Soft UV Nanoimprint Lithography: A Versatile Tool for Nanostructuration at the 20nm Scale

Abstract

1.1Why soft UV nanoimprint lithography? Since the pioneering work of Whitesides and coworkers on microContact Printing (mCP) and Soft Lithography (Kumar & Whitesides (1993)) (Xia & Whitsides (1998)), considerable progress has been made in the last years and Soft Lithography is now a well consolidated technology utilized in academic and industrial laboratories (Rogers & Nuzzo (2005)). These printing methods use a flexible elastomer material named PDMS (poly(dimethylsiloxane)) to transfer molecules on a surface thus creating localized chemical patterns (Cerf & Vieu (2010)). The PDMS stamp inked with the desired molecules is placed in contact with the substrate to perform the molecular transfer. mCP has received large attention for biological applications since this soft transfer occurs in a gentlemanner which allows the biomolecules to be transferred without any damage. In addition, this powerful technique is cheap because the soft PDMS stamp can be replicated an indefinite number of times by simply pouring the PDMS prepolymer onto a single expensive siliconmaster mold that containsmicro or nanostructures. Since the flexibility of the elastomeric stamp ensures a perfect conformal adhesion within the substrate, thus allowing replication on large areas up to several tens of cm2, the use of such flexible PDMS stamps was also efficiently applied to another low-cost and high-throughput manufacturing technique called Soft UV Nanoimprinting Lithography (Soft UV-NIL). This technique creates a thickness contrast by embossing thin polymeric films, highlighting the advantages of using a flexible PDMS stamp. Historically, Nanomprint Lithography (NIL) in its original version was proposed by Stephen Chou in 1995 (Chou et al. (1995)) as an alternative technique for the embossing of high resolution patterns in thermoplastic materials. The patterning of features as small as 10 nm has been demonstrated from the beginning (Chou (1997)). This nanoimprint process, usually referred to as thermal-assisted NIL (T-NIL), is based on the use of a hard mold, namely a silicon wafer. As schematically shown in Figure 1, this hard mold containing nanoscale surface-relief features is pressed at high pressure (50-100 bar) onto a thin polymeric resist film. The resist is held some 90-100 C above its glass-transition temperature (Tg) for fewminutes to allow the flowing of the polymer in the mold nanocavities. The thin residual layer of polymer intentionally left to prevent the direct contact between the substrate and the rigid mold is Soft UV Nanoimprint Lithography: A Versatile Tool for Nanostructuration at the 20nm Scale