscope image of nanochannel pattern on a photoresist layer transferred. Interes- tingly, the cross section of the nanochannel is an asymmetric peak shape, which resembles the cross-section shape of roof- collapsed channels as previously reported. 7-8 The reason of asym- metry is that a roof collapsed nanochannel has two sides: one side used to be a nanoslit wall and the other side used to be a nanoslit roof. When a roof collapsed, the wall side is relatively steeper and the roof side is leaned with a tail. A microscope