High density nanostructure transfer in soft molding using polyurethane acrylate molds and polyelectrolyte multilayers

Here we present an alternative, new, unconventional lithographic technique developed to create dense and multilevel nanostructure pattern transfer using a highly accurate polyurethane acrylate (PU, MINS101m, Minuta Tech.) mold and a polyelectrolyte multilayer as the adhesion promotion layer. Specifically, we demonstrate the pattern transfer of periodic 80 nm lines with 400 nm height and complex and multilevel nanostructures to a polymer layer on various substrates, such as Si or SiO2 wafers, glass and flexible polymer films. This new, unconventional lithographic technique presented here would open the door to a variety of applications in the fields of electronic, optical and biological devices. The fabrication of electronic, optical and biological devices requires the patterning of surfaces on microand nanometre scales. Currently the dominant technology for microand nanostructure fabrication is photolithography. However, the cost of short-wavelength light sources and photosensitive polymer (photoresist) increases rapidly with diminishing resolution limit. As the required feature size is reduced further, photolithography will eventually reach its limits. Unconventional lithographic techniques such as soft lithography [1] and imprint lithography [2] have been developed as alternatives for photolithography and utilized successfully in a number of applications in materials science [3, 4], including flexible displays [5]. Over the past decade extensive efforts have been made to replace photolithography with unconventional patterning techniques. A number of promising low-cost techniques that have been developed recently involve a form of soft lithography [1]; such methods involve the use of an elastomeric master that is used to transfer a pattern, including microcontact printing, replica molding, micro-transfer molding, and micro-molding in capillaries. Recent additions to these nonlithographic approaches include ‘dip-pen’ 3 Author to whom any correspondence should be addressed. nanolithography [6], soft molding [7, 8], capillary force lithography [9], polymer-on-polymer stamping [10] and patterning techniques using de-wetting of a thin polymer layer [11]. The majority of these soft lithography techniques utilize a polydimethylsiloxane (PDMS) mold, which presents some advantages. In particular, PDMS reproduces nanoscale structures with great fidelity. Also, there is generally poor adhesion between the PDMS mold surface and the polymer to be molded [1], allowing complete release from the mold. However, with regard to nanostructure fabrication, the use of a PDMS mold has some disadvantages. Although softlithography methods such as replica molding, micro-molding in capillaries and others can make isolated line and ring structures at the nanometre scale, it is generally difficult to produce a densely arrayed nanopattern (e.g. alternating 100 nm line/space) because the PDMS mold collapses laterally [1]. Recently, researchers in the Whitesides group improved the pattern fidelity using a hard PDMS mixture; the process involves the construction of a two-component stamp consisting of a soft, thin PDMS layer and a thicker, harder, more highly cross-linked polysiloxane as a support [12]. This approach alleviates a number of the issues of collapse, but requires the construction of a more complex elastomeric mold. 0957-4484/03/101140+05$30.00 © 2003 IOP Publishing Ltd Printed in the UK 1140 High density nanostructure transfer in soft molding using polyurethane acrylate molds and polyelectrolyte multilayers Figure 1. An illustration of the procedure for fabricating a PU mold from the master. SEM images of (a) a replicated PU mold and (b) a master. Optical microscopy images (500×) show the good fidelity of the PU mold in nanostructure replication: (c) is the laterally collapsed PDMS mold; (d) is the PU mold in replication of the same master; (e) shows periodic 80 nm lines in the PU mold. The use of a completely hard mold has been demonstrated with imprint lithography, which has the unique feature that it can be used to create especially dense nanostructures [2]. Although imprint lithography is a promising technique for nanostructure fabrication, some challenges to overcome include problems in release due to strong adhesion between the hard mold and the polymer layer on the substrate [13], variations due to changes in the pattern density [13, 14], and limited material transport [15]. Imprint lithography with a high-aspect-ratio mold can induce deformation of the polymer layer on the substrate by adhering to the stamp [14] and uneven substrates can be broken by the high pressure required for the technique. In this report, we present a simple method for multistep nanopattern fabrication with a high feature density. This method overcomes the difficulties and shortcomings of PDMS-based soft lithography and hard-mold-based imprint lithography. We use a combination of a UV-curable polyurethane acrylate mold, the soft-molding process utilizing water soluble sulfonated polystyrene (SPS), and multilayer deposition [16] for the creation of an adhesion promotion layer between the polymer layer and the substrate. The first step is to fabricate a highly accurate elastomer mold using UV-curable polyurethane acrylate (MINS101m, Minuta Tech.). Figure 1 illustrates schematically the process of forming the polyurethane acrylate (PU) mold, and a comparison between PUand PDMS-molded stamps. The PU mold is harder than the general PDMS mold (the Young’s modulus of the PU mold is ∼1.7 × 109 N m−2 at 15 ◦C). However, the PU mold softens dramatically upon reaching a temperature of 50 ◦C. At this softening temperature, the mold becomes conformal with the underlying polymer film that is to be molded. A comparison of the master (figure 1(a)) with the PU mold (figure 1(b)) reveals that the original master is replicated with high fidelity. The PU mold has two distinct differences from the PDMS molds typically used in soft lithography. First, in the PU mold it is possible to make a highly accurate mold with a defect-free, dense nanopattern. Figures 1(c)–(e) show the effectiveness of the PU mold with dense nanostructures. The nanostructures in a PDMS mold